Issue with type casting the argument of the constructor

Question

#include <iostream>

struct Foo
{
    Foo(int d) : x(d) {}
    int x;
};

int main() 
{ 
    double x = 3.14;

    Foo f( int(x) );

    std::cout << f.x << std::endl;

    return 0;
}

When I compile this code I get the following error:

[Error] request for member 'x' in 'f', which is of non-class type 'Foo(int)'

Suppose that in int main I remove int in Foo f(int(x)). I mean if I write just like this:

     Foo f(x);

the code compiled correctly and got the output as 3.

So what happens if we type cast the argument like Foo f(int(x)) to invoke the constructor?

Answer 1

Foo f(int(x));

It is not a type cast, it's a function declaration - function f that takes an int called x and returns a Foo.

The grammar allows a (theoretically unlimited) set of parentheses around an identifier in a declaration. It is the same as if you wrote

Foo f(int x);

or

Foo f( int (((x))) );

As you already figured out, you don't need to cast, as conversion between a double and and int is implicit. But if you really wanted, you could say static_cast<int>(x) instead or

Foo f((int (x)));
//    ^       ^

which makes it an expression instead of declaration.