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Scope resolution operator

cppreference Scope resolution operator

microsoft Scope resolution operator: ::

stackoverflow Using :: in C++

A

The :: are used to dereference scopes.

Use a name in global namespace

在 cppreference Scope resolution operator 中有如下说明:

The expression ::tolower names the function tolower in the global namespace.

在 cppreference new expression # Allocation 中有如下说明:

As described in allocation function, the C++ program may provide global and class-specific replacements for these functions. If the new-expression begins with the optional :: operator, as in ::new T or ::new T[n], class-specific replacements will be ignored (the function is looked up in global scope). Otherwise, if T is a class type, lookup begins in the class scope of T.

在 cppreference Qualified name lookup 中有如下说明:

If there is nothing on the left hand side of the ::, the lookup considers only declarations made in the global namespace scope (or introduced into the global namespace by a using declaration). This makes it possible to refer to such names even if they were hidden by a local declaration

其中给出的例子非常好。

stackoverflow What does the “::” mean in “::tolower”?

I've seen code like this:

std::string str = "wHatEver";
std::transform(str.begin(), str.end(), str.begin(), ::tolower);

And I have a question: what does mean :: before tolower?

and std::tolower not works, but ::tolower works OK

A

Means that it is explicitly using the tolower in the global namespace (which is presumably the stdc lib one).

NOTE: disambiguation(消除歧义)

Example:

void foo()
{
    // This is your global foo
}

namespace bar
{
void foo()
{
    // This is bar's foo
}
}

using namespace bar;

void test()
{
    foo();   // Ambiguous - which one is it?
    ::foo(); // This is the global foo()
}
int main()
{
    test();

}
// g++ test.cpp

NOTE: 上述代码编译报错如下:

test.cpp: In function ‘void test()’:
test.cpp:18:6: error: call of overloaded ‘foo()’ is ambiguous
  foo();   // Ambiguous - which one is it?
      ^
test.cpp:18:6: note: candidates are:
test.cpp:1:6: note: void foo()
 void foo()
      ^
test.cpp:8:6: note: void bar::foo()
 void foo()