struct field assignment

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15 Structures

A structure is a user-defined data type that holds various fields of data. Each field has a name and a data type specified in the structure’s definition.

Here we define a structure suitable for storing a linked list of integers. Each list item will hold one integer, plus a pointer to the next item.

The structure definition has a type tag so that the code can refer to this structure. The type tag here is intlistlink . The definition refers recursively to the same structure through that tag.

You can define a structure without a type tag, but then you can’t refer to it again. That is useful only in some special contexts, such as inside a typedef or a union .

The contents of the structure are specified by the field declarations inside the braces. Each field in the structure needs a declaration there. The fields in one structure definition must have distinct names, but these names do not conflict with any other names in the program.

A field declaration looks just like a variable declaration. You can combine field declarations with the same beginning, just as you can combine variable declarations.

This structure has two fields. One, named datum , has type int and will hold one integer in the list. The other, named next , is a pointer to another struct intlistlink which would be the rest of the list. In the last list item, it would be NULL .

This structure definition is recursive, since the type of the next field refers to the structure type. Such recursion is not a problem; in fact, you can use the type struct intlistlink * before the definition of the type struct intlistlink itself. That works because pointers to all kinds of structures really look the same at the machine level.

After defining the structure, you can declare a variable of type struct intlistlink like this:

The structure definition itself can serve as the beginning of a variable declaration, so you can declare variables immediately after, like this:

But that is ugly. It is almost always clearer to separate the definition of the structure from its uses.

Declaring a structure type inside a block (see Blocks ) limits the scope of the structure type name to that block. That means the structure type is recognized only within that block. Declaring it in a function parameter list, as here,

(assuming that struct foo is not already defined) limits the scope of the structure type struct foo to that parameter list; that is basically useless, so it triggers a warning.

Standard C requires at least one field in a structure. GNU C does not require this.

Autor: Codenga 12.05.2023

How to use Structures (Struct) in C

Structures (Structs) are a great way to group related information together. If you have a set of variables in your code that store logically connected data, consider combining them into a single entity - that is, a Structure. In this article, you will learn the most important techniques for using Structures in the C language.

A basic Structure

Let’s start with a simple example:

Pay attention to the keyword "struct". Our variables describing a mechanical vehicle (i.e., Car ) should be enclosed in curly braces, and the entire structure should be terminated with a semicolon. Inside the structure, there are variables of various types: integer types ( int ) and a pointer (char*). This will be our very basic structure, grouping information about a car.

Access the values

You need to imagine that a structure is actually a new data type. In the example below, we will show you how to define types for structures and how to set values:

And here is the result:

Let's analyze what happened in the main() function. At the beginning, we define a variable named "audi" which is of type struct Car. With the variable "audi," we can modify the elements of the structure. To access a structure element, we use the dot operator:

So we got this simple syntax:

As you can see, there's nothing difficult here. A structure (Struct) is simply a set of related data. Access to a field within a structure can be obtained using the dot operator.

Structures and functions

Now it's time for a more advanced technique. Sometimes there is a need to pass a structure to a function. Passing a structure to a function involves making a copy of it and passing it as an argument. Here's a code snippet to demonstrate this:

In this example, the structure "Person" is passed to the function "displayPerson()" as an argument by value, which means it is copied into a local variable "person" (the function parameter). Inside the function, the fields "name" and "age" can be accessed using the dot operator ".".

It's important to note that for large structures, passing them by value can be inefficient in terms of memory consumption and time. In such cases, it is recommended to use pointers.

You already know another useful technique for working with structures. It's time for the next example. Our previous function had the ability to print information from a structure. Of course, there is nothing stopping us from returning a structure from a function. Here is an example that demonstrates such behavior.

In this example, the function "updatePerson()" takes the structure "Person" as an argument, named "person" (by value). It modifies the fields of the structure and then returns the modified structure as the return value.

In the "main()" function, we initialize a structure "p1" and then call the "updatePerson()" function, passing "p1" as an argument. After calling the function, we read the fields of the returned structure "p2" and display them on the screen.

The result is as follows:

These examples illustrate the potential that lies in using structures and functions. As you can see, structures can be passed as arguments and can also be returned by functions.

Structures and pointers

Pointers are an essential part of the C language , and they can also be used to work with structures. Take a look at the example below:

In the above code, we have a crucial line of code where the pointer "ptr" is assigned the address of the "point" object:

After such an assignment, using the pointer "ptr" , we can access the fields of the structure using the "->" operator:

Structures allow you to easily group related data together. You can pass them as parameters to functions, return them from functions, create pointers to structures, and more. With structures, you can achieve clean and organized code where data is logically grouped together.

If you want to master Structures and many other practical techniques in C, enlist for the C Developer Career Path. With this career path, you will learn the C language from the basics to an intermediate-advanced level. The path concludes with an exam that grants you the Specialist Certificate, enabling you to demonstrate your proficiency with the C language.

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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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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

For example,

Here, a derived type struct Person is defined. Now, you can create variables of this type.

Create struct Variables

When a struct type is declared, no storage or memory is allocated. To allocate memory of a given structure type and work with it, we need to create variables.

Here's how we create structure variables:

Another way of creating a struct variable is:

In both cases,

• person1 and person2 are struct Person variables
• p[] is a struct Person array of size 20.

Access Members of a Structure

There are two types of operators used for accessing members of a structure.

• . - Member operator
• -> - Structure pointer operator (will be discussed in the next tutorial)

Suppose, you want to access the salary of person2 . Here's how you can do it.

Example 1: C structs

In this program, we have created a struct named Person . We have also created a variable of Person named person1 .

In main() , we have assigned values to the variables defined in Person for the person1 object.

Notice that we have used strcpy() function to assign the value to person1.name .

This is because name is a char array ( C-string ) and we cannot use the assignment operator = with it after we have declared the string.

Finally, we printed the data of person1 .

• Keyword typedef

We use the typedef keyword to create an alias name for data types. It is commonly used with structures to simplify the syntax of declaring variables.

For example, let us look at the following code:

We can use typedef to write an equivalent code with a simplified syntax:

Example 2: C typedef

Here, we have used typedef with the Person structure to create an alias person .

Now, we can simply declare a Person variable using the person alias:

• Nested Structures

You can create structures within a structure in C programming. For example,

Suppose, you want to set imag of num2 variable to 11 . Here's how you can do it:

Example 3: C Nested Structures

Why structs in c.

Suppose you want to store information about a person: his/her name, citizenship number, and salary. You can create different variables name , citNo and salary to store this information.

What if you need to store information of more than one person? Now, you need to create different variables for each information per person: name1 , citNo1 , salary1 , name2 , citNo2 , salary2 , etc.

A better approach would be to have a collection of all related information under a single name Person structure and use it for every person.

More on struct

• Structures and pointers
• Passing structures to a function

Table of Contents

• C struct (Introduction)
• Create struct variables
• Access members of a structure
• Example 1: C++ structs

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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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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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• C Structures vs C++ Structure

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Declares a class data member with explicit size, in bits. Adjacent bit-field members may (or may not) be packed to share and straddle the individual bytes.

A bit-field declaration is a class data member declaration which uses the following declarator:

The type of the bit-field is introduced by the decl-specifier-seq of the declaration syntax .

[ edit ] Explanation

The type of a bit-field can only be integral or (possibly cv-qualified) enumeration type, an unnamed bit-field cannot be declared with a cv-qualified type.

A bit-field cannot be a static data member .

There are no bit-field prvalues : lvalue-to-rvalue conversion always produces an object of the underlying type of the bit-field.

The number of bits in a bit-field sets the limit to the range of values it can hold:

Possible output:

Multiple adjacent bit-fields are usually packed together (although this behavior is implementation-defined):

The special unnamed bit-field of size zero can be forced to break up padding. It specifies that the next bit-field begins at the beginning of its allocation unit:

If the specified size of the bit-field is greater than the size of its type, the value is limited by the type: a std:: uint8_t b : 1000 ; would still hold values between 0 and 255. the extra bits are padding bits .

Because bit-fields do not necessarily begin at the beginning of a byte, address of a bit-field cannot be taken. Pointers and non-const references to bit-fields are not possible. When initializing a const reference from a bit-field, a temporary is created (its type is the type of the bit-field), copy initialized with the value of the bit-field, and the reference is bound to that temporary.

[ edit ] Notes

The following properties of bit-fields are implementation-defined :

• The value that results from assigning or initializing a signed bit-field with a value out of range, or from incrementing a signed bit-field past its range.
• Everything about the actual allocation details of bit-fields within the class object.
• For example, on some platforms, bit-fields don't straddle bytes, on others they do.
• Also, on some platforms, bit-fields are packed left-to-right, on others right-to-left.

In the C programming language, the width of a bit-field cannot exceed the width of the underlying type, and whether int bit-fields that are not explicitly signed or unsigned are signed or unsigned is implementation-defined. For example, int b : 3 ; may have the range of values 0 .. 7 or - 4 .. 3 in C, but only the latter choice is allowed in C++.

[ edit ] Defect reports

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

[ edit ] References

• C++23 standard (ISO/IEC 14882:2023):
• 11.4.10 Bit-fields [class.bit]
• C++20 standard (ISO/IEC 14882:2020):
• 11.4.9 Bit-fields [class.bit]
• C++17 standard (ISO/IEC 14882:2017):
• 12.2.4 Bit-fields [class.bit]
• C++14 standard (ISO/IEC 14882:2014):
• 9.6 Bit-fields [class.bit]
• C++11 standard (ISO/IEC 14882:2011):
• C++03 standard (ISO/IEC 14882:2003):
• C++98 standard (ISO/IEC 14882:1998):

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Next: Unions , Previous: Overlaying Different 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.