A Function with Constant Argument(s)

By a constant argument, it means that the function cannot modify the argument. If you pass a constant value to the function, then the function cannot modify the value as the value is constant. For example see the following sample program:

#include <iostream.h>

#include <string.h>

#include <conio.h>

int len(const char[]);

int len(const char str[])

{

 str[1]=str[1]+32;

return(strlen(str));

}

void main()

{

int k=0;

k=len("My String");

cout<<"Length of the string = "<<k;

getch();

}

On compiling the above program you will get the error message as – “Cannot modify a const object”. Because I am trying to modify the value of a constant argument in the line str[1]=str[1]+32; ( Trying to add 32 with the character at the index 1 of the character array str). Thus we have seen that const qualifier in function prototype and so in definition, tell the compiler that the function should not modify the argument. The constant arguments are useful when the functions are called by reference.