C++ LANGUAGE VARIABLE DECLARATION 17

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C++ LANGUAGE VARIABLE DECLARATION

Variable Declaration in C++ 


A variable declaration provides assurance to the compiler 
that there is one variable existing with the given type 
and name so that compiler proceed for further compilation 
without needing complete detail about the variable. 
A variable declaration has its meaning at the time of compilation 
only, compiler needs actual variable declaration at the time of 
linking of the program. 

A variable declaration is useful when you are using multiple 
files and you define your variable in one of the files which 
will be available at the time of linking of 
the program. You will use extern keyword to declare a variable 
at any place. Though you can declare a variable multiple times 
in your C++ program, but it can be defined only once in a file, 
a function or a block of code. 

Example: 

Try the following example where a variable has been declared at 
the top, but it has been defined inside the main function:
 
#include <iostream> 

using namespace std; 
 
// Variable declaration: 
extern int a, b; 
extern int c; 
extern float f; 
   
int main () 
{ 
  // Variable definition: 
  int a, b; 
  int c; 
  float f; 
  
  // actual initialization 
  a = 10; 
  b = 20; 
  c = a + b; 
  
  cout << c << endl ; 
 
  f = 70.0/3.0; 
  cout << f << endl ; 
  
  return 0; 
} 
 
When the above code is compiled and executed, 
it produces the following result: 
30 
23.3333 
Same concept applies on function declaration where you 
provide a function name at the time of its declaration and 
its actual definition can be given anywhere else. For example: 

// function declaration 
int func(); 
 
int main() 
{ 
    // function call 
    int i = func(); 
} 
 
// function definition 
int func() 
{ 
    return 0; 
}