Why can't I call operator() on temporary objects directly?

Question

What I want to do can be summarized into the following code:

struct A{};

struct B{
    A& a;
    B(A& a) noexcept : a(a){}
    int operator()(int) {}
};

int main(){
    A a;
    B(a)(2);
}

And my compiler (g++ 6) rejected the code complaining that a shadows a parameter. However, if I try to explicitly call operator(), it works as expected.

It seems that g++ will ignore the parentheses and see the statement as a declaration.

Is this the specified or expected behavior?

Answer 1

This is one of those icky parsing rules which catches you every now and again. As you suggest, B(a)(2); is actually equivalent to B a(2);, so your code tries to initialize a B with an int.

To fix this, you can use C++11's uniform initialization:

B{a}(2);