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    Difference between Definition and Declaration

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    Difference between Definition and Declaration

    Difficulty Level : Easy

    Last Updated : 11 Jul, 2022

    Declaration of a variable is for informing the compiler of the following information: name of the variable, type of value it holds, and the initial value if any it takes. i.e., declaration gives details about the properties of a variable. Whereas, Definition of a variable says where the variable gets stored. i.e., memory for the variable is allocated during the definition of the variable. In C language definition and declaration for a variable takes place at the same time. i.e. there is no difference between declaration and definition. For example, consider the following declaration

    int a;

    Here, the information such as the variable name: a, and data type: int, is sent to the compiler which will be stored in the data structure known as the symbol table. Along with this, a memory of size 2 bytes(depending upon the type of compiler) will be allocated. Suppose, if we want to only declare variables and not define it i.e. we do not want to allocate memory, then the following declaration can be used

    extern int a;

    In this example, only the information about the variable is sent and no memory allocation is done. The above information tells the compiler that the variable a is declared now while memory for it will be defined later in the same file or in a different file. Declaration of a function provides the compiler with the name of the function, the number and type of arguments it takes, and its return type. For example, consider the following code,

    int add(int, int);

    Here, a function named add is declared with 2 arguments of type int and return type int. Memory will not be allocated at this stage. Definition of the function is used for allocating memory for the function. For example, consider the following function definition,

    int add(int a, int b)

    { return (a+b); }

    During this function definition, the memory for the function add will be allocated. A variable or a function can be declared any number of times but, it can be defined only once. The above points are summarized in the following table as follows:

    Declaration Definition

    A variable or a function can be declared any number of times A variable or a function can be defined only once

    Memory will not be allocated during declaration Memory will be allocated

    int f(int);

    The above is a function declaration. This declaration is just for informing the compiler that a function named f with return type and argument as int will be used in the function.

    int f(int a) { return a; }

    The system allocates memory by seeing the above function definition.

    In declaration, the data type of the variable is known In definition, value stored in the variable is specified.

    स्रोत : www.geeksforgeeks.org

    What is the Difference Between Function Declaration and Function Definition in C Programming

    The main difference between Function Declaration and Function Definition in C Programming is that the function declaration indicates what the function is and function definition indicates what the function does. Function declaration helps to indicate the compiler about the function and how to call

    What is the Difference Between Function Declaration and Function Definition in C Programming

    February 28, 2019by Lithmee4 min read

    7Share

    The main difference between Function Declaration and Function Definition in C Programming is that Function declaration indicates what the function is and Function Definition indicates what the function does.

    C is a high-level general purpose programming language developed by Dennis Richie. It is the foundation programming language of many other languages such as C++, Python, Java, PHP, etc. A function is an important concept in C programming. It is a set of statements that performs a particular task. Rather than writing all the statements inside the main program, the programmer can create functions and call them within the main program. It makes the program more readable. After the execution of the function, the control passes back to the main program. A function has a declaration and definition. The declaration is a prototype whhile the definition contains the actual implementation.

    Key Areas Covered

    1. What is Function Declaration in C Programming

    – Definition, Functionality

    2. What is Function Definition in C Programming

    – Definition, Functionality

    3. Difference Between Function Declaration and Function Definition in C Programming

    – Comparison of Key Differences

    Key Terms

    C Programming, Function Declaration, Function Definition

    What is Function Declaration in C Programming

    Function declaration indicates the compiler about a specific function. The syntax is as follows.

    return_type function_name (parameter list);

    The function_name helps to identify a function. The parameter list consist of what is passed to the function to perform the task. The return_type refers to the data type of the output of the function. If the function gives an integer, the return type is int. If it is a double value, the return type is double. If the function does not return any value, then it is a void.

    For example, assume a function that multiplies two integers. The function declaration is as follows.

    int multiply (int num1, int num2);

    The function is called multiply. It gets two integers called num1 and num2. The return type is int. Therefore, the function outputs an integer value. Finally, the declaration ends with a semicolon.

    It is also possible to mention only the data types of the parameters in the declaration as follows.

    int multiply (int, int);

    What is Function Definition in C Programming

    Function definition refers to the implementation of the function. In other words, it represents the actual statements that the function performs. The syntax is as follows.

    return_type function_name (parameter_list){

    // statements of the function

    }

    The function_name helps to identify the function. When invoking the function, the values pass to that function. These values copy to the parameters. There can be single or multiple parameters with their corresponding data types. The return type explains the output of the function. The programmer can write the statements of the function inside the curly braces.

    An example program is as follows.

    Figure 1 : C program with function declaration and definition

    In the above program, line 3 displays the function declaration. It provides information about the function to the compiler such as name, parameters, etc. In the main method, there are two integers: num1 and num2. In line 9, these values are passed to multiply. The function is executed. The num1 value copies to x and num2 value copies to y. Then the result is returned and stored to the variable ans. Finally, the printf statement displays the value on the console.

    In the above program, line 3 displays the function declaration whereas line 14 to line 16 displays the function definition.

    Difference Between Function Declaration and Function Definition in C Programming

    Definition

    Function declaration is a prototype that specifies the function name, return types and parameters without the function body. Function Definition, on the other hand, refers to the actual function that specifies the function name, return types and parameters with the function body. Thus, this is the main difference between function declaration and function definition.

    Functionality

    Functionally, the difference between function declaration and function definition is that the function declaration helps to indicate the compiler about the function and how to call that function, etc. while Function definition helps to write what the function should perform. It is the actual implementation of the function.

    Consist of

    Function declaration contains the function name, parameter list, and, return type while Function definition contains the function name, parameter list, return type, function body (statements of the function). This is another difference between function declaration and function definition.

    स्रोत : pediaa.com

    Explain the Difference Between Definition and Declaration

    Explain the Difference Between Definition and Declaration - In this post, we will understand the difference between definition and declaration.DefinitionDefinit ...

    Explain the Difference Between Definition and Declaration

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    In this post, we will understand the difference between definition and declaration.

    Definition

    Definition, with respect to a function indicates that the body of the function has been developed.

    With respect to variable, it means a value has been associated/defined with that variable.

    A function can be defined only once.

    Once a function or variable has been defined, it is ready to be used.

    A variable can be re-defined multiple times, as and when required. This depends on the language and the scopes.

    Memory is allocated when function or a variable is defined.

    Below is an example of variable definition

    sum = 0

    A variable named ‘sum’ is assigned to 0.

    Below is an example of function definition

    def add_val(param_1, param_2)

    c = param_1 + param_2

    return c

    The ‘add_val’ is a method, and ‘param_1’ and ‘param_2’ are parameters passed to it. There is a definition of this method too.

    Declaration

    Function declaration basically means that a name has been given and the parameters have been mentioned.

    No value is associated with a variable when it is just declared.

    Its body hasn’t been developed yet.

    Once a function or variable has been declared, it is not yet ready to be used.

    A function or variable can be declared any number of times.

    Memory isn’t allocated when a function or variable is declared.

    Below is an example of variable declaration −

    sum_val

    A variable named ‘sum_val’ is declared.

    Below is an example of function declaration −

    add_val(p_1, p_2)

    The ‘add_val’ is a method, and ‘p_1’ and ‘p_2’ are parameters passed to it. There is no definition of the method.

    AmitDiwan

    Updated on 23-Mar-2021 07:13:31

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