What is the difference between a definition and a declaration in C++?

In C++, declaration and definition are often confused. A declaration tells the compiler about the name and type, while a definition allocates memory or provides implementation. In this article, we will understand their differences with examples.

What is a Declaration in C++?

A declaration means (in C or C++) that you are telling the compiler about type, size and in case of function declaration, type and size of its parameters of any variable, or user-defined type or function in your program. No space is reserved in memory for any variable in case of the declaration.

Following is the syntax to the declaration of variable, function and structure in C++:

// Variable
extern data_type variable_name;

// Structure
struct StructName;

// Function
return_type function_name(parameter_list);

Following is the declarations in C++:

// Declaring a variable a without defining it
extern int a;

// Declaring a struct
struct _tagExample { int a; int b; };

// Declaring a function
int myFunc (int a, int b);

Example

In this example, the Declaration (extern, function prototype) tells the compiler: "This variable/function exists somewhere.", but does not allocate memory or provide implementation to the program. So, that is reason the program fails at runtime.

#include<iostream>
using namespace std;
// Declaration of a variable
extern int num;
// Declaration of a function
int add(int, int);
int main() {
   int result = add(num, 10);
   cout<<"Result = "<<result<<endl;
   return 0;
}

Following is the output to the above program:

/usr/bin/ld: /tmp/ccztBu7g.o: warning: relocation against 'num' in read-only section '.text'
/usr/bin/ld: /tmp/ccztBu7g.o: in function 'main':
main.cpp:(.text+0xe): undefined reference to 'num'
/usr/bin/ld: main.cpp:(.text+0x1a): undefined reference to 'add(int, int)'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status

What is Definition in C++?

The Definition on the other hand means that in additions to all the things that declaration does, space is additionally reserved in memory. You can say "DEFINITION = DECLARATION + SPACE RESERVATION".

Following is the syntax to the definition of variable, function and structure in C++:

// Variable
data_type variable_name;
data_type variable_name = value;

// structure
struct StructName {
    // members
};

// Function
return_type function_name(parameter_list) {
    // Function body
}

Following is the defining a definitions in C++.

// defining a variable
int a;
int b = 0;

// defining a function 
int myFunc (int a, int b) { return a + b; }

// defining a struct
struct _tagExample example;

Example

In this program, the variable 'num' is defined with a value of 60, and the function 'add()' is also fully defined with its logic. Both memory for num and code for add() are available, so the program runs correctly and prints the result of 60+10=70.

#include<iostream>
using namespace std;
// Definition (memory allocated)
int num = 60;  
// Definition (implementation provided)
int add(int a, int b) {
    return a + b;
}
int main() {
    int result = add(num, 10);
    // Now, num and add() exist in memory!
    cout<<"Result = "<<result<<endl;
    return 0;
}

Following is the output to the above program:

Result = 70