emplicitly and explicitly function declaration

[masiht]

Can anyone please tell me what is the difference between declaring a function explicitly and implicitly ?

[Gumbercules]

explicit is used for constructors to signify that it won't automatically make the conversion to the class type, ie you have to explicitly use the constructor

[masiht]

and what is implicit function declaration ?

[PanGalactic]

what is the difference between declaring a function explicitly and implicitly?

Functions can only be declared explicitly.

So says the standard.

So say we all.

The keyword "explicit" has nothing to do with explicit function declarations.

[Bazzy]

From http://en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B :

implicit function declarations (using functions that have not been declared) are not allowed in C++

[Duthomhas]

An implicit function is one where the compiler assumes that the function is declared&defined elsewhere. That is, the compiler doesn't know anything more about the function than what it can figure out when you call it.

int main()
  {
  printf( "Hello world!\n" );  /* printf() !!?? what's that? */
  return 0;
  }

In the example, since printf() was never declared, the compiler had to try to figure out what kind of function it is. It looks like

void printf( const char* );

We know different, but that is close enough so it works. (Usually. A C program is automatically linked with the actual printf() function.)

Now, if we want to fix the above, we can declare the function properly -- that is, explicitly:

int printf( const char*, ... );  /* here the function is explicitly declared */

int main()
  {
  printf( "Hello world!\n" );  /* now it can be used, since the compiler knows about it */
  return 0;
  }

Keep in mind that the function must still be defined somewhere. In the case of printf() it is defined in the standard library that is automatically linked with your C program. For functions you create yourself you must also declare them:

void print_sum( int a, int b );  /* explicit declaration */

int main()
  {
  print_sum( 27, 15 );  /* happy compiler */
  return 0;
  }

void print_sum( int a, int b )  /* definition */
  {
  printf( "%d + %d = %d\n", a, b, a + b );  /* implicit function use */
  }

Hope this helps.

[masiht]

So you mean if a function is used withour declaring it , It will be implicit function .

If a function is declared and then used somewhere in the program it will be explicitly.

[Duthomhas]

Yep. :-)

[masiht]

Thanks Duoas !