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c++syntaxmost-vexing-parse

Why does this call the default constructor?

struct X
{
    X()    { std::cout << "X()\n";    }
    X(int) { std::cout << "X(int)\n"; }
};

const int answer = 42;

int main()
{
    X(answer);
}

I would have expected this to print either

  • X(int), because X(answer); could be interpreted as a cast from int to X, or
  • nothing at all, because X(answer); could be interpreted as the declaration of a variable.

However, it prints X(), and I have no idea why X(answer); would call the default constructor.

BONUS POINTS: What would I have to change to get a temporary instead of a variable declaration?

Solution

  • nothing at all, because X(answer); could be interpreted as the declaration of a variable.

    Your answer is hidden in here. If you declare a variable, you invoke its default ctor (if non-POD and all that stuff).

    On your edit: To get a temporary, you have a few options: