Why there is not any compiler errors?

[LarryChen]

Here is code,

void foo(int x);
void foo(int x);

int main()
{
	return 0;
}

I would expect that there is compiler error for multiple definitions of function foo. But it turns out to be just fine. Why? Thanks.

[VictorN]

There are not the definitions but declarations.
Try to define (implement) this function.

[SeventhStar]

Basically you're declaring the same function and not using it. Here's what the build process will "say" for your code (comments):

void foo(int x); // there is a function named foo
void foo(int x); // there is a function named foo (I already know that, but it doesn't matter)

int main()
{
	// ok... do nothing
	return 0;
}

// the function foo was never used, so I don't need anything more

If you had called foo in main (say foo(42), there would have been a linker error.