Why should we use "::" operator on global functions / objects?

Question

I have seen this used in various places. C++ programmers will often use the :: operator right before a global function call.

e.g

::glGenBuffers( 1, &id );

Why is this? Why not just use:

glGenBuffers( 1, &id );

Answer 1

To avoid accidental namespace clashing. For example if you current namespace would have glGenBuffers which does something different from the "good" glGenBuffers with :: you can specify to call the glGenBuffers which is in the global namespace.

Answer 2

The problem is 1) names in inner scopes hide names in outer scopes and 2) there can be ambiguous of function calls when using directive is used.

For example (ambiguity)

#include <algorithm>

using namespace std;

void swap( int &x, int &y )
{
   int tmp = x;
   x = y;
   y = tmp;
}

int main()
{
   int x = 5, y = 10;

   //swap( x, y ); compiler error: what function to call?

   ::swap( x, y ); // the user defined function is called

   std::swap( x, y ); // the standard function is called.
}

Another example of hidding names

#include <iostream>

void f() { std::cout << "::f()\n"; }

namespace MyNamespace
{
   void f() { std::cout << "MyNamespace::f()\n"; }

   void g() { f(); } // calls MyNamespace::f()

   void h() { ::f(); } // calls ::f()
}


int main()
{
   MyNamespace::g();
   MyNamespace::h();
}