Why does C++ need the scope resolution operator?

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Because someone in the C++ standards committee thought that it was a good idea to allow this code to work:

struct foo
{
  int blah;
};

struct thingy
{
  int data;
};

struct bar : public foo
{
  thingy foo;
};

int main()
{
  bar test;
  test.foo.data = 5;
  test.foo::blah = 10;
  return 0;
}

Basically, it allows a member variable and a derived class type to have the same name. I have no idea what someone was smoking when they thought that this was important. But there it is.

When the compiler sees ., it knows that the thing to the left must be an object. When it sees ::, it must be a typename or namespace (or nothing, indicating the global namespace). That’s how it resolves this ambiguity.

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