C ++：运算符重载：类内和类外。与preincrement运算符的歧义

Check it out this code:

看看这段代码:

struct A
{
    A operator+(A const& a) { cout << 1 << endl; return A(); }
    A& operator++() { cout << 2 << endl; return *this; }
    A operator++(int) { cout << 3 << endl; return *this; }
    bool operator!() { cout << 4 << endl; return true; }

};

A operator+(A const& a, A const& b)
{ cout << 5 << endl; return A(); }

A& operator++(A& a) { cout << 6 << endl; return a; }
A operator++(A const& a, int) { cout << 7 << endl; return A(); }
bool operator!(A const& a) { cout << 8 << endl; return false; }

int main()
{
    A a, b;

    a + b; // Prints 1 instead 5
    ++a;   // Ambiguity
    a++;   // Prints 3 instead 7
    !a;    // Prints 4 instead 8

    return 0;
}
struct A
{