The namespace starts with :: in C++ [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicate:

What's the purpose of a leading “::” in a C++ method call

This gtest has the example code.

::testing::Ass开发者_运维百科ertionResult IsEven(int n) {
  if ((n % 2) == 0)
    return ::testing::AssertionSuccess();
  else
    return ::testing::AssertionFailure() << n << " is odd";
}

How does it work? If the namespace is testing, isn't it testing::AssertionResult is the right usage?

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The :: prefix refers to the global namespace, so this is like an absolute versus relative path specification.