C++ Constructor not been called

Question

I was pretty sure the calling of B(A()) will call the defined constructor of B in my code. But I was surprised to find out that it didn't call to my constructor B and of course didn't print "Constructor B".

So, to which constructor this code is calling (what is the signature of this constructor that it's calling to)?

struct A
{
    
};

struct B
{
    B(const A a) { std::cout << "Constructor B" << std::endl; }
};

int main()
{
    B(A());
    return 0;
}

Does this answer your question? [Most vexing parse](https://stackoverflow.com/questions/5926103/most-vexing-parse) — Oliv, Jul 18 '20 at 09:01

score 1 · Accepted Answer · answered Jul 18 '20 at 13:05

It doesn't construct an object with type B as you expected, then no constructor is called.

When declaring variables we can add (unnecessary) parentheses around the variable name, i.e. int (a); has the same effect as int a;. Similarly, B(A()); is same as B A();, which declares a function named A, which takes no parameters and returns B.

As the workaround you can change () to {}, e.g.

B(A{});
B{A()};
B{A{}};

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C++ Constructor not been called

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