C Functions

C structures, c structures (structs).

Structures (also called structs) are a way to group several related variables into one place. Each variable in the structure is known as a member of the structure.

Unlike an array , a structure can contain many different data types (int, float, char, etc.).

Create a Structure

You can create a structure by using the struct keyword and declare each of its members inside curly braces:

To access the structure, you must create a variable of it.

Use the struct keyword inside the main() method, followed by the name of the structure and then the name of the structure variable:

Create a struct variable with the name "s1":

Access Structure Members

To access members of a structure, use the dot syntax ( . ):

Now you can easily create multiple structure variables with different values, using just one structure:

What About Strings in Structures?

Remember that strings in C are actually an array of characters, and unfortunately, you can't assign a value to an array like this:

An error will occur:

However, there is a solution for this! You can use the strcpy() function and assign the value to s1.myString , like this:

Simpler Syntax

You can also assign values to members of a structure variable at declaration time, in a single line.

Just insert the values in a comma-separated list inside curly braces {} . Note that you don't have to use the strcpy() function for string values with this technique:

Note: The order of the inserted values must match the order of the variable types declared in the structure (13 for int, 'B' for char, etc).

Copy Structures

You can also assign one structure to another.

In the following example, the values of s1 are copied to s2:

Modify Values

If you want to change/modify a value, you can use the dot syntax ( . ).

And to modify a string value, the strcpy() function is useful again:

Modifying values are especially useful when you copy structure values:

Ok, so, how are structures useful?

Imagine you have to write a program to store different information about Cars, such as brand, model, and year. What's great about structures is that you can create a single "Car template" and use it for every cars you make. See below for a real life example.

Real-Life Example

Use a structure to store different information about Cars:

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Structure assignment and its pitfall in c language.

Jan 28 th , 2013 9:47 pm

There is a structure type defined as below:

If we want to assign map_t type variable struct2 to sturct1 , we usually have below 3 ways:

Consider above ways, most of programmer won’t use way #1, since it’s so stupid ways compare to other twos, only if we are defining an structure assignment function. So, what’s the difference between way #2 and way #3? And what’s the pitfall of the structure assignment once there is array or pointer member existed? Coming sections maybe helpful for your understanding.

The difference between ‘=’ straight assignment and memcpy

The struct1=struct2; notation is not only more concise , but also shorter and leaves more optimization opportunities to the compiler . The semantic meaning of = is an assignment, while memcpy just copies memory. That’s a huge difference in readability as well, although memcpy does the same in this case.

Copying by straight assignment is probably best, since it’s shorter, easier to read, and has a higher level of abstraction. Instead of saying (to the human reader of the code) “copy these bits from here to there”, and requiring the reader to think about the size argument to the copy, you’re just doing a straight assignment (“copy this value from here to here”). There can be no hesitation about whether or not the size is correct.

Consider that, above source code also has pitfall about the pointer alias, it will lead dangling pointer problem ( It will be introduced below section ). If we use straight structure assignment ‘=’ in C++, we can consider to overload the operator= function , that can dissolve the problem, and the structure assignment usage does not need to do any changes, but structure memcpy does not have such opportunity.

The pitfall of structure assignment:

Beware though, that copying structs that contain pointers to heap-allocated memory can be a bit dangerous, since by doing so you’re aliasing the pointer, and typically making it ambiguous who owns the pointer after the copying operation.

If the structures are of compatible types, yes, you can, with something like:

} The only thing you need to be aware of is that this is a shallow copy. In other words, if you have a char * pointing to a specific string, both structures will point to the same string.

And changing the contents of one of those string fields (the data that the char points to, not the char itself) will change the other as well. For these situations a “deep copy” is really the only choice, and that needs to go in a function. If you want a easy copy without having to manually do each field but with the added bonus of non-shallow string copies, use strdup:

This will copy the entire contents of the structure, then deep-copy the string, effectively giving a separate string to each structure. And, if your C implementation doesn’t have a strdup (it’s not part of the ISO standard), you have to allocate new memory for dest_struct pointer member, and copy the data to memory address.

Example of trap:

Below diagram illustrates above source memory layout, if there is a pointer field member, either the straight assignment or memcpy , that will be alias of pointer to point same address. For example, b.alias and c.alias both points to address of a.alias . Once one of them free the pointed address, it will cause another pointer as dangling pointer. It’s dangerous!!

  • Recommend use straight assignment ‘=’ instead of memcpy.
  • If structure has pointer or array member, please consider the pointer alias problem, it will lead dangling pointer once incorrect use. Better way is implement structure assignment function in C, and overload the operator= function in C++.
  • stackoverflow.com: structure assignment or memcpy
  • stackoverflow.com: assign one struct to another in C
  • bytes.com: structures assignment
  • wikipedia: struct in C programming language

cppreference.com

Struct declaration.

A struct is a type consisting of a sequence of members whose storage is allocated in an ordered sequence (as opposed to union, which is a type consisting of a sequence of members whose storage overlaps).

The type specifier for a struct is identical to the union type specifier except for the keyword used:

[ edit ] Syntax

[ edit ] explanation.

Within a struct object, addresses of its elements (and the addresses of the bit-field allocation units) increase in order in which the members were defined. A pointer to a struct can be cast to a pointer to its first member (or, if the member is a bit-field, to its allocation unit). Likewise, a pointer to the first member of a struct can be cast to a pointer to the enclosing struct. There may be unnamed padding between any two members of a struct or after the last member, but not before the first member. The size of a struct is at least as large as the sum of the sizes of its members.

[ edit ] Forward declaration

A declaration of the following form

hides any previously declared meaning for the name name in the tag name space and declares name as a new struct name in current scope, which will be defined later. Until the definition appears, this struct name has incomplete type .

This allows structs that refer to each other:

Note that a new struct name may also be introduced just by using a struct tag within another declaration, but if a previously declared struct with the same name exists in the tag name space , the tag would refer to that name

[ edit ] Keywords

[ edit ] notes.

See struct initialization for the rules regarding the initializers for structs.

Because members of incomplete type are not allowed, and a struct type is not complete until the end of the definition, a struct cannot have a member of its own type. A pointer to its own type is allowed, and is commonly used to implement nodes in linked lists or trees.

Because a struct declaration does not establish scope , nested types, enumerations and enumerators introduced by declarations within struct-declaration-list are visible in the surrounding scope where the struct is defined.

[ edit ] Example

Possible output:

[ edit ] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

[ edit ] References

  • C23 standard (ISO/IEC 9899:2023):
  • 6.7.2.1 Structure and union specifiers (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 6.7.2.1 Structure and union specifiers (p: 81-84)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.7.2.1 Structure and union specifiers (p: 112-117)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.7.2.1 Structure and union specifiers (p: 101-104)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.5.2.1 Structure and union specifiers

[ edit ] See also

  • struct and union member access
  • struct initialization
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Next: Unions , Previous: Overlaying Structures , Up: Structures   [ Contents ][ Index ]

15.13 Structure Assignment

Assignment operating on a structure type copies the structure. The left and right operands must have the same type. Here is an example:

Notionally, assignment on a structure type works by copying each of the fields. Thus, if any of the fields has the const qualifier, that structure type does not allow assignment:

See Assignment Expressions .

When a structure type has a field which is an array, as here,

structure assigment such as r1 = r2 copies array fields’ contents just as it copies all the other fields.

This is the only way in C that you can operate on the whole contents of a array with one operation: when the array is contained in a struct . You can’t copy the contents of the data field as an array, because

would convert the array objects (as always) to pointers to the zeroth elements of the arrays (of type struct record * ), and the assignment would be invalid because the left operand is not an lvalue.

  • How to Initialize a Struct in C

Use Designated Initializers to Initialize a Struct in C

Use compound literals to initialize a struct in c, use explicit assignments to initialize a struct in c, use constructors (in c99 and later) to initialize a struct in c.

How to Initialize a Struct in C

Structs in C provide a powerful mechanism for grouping variables of diverse data types under a unified name. Properly initializing a struct is crucial for ensuring the correct functioning of your programs.

In this article, we’ll explore various methods to initialize a struct in C, each offering its advantages.

In C programming, structs serve as a pivotal tool for grouping variables of diverse data types under a unified name. When it comes to initializing a struct, one particularly versatile technique is the use of designated initializers.

This method allows for explicit specification of which members of the struct to initialize, offering clarity and flexibility in code organization.

The syntax for designated initializers involves specifying the member name followed by an equal sign and the value to assign. The general form looks like this:

This syntax allows initializing specific members of the struct while leaving others uninitialized, fostering flexibility in struct initialization.

Let’s dive into an example featuring a Person struct, representing an individual’s first name, last name, age, and a boolean indicating their vitality. Designated initializers will be employed to create an instance of this struct.

In this code snippet, we include necessary header files ( stdbool.h , stdio.h , stdlib.h , string.h ) and define a Person struct using typedef . The struct encapsulates members for the first name ( firstname ), last name ( lastname ), age, and a boolean indicating whether the person is alive ( alive ).

In the main function, we create an instance of the Person struct named me . Using designated initializers, we initialize each member of the struct with specific values.

The first name is set to John , the last name to McCarthy , the age to 24 , and the alive status to true .

The subsequent printf statement displays the information stored in the me struct. It prints the first name, last name, and age using the %s and %d format specifiers, respectively.

The last part of the output depends on the boolean value of the alive member. If alive is true , Yes is printed; otherwise, No is printed.

Finally, the program exits with EXIT_SUCCESS , indicating successful execution.

Another versatile technique to initialize a struct is using compound literals. This method allows for the creation of temporary instances of a struct with specific initial values.

The syntax for compound literals involves enclosing a list of values within parentheses and casting them to the desired struct type. The general form looks like this:

This syntax allows for the creation of an unnamed temporary instance of the struct with the specified initial values.

Let’s delve into a practical example using the Person struct:

In this example, we have a Person struct representing an individual’s first name, last name, age, and alive status. The main function initializes an instance of this struct named me using a compound literal.

The compound literal (Person){.firstname = "John\0", .lastname = "McCarthy\0", .age = 24, .alive = 1} creates a temporary instance of the Person struct with each member explicitly initialized.

This method combines clarity and conciseness, eliminating the need for a separate declaration and assignment step.

The subsequent printf statement displays the information stored in the me struct, presenting the first name, last name, age, and alive status. The output, in this case, would be:

Using compound literals in this manner enhances the readability of the code, making it clear and expressive. This approach is particularly advantageous when immediate struct initialization is preferred, providing a streamlined and elegant solution for struct initialization in C.

When initializing struct members in C, an alternative method involves declaring a variable and assigning each member with its corresponding value individually. This approach is characterized by explicit assignments, making it clear and straightforward.

It’s worth noting that when dealing with char arrays, assigning them directly with strings isn’t allowed in C. Instead, explicit copying using functions like memcpy or memmove is necessary (refer to the manual for more details).

Additionally, care should be taken to ensure that the length of the array is not less than the string being stored.

Here’s an illustrative example using a Person struct:

As we can see in this example, each member of the me2 instance is assigned a value individually. Here, the use of memcpy ensures the proper copying of string values into char arrays, respecting the necessary length constraints.

The subsequent printf statement remains unchanged, displaying the information stored in the me2 struct. The output, as before, will be:

While this approach is valid, it often involves more lines of code and can be less concise compared to using compound literals, as demonstrated in the previous section.

Compound literals provide a more streamlined and expressive way to initialize structs in C, especially when dealing with immediate initialization or temporary instances.

The concept of constructors, while not native, like in some other languages, can be emulated to achieve struct initialization.

C99 and later versions allow for a more expressive and organized approach by enabling the use of designated initializers within functions. This technique can be likened to a constructor, allowing for structured and customizable initialization of structs.

The syntax for creating a constructor-like function involves defining a function that returns an instance of the struct with designated initializers.

Here’s an example using a Person struct:

In this example, we define a Person struct with members for the first name, last name, age, and alive status. Here, the createPerson function serves as a constructor-like function, initializing and configuring a Person struct with specific values.

The function uses strncpy to copy the first and last names into the struct’s firstname and lastname members, respectively. It also ensures null-termination of the strings to prevent potential buffer overflows.

The age and alive members are then set according to the provided arguments.

In the main function, the createPerson function is used to initialize a Person struct named me . The subsequent printf statement displays the information stored in the me struct, showcasing the first name, last name, age, and alive status.

This constructor-like approach offers a structured and modular way to initialize structs in C, providing a clear and organized solution for customizable struct initialization.

Initializing structs in C involves a range of techniques, each catering to different preferences and scenarios.

Designated initializers offer clarity and flexibility, whereas compound literals provide immediate initialization. Explicit assignments ensure straightforward initialization and constructors (in C99 and later) allow for a structured approach.

Choose the method that aligns with your coding style and the specific requirements of your program. Whether you prioritize clarity, conciseness, or modularity, understanding these initialization techniques will empower you to write more efficient and maintainable C code.

Jinku Hu avatar

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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The structure in C is a user-defined data type that can be used to group items of possibly different types into a single type. The struct keyword is used to define the structure in the C programming language. The items in the structure are called its member and they can be of any valid data type.

C Structure Declaration

We have to declare structure in C before using it in our program. In structure declaration, we specify its member variables along with their datatype. We can use the struct keyword to declare the structure in C using the following syntax:

The above syntax is also called a structure template or structure prototype and no memory is allocated to the structure in the declaration.

C Structure Definition

To use structure in our program, we have to define its instance. We can do that by creating variables of the structure type. We can define structure variables using two methods:

1. Structure Variable Declaration with Structure Template

2. structure variable declaration after structure template, access structure members.

We can access structure members by using the ( . ) dot operator.

In the case where we have a pointer to the structure, we can also use the arrow operator to access the members.

Initialize Structure Members

Structure members cannot be initialized with the declaration. For example, the following C program fails in the compilation.

The reason for the above error is simple. When a datatype is declared, no memory is allocated for it. Memory is allocated only when variables are created.

We can initialize structure members in 3 ways which are as follows:

  • Using Assignment Operator.
  • Using Initializer List.
  • Using Designated Initializer List.

1. Initialization using Assignment Operator

2. initialization using initializer list.

In this type of initialization, the values are assigned in sequential order as they are declared in the structure template.

3. Initialization using Designated Initializer List

Designated Initialization allows structure members to be initialized in any order. This feature has been added in the C99 standard .

The Designated Initialization is only supported in C but not in C++.

Example of Structure in C

The following C program shows how to use structures

typedef for Structures

The typedef keyword is used to define an alias for the already existing datatype. In structures, we have to use the struct keyword along with the structure name to define the variables. Sometimes, this increases the length and complexity of the code. We can use the typedef to define some new shorter name for the structure.

Nested Structures

C language allows us to insert one structure into another as a member. This process is called nesting and such structures are called nested structures . There are two ways in which we can nest one structure into another:

1. Embedded Structure Nesting

In this method, the structure being nested is also declared inside the parent structure.

2. Separate Structure Nesting

In this method, two structures are declared separately and then the member structure is nested inside the parent structure.

One thing to note here is that the declaration of the structure should always be present before its definition as a structure member. For example, the declaration below is invalid as the struct mem is not defined when it is declared inside the parent structure.

Accessing Nested Members

We can access nested Members by using the same ( . ) dot operator two times as shown:

Example of Structure Nesting

Structure pointer in c.

We can define a pointer that points to the structure like any other variable. Such pointers are generally called Structure Pointers . We can access the members of the structure pointed by the structure pointer using the ( -> ) arrow operator.

Example of Structure Pointer

Self-referential structures.

The self-referential structures in C are those structures that contain references to the same type as themselves i.e. they contain a member of the type pointer pointing to the same structure type.

Example of Self-Referential Structures

Such kinds of structures are used in different data structures such as to define the nodes of linked lists, trees, etc.

C Structure Padding and Packing

Technically, the size of the structure in C should be the sum of the sizes of its members. But it may not be true for most cases. The reason for this is Structure Padding.

Structure padding is the concept of adding multiple empty bytes in the structure to naturally align the data members in the memory. It is done to minimize the CPU read cycles to retrieve different data members in the structure.

There are some situations where we need to pack the structure tightly by removing the empty bytes. In such cases, we use Structure Packing. C language provides two ways for structure packing:

  • Using #pragma pack(1)
  • Using __attribute((packed))__

Example of Structure Padding and Packing

As we can see, the size of the structure is varied when structure packing is performed.

To know more about structure padding and packing, refer to this article – Structure Member Alignment, Padding and Data Packing .

Bit Fields are used to specify the length of the structure members in bits. When we know the maximum length of the member, we can use bit fields to specify the size and reduce memory consumption.

Syntax of Bit Fields

 example of bit fields.

As we can see, the size of the structure is reduced when using the bit field to define the max size of the member ‘a’.

Uses of Structure in C

C structures are used for the following:

  • The structure can be used to define the custom data types that can be used to create some complex data types such as dates, time, complex numbers, etc. which are not present in the language.
  • It can also be used in data organization where a large amount of data can be stored in different fields.
  • Structures are used to create data structures such as trees, linked lists, etc.
  • They can also be used for returning multiple values from a function.

Limitations of C Structures

In C language, structures provide a method for packing together data of different types. A Structure is a helpful tool to handle a group of logically related data items. However, C structures also have some limitations.

  • Higher Memory Consumption: It is due to structure padding.
  • No Data Hiding: C Structures do not permit data hiding. Structure members can be accessed by any function, anywhere in the scope of the structure.
  • Functions inside Structure: C structures do not permit functions inside the structure so we cannot provide the associated functions.
  • Static Members: C Structure cannot have static members inside its body.
  • Construction creation in Structure: Structures in C cannot have a constructor inside Structures.

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Intro to C for CS31 Students

Part 2: structs & pointers.

  • Pointers and Functions C style "pass by referece"
  • Dynamic Memory Allocation (malloc and free)
  • Pointers to Structs

Defining a struct type

Declaring variables of struct types, accessing field values, passing structs to functions, rules for using pointer variables.

  • Next, initialize the pointer variable (make it point to something). Pointer variables store addresses . Initialize a pointer to the address of a storage location of the type to which it points. One way to do this is to use the ampersand operator on regular variable to get its address value: int x; char ch; ptr = &x; // ptr get the address of x, pointer "points to" x ------------ ------ ptr | addr of x|--------->| ?? | x ------------ ------ cptr = &ch; // ptr get the address of ch, pointer "points to" ch cptr = &x; // ERROR! cptr can hold a char address only (it's NOT a pointer to an int) All pointer variable can be set to a special value NULL . NULL is not a valid address but it is useful for testing a pointer variable to see if it points to a valid memory address before we access what it points to: ptr = NULL; ------ ------ cptr = NULL; ptr | NULL |-----| cptr | NULL |----| ------ ------
  • Use *var_name to dereference the pointer to access the value in the location that it points to. Some examples: int *ptr1, *ptr2, x, y; x = 8; ptr1 = NULL; ptr2 = &x; ------------ ------ ptr2 | addr of x|--------->| 8 | x ------------ ------ *ptr2 = 10; // dereference ptr2: "what ptr2 points to gets 10" ------------ ----- ptr2 | addr of x|--------->| 10 | x ------------ ----- y = *ptr2 + 3; // dereference ptr2: "y gets what ptr2 points to plus 3" ----- ----- ptr1 = ptr2; // ptr1 gets address value stored in ptr2 ------------ ----- ptr2 | addr of x |--------->| 10 | x ------------ ----- /\ ------------ | ptr1 | addr of x |-------------- ------------ // TODO: finish tracing through this code and show what is printed *ptr1 = 100; ptr1 = &y; *ptr1 = 80; printf("x = %d y = %d\n", x, y); printf("x = %d y = %d\n", *ptr2, *ptr1);
  • and what does this mean with respect to the value of each argument after the call?
  • Implement a program with a swap function that swaps the values stored in its two arguments. Make some calls to it in main to test it out.

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Pointers, the heap, and functions, linked lists in c.

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Naman Tamrakar

Posted on Jul 12, 2022

Ways to copy struct in c/c++

Today I am gonna show some of the ways to copy struct in c/c++ .

In this approach we will copy attribute by attribute of struct into another struct to make a copy. The rest code is self explainable.

Disadvantage in this approach

Although It is very clear that this code is written for copying the struct. But as the size of struct increase It becomes more difficult to type all the attributes and So the chance of typo also increases.

So what's the solution for this. Let's discuss our next approach.

Instead of copying attribute by attributes We can also copy whole struct using memcpy and rest code will be same. Just copy whole struct in one line only.

We can also use assignment operator to make copy of struct.

A lot of people don't even realize that they can copy a struct this way because one can't do same it with an array. Similarly one can return struct from a function and assign it but not array. The compiler do all the copying stuff behind the scene for us.

NOTE : When dealing with struct pointers approach 3 doesn't work as assigning one pointer to another doesn't make copy instead just point to new memory address. So if we modify new variable, original one is also affected.

❤️Thank you so much for reading it completely. If you find any mistake please let me know in comments.

Also consider sharing and giving a thumbs up If this post help you in any way.

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5.3. Structures

  • Declaration and definition
  • Stack And Heap
  • new And delete
  • Dot And Arrow

Programmers often use the term "structure" to name two related concepts. They form structure specifications with the struct keyword and create structure objects with a variable definition or the new operator. We typically call both specifications and objects structures, relying on context to clarify the the specific usage.

Structures are an aggregate data type whose contained data items are called fields or members . Once specified, the structure name becomes a new data type or type specifier in the program. After the program creates or instantiates a structure object, it accesses the fields with one of the selection operators . Referring to the basket analogy suggested previously, a structure object is a basket, and the fields or members are the items in the basket. Although structures are more complex than the fundamental types, programmers use the same pattern when defining variables of either type.

Structure Specifications: Fields And Members

A program must specify or declare a structure before using it. The specification determines the number, the type, the name, and the order of the data items contained in the structure.

Structure Definitions And Objects

We can use a blueprint as another analogy for a structure. Blueprints show essential details like the number and size of bedrooms, the location of the kitchen, etc. Similarly, structure specifications show the structure fields' number, type, name, and order. Nevertheless, a blueprint isn't a house; you can't live in it. However, given the blueprint, we can make as many identical houses as we wish, and while each one looks like the others, it has a different, distinct set of occupants and a distinct street address. Similarly, each object created from a structure specification looks like the others, but each can store distinct values and has a distinct memory address.

  • C++ code that defines three new student structure variables or objects named s1 , s2 , and s3 . The program allocates the memory for the three objects on the stack.
  • The pointer variables s4 , s5 , and s6 are defined and allocated automatically on the stack. However, the structure objects they point to are allocated dynamically on the heap.
  • An abstract representation of three student objects on the stack. Each object has three fields.
  • An abstract representation of three student pointer variables pointing to three student objects allocated dynamically on the heap with the new operator.

Initializing Structures And Fields

Figure 3 demonstrates that we are not required to initialize structure objects when we create them - perhaps the program will read the data from the console or a data file. However, we can initialize structures when convenient. C++ inherited an aggregate field initialization syntax from the C programming language, and the 2011 ANSI standard extends that syntax, making it conform to other major programming languages. More recently, the ANSI 2020 standard added a variety of default field initializations. The following figures illustrate a few of these options.

  • The field order within the structure and the data order within the initialization list are significant and must match. The program saves the first list value in the first structure field until it saves the last list value in the last structure field. If there are fewer initializers (the elements inside the braces) than there are fields, the excess fields are initialized to their zero-equivalents . Having more initializers than fields is a compile-time error.
  • Similar to (a), but newer standards no longer require the = operator.
  • Historically, en block structure initialization was only allowed in the structure definition, but the ANSI 2015 standard removed that restriction, allowing en bloc assignment to previously defined variables.
  • Abstract representations of the structure objects or variables created and initialized in (a), (b), and (c). Each object has three fields. Although every student must have these fields, the values stored in them may be different.
  • Another way to conceptualize structures and variables is as the rows and columns of a simple database table. Each column represents one field, and each row represents one structure object. The database schema fixes the number of columns, but the program can add any number of rows to the table.
  • Initializing a structure object when defining it. The = operator is now optional.
  • Designated initializers are more flexible than the non-designated initializers illustrated in Figure 4, which are matched to structure fields by position (first initializer to the first field, second initializer to the second field, etc.). Designated initializers may skip a field - the program does not initialize name in this example.
  • Programmers may save data to previously defined structure objects, possibly skipping some fields, with designators.

The next step is accessing the individual fields within a structure object.

Field Selection: The Dot And Arrow Operators

A basket is potentially beneficial, but so far, we have only seen how to put items into it, and then only en masse . To achieve its full potential, we need some way to put items into the basket one at a time, and there must be some way to retrieve them. C++ provides two selection operators that join a specific structure object with a named field. The combination of an object and a field form a variable whose type matches the field type in the structure specification.

The Dot Operator

The C++ dot operator looks and behaves just like Java's dot operator and performs the same tasks in both languages.

  • The general dot operator syntax. The left-hand operand is a structure object, and the right-hand operator is the name of one of its fields.
  • Examples illustrating the dot operator based on the student examples in Figure 3(a).
  • The dot operator's left operand names a specific basket or object, and the right-hand operand names the field. If we view the structure as a table, the left-hand operand selects the row, and the right-hand operand selects the column. So, we can visualize s2 . name as the intersection of a row and column.

The Arrow Operator

Java only has one way of creating objects and only needs one operator to access its fields. C++ can create objects in two ways (see Figure 3 above) and needs an additional field access operator, the arrow: -> .

  • The general arrow operator syntax. The left-hand operand is a pointer to a structure object, and the right-hand operator is the name of one of the object's fields.
  • Examples illustrate the arrow operator based on the student examples in Figure 3(b).

Choosing the correct selection operator

A picture illustrating the order (left to right) of a structure variable name, the location of the selection operator, and a structure field name.

  • Choose the arrow operator if the left-hand operand is a pointer.
  • Otherwise, choose the dot operator.

Moving Structures Within A Program

A basket is a convenient way to carry and move many items by holding its handle. Similarly, a structure object is convenient for a program to "hold" and move many data items with a single (variable) name. Specifically, a program can assign one structure to another (as long as they are instances of the same structure specification), pass them as arguments to functions, and return them from functions as the function's return value.

Assignment is a fundamental operation regardless of the kind of data involved. The C++ code to assign one structure to another is simple and should look familiar:

  • C++ code defining a new structure object and copying an existing object to it by assignment.
  • An abstract representation of a new, empty structure.
  • An abstract representation of how a program copies the contents of an existing structure to another structure.

Function Argument

Once the program creates a structure object, passing it to a function looks and behaves like passing a fundamental-type variable. If we view the function argument as a new, empty structure object, we see that argument passing copies one structure object to another, behaving like an assignment operation.

  • The first part of the C++ code fragment defines a function (similar to a Java method). The argument is a student structure. The last statement (highlighted) passes a previously created and initialized structure as a function argument.
  • In this abstract representation, a program copies the data stored in s2 to the function argument temp . The braces forming the body of the print function create a new scope distinct from the calling scope where the program defines s2 . The italicized variable names label and distinguish the illustrated objects.

Function Return Value

Just as a program can pass a structure into a function as an argument, so it can return a structure as the function's return value, as demonstrated by the following example:

  • C++ code defining a function named read that returns a structure. The program defines and initializes the structure object, temp , in local or function scope with a series of console read operations and returns it as the function return value. The assignment operator saves the returned structure in s3 .
  • A graphical representation of the return process. Returning an object with the return operator copies the local object, temp , to the calling scope, where the program saves it in s1 .

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Store Information of a Student Using Structure

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C Program to Store Information of Students Using Structure

To understand this example, you should have the knowledge of the following C programming topics:

Store Information in Structure and Display it

In this program, a structure student is created. The structure has three members: name (string), roll (integer) and marks (float).

Then, we created an array of structures s having 5 elements to store information of 5 students.

Using a for loop, the program takes the information of 5 students from the user and stores it in the array of structure. Then using another for loop, the information entered by the user is displayed on the screen.

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13.5 — Introduction to structs, members, and member selection

There are many instances in programming where we need more than one variable in order to represent something of interest. As we discussed in the introduction to the previous chapter ( 12.1 -- Introduction to compound data types ), a fraction has a numerator and denominator that are linked together into a single mathematical object.

Alternatively, lets say we want to write a program where we need to store information about the employees in a company. We might be interested in keeping track of attributes such as the employee’s name, title, age, employee id, manager id, wage, birthday, hire date, etc…

If we were to use independent variables to track all of this information, that might look something like this:

However, there are a number of problems with this approach. First, it’s not immediately clear whether these variables are actually related or not (you’d have to read comments, or see how they are used in context). Second, there are now 12 variables to manage. If we wanted to pass this employee to a function, we’d have to pass 12 arguments (and in the correct order), which would make a mess of our function prototypes and function calls. And since a function can only return a single value, how would a function even return an employee?

And if we wanted more than one employee, we’d need to define 12 more variables for each additional employee (each of which would require a unique name)! This clearly doesn’t scale at all. What we really need is some way to organize all of these related pieces of data together, to make them easier to manage.

Fortunately, C++ comes with two compound types designed to solve such challenges: structs (which we’ll introduce now) and classes (which we’ll explore soon). A struct (short for structure ) is a program-defined data type ( 13.1 -- Introduction to program-defined (user-defined) types ) that allows us to bundle multiple variables together into a single type. As you’ll see shortly, this makes management of related sets of variables much simpler!

Defining structs

Because structs are a program-defined type, we first have to tell the compiler what our struct type looks like before we can begin using it. Here is an example of a struct definition for a simplified employee:

The struct keyword is used to tell the compiler that we’re defining a struct, which we’ve named Employee (since program-defined types are typically given names starting with a capital letter).

Then, inside a pair of curly braces, we define the variables that each Employee object will contain. In this example, each Employee we create will have 3 variables: an int id , an int age , and a double wage . The variables that are part of the struct are called data members (or member variables ).

In everyday language, a member is a individual who belongs to a group. For example, you might be a member of the basketball team, and your sister might be a member of the choir.

In C++, a member is a variable, function, or type that belongs to a struct (or class). All members must be declared within the struct (or class) definition.

We’ll use the term member a lot in future lessons, so make sure you remember what it means.

Just like we use an empty set of curly braces to value initialize ( 1.4 -- Variable assignment and initialization ) normal variables, the empty curly braces after each member variable ensures that the member variables inside our Employee are value initialized when an Employee is created. We’ll talk more about this when we cover default member initialization in a few lessons ( 13.7 -- Default member initialization ).

Finally, we end the type definition with a semicolon.

As a reminder, Employee is just a type definition -- no objects are actually created at this point.

Defining struct objects

In order to use the Employee type, we simply define a variable of type Employee :

This defines a variable of type Employee named joe . When the code is executed, an Employee object is instantiated that contains the 3 data members. The empty braces ensures our object is value-initialized.

Just like any other type, it is possible to define multiple variables of the same struct type:

Accessing members

Consider the following example:

In the above example, the name joe refers to the entire struct object (which contains the member variables). To access a specific member variable, we use the member selection operator ( operator. ) in between the struct variable name and the member name. For example, to access Joe’s age member, we’d use joe.age .

Struct member variables work just like normal variables, so it is possible to do normal operations on them, including assignment, arithmetic, comparison, etc…

This prints:

One of the biggest advantages of structs is that we only need to create one new name per struct variable (the member names are fixed as part of the struct type definition). In the following example, we instantiate two Employee objects: joe and frank .

In the above example, it is very easy to tell which member variables belong to Joe and which belong to Frank. This provides a much higher level of organization than individual variables would. Furthermore, because Joe’s and Frank’s members have the same names, this provides consistency when you have multiple variables of the same struct type.

We’ll continue our exploration of structs in the next lesson, including a look at how to initialize them.

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  2. struct Basics

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  3. Structures in C with Examples

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  4. What Is Structures In C: How to Create & Declare Them

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  5. C++ Tutorial

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  6. What Is Structures In C: How to Create & Declare Them

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COMMENTS

  1. Assign one struct to another in C

    Assign one struct to another in C Ask Question Asked 14 years ago Modified 2 years, 10 months ago Viewed 246k times 182 Can you assign one instance of a struct to another, like so: struct Test t1; struct Test t2; t2 = t1; I have seen it work for simple structures, bu does it work for complex structures?

  2. C Structures (structs)

    You can create a structure by using the struct keyword and declare each of its members inside curly braces: struct MyStructure { // Structure declaration int myNum; // Member (int variable) char myLetter; // Member (char variable) }; // End the structure with a semicolon To access the structure, you must create a variable of it.

  3. C struct (Structures)

    In C programming, a struct (or structure) is a collection of variables (can be of different types) under a single name. Define Structures Before you can create structure variables, you need to define its data type. To define a struct, the struct keyword is used. Syntax of struct struct structureName { dataType member1; dataType member2; ... };

  4. Structure Assignment and Its Pitfall in C Language

    The pitfall of structure assignment: Beware though, that copying structs that contain pointers to heap-allocated memory can be a bit dangerous, since by doing so you're aliasing the pointer, and typically making it ambiguous who owns the pointer after the copying operation. If the structures are of compatible types, yes, you can, with ...

  5. Struct and union initialization

    If the nested initializer does not begin with an opening brace, only enough initializers from the list are taken to account for the elements or members of the member array, struct or union; any remaining initializers are left to initialize the next struct member: struct example ex ={80, , , , } When designators are nested, the designators for ...

  6. Struct declaration

    A struct is a type consisting of a sequence of members whose storage is allocated in an ordered sequence (as opposed to union, which is a type consisting of a sequence of members whose storage overlaps). type specifier for a struct is identical to the type specifier except for the keyword used: Syntax Explanation 3Forward declaration Keywords

  7. Structure Assignment (GNU C Language Manual)

    This is the only way in C that you can operate on the whole contents of a array with one operation: when the array is contained in a struct. You can't copy the contents of the data field as an array, because r1.data = r2.data;

  8. How to Initialize a Struct in C

    In the main function, the createPerson function is used to initialize a Person struct named me. The subsequent printf statement displays the information stored in the me struct, showcasing the first name, last name, age, and alive status. Name: Jane Delaney. Age: 27. Alive: Yes.

  9. c++

    105k 38 210 254 asked Apr 14, 2012 at 13:30 mohrphium 353 1 4 13 C99 has compound literals that allow for this (sort-of), but I don't know whether it made it into C++ - dreamlax Apr 14, 2012 at 13:38 1 In C++11, brace initialization works for both individual board s as well as for the array. - Kerrek SB Apr 14, 2012 at 13:52

  10. C structs and Pointers (With Examples)

    Run Code When you run the program, the output will be: Enter the number of persons: 2 Enter first name and age respectively: Harry 24 Enter first name and age respectively: Gary 32 Displaying Information: Name: Harry Age: 24 Name: Gary Age: 32 In the above example, n number of struct variables are created where n is entered by the user.

  11. C Structures

    We can use the struct keyword to declare the structure in C using the following syntax: Syntax struct structure_name { data_type member_name1; data_type member_name1; .... .... }; The above syntax is also called a structure template or structure prototype and no memory is allocated to the structure in the declaration. C Structure Definition

  12. C Struct and Functions

    Passing struct by reference. You can also pass structs by reference (in a similar way like you pass variables of built-in type by reference). We suggest you to read pass by reference tutorial before you proceed.. During pass by reference, the memory addresses of struct variables are passed to the function.

  13. CS31: Intro to C Structs and Pointers

    C Stucts and Pointers. This is the second part of a two part introduction to the C programming language. It is written specifically for CS31 students. The first part covers C programs, compiling and running, variables, types, operators, loops, functions, arrays, parameter passing (basic types and arrays), standard I/O (printf, scanf), and file ...

  14. c

    A structure type is defined as: typedef struct student{ int id; char* name; double score; } Student; I construct a variable of type Student and I want to assign values to it. How can I do that efficiently? int main(){ Student s1; int id = 3; char* name = getName(id); double score = getScore(id); /*Error s1 = {id, name, score}; */

  15. Ways to copy struct in c/c++

    Just copy whole struct in one line only. int main() { User user = {"Naman", MALE, 21, 5}, user1; // Approach 2 memcpy(&user1, &user, sizeof(user)); // ^dest ^src ^size to copy printf("user: "); print_user(&user); printf("user copy: "); print_user(&user1); return 0; } Output

  16. 5.3. Structures

    The C++ code to assign one structure to another is simple and should look familiar: student s4; s4 = s1; (a) (b) (c) The assignment operator works with structures. Used with structure operands, the assignment operator copies a block of bits from one memory address (the address of the right-hand object) to another (the address of the left-hand ...

  17. C Program to Store Information of Students Using Structure

    The structure has three members: name (string), roll (integer) and marks (float). Then, we created an array of structures s having 5 elements to store information of 5 students. Using a for loop, the program takes the information of 5 students from the user and stores it in the array of structure.

  18. C Structure Assignment

    C Structure Assignment | Assign one Struct to another in C - C Programming Tutorial 81 - YouTube 0:00 / 15:22 Notes for You:: C Structure Assignment.Example Code:#include...

  19. c

    67 I'm trying to understand how to solve this trivial problem in C, in the cleanest/safest way. Here's my example: #include <stdio.h> int main(int argc, char *argv[]) { typedef struct { char name[20]; char surname[20]; int unsigned age; } person; // Here I can pass strings as values...how does it work? person p = {"John", "Doe", 30};

  20. 13.5

    In everyday language, a member is a individual who belongs to a group. For example, you might be a member of the basketball team, and your sister might be a member of the choir. In C++, a member is a variable, function, or type that belongs to a struct (or class). All members must be declared within the struct (or class) definition.

  21. c

    4 Answers Sorted by: 4 RA.array = {1, 2, 3, 4, 5}; memcpy (RA.array, (int []) {1, 2, 3, 4, 5}, sizeof RA.array); RA.array = arr; memcpy (RA.array, arr, sizeof arr); // better: sizeof RA.array