Visual Studio C++ compiler weird behaviour

Question

I'm just curious to know why this small piece of code compiles correctly (and without warnings) in Visual Studio . Maybe the result is the same with GCC and Clang , but unfortunately I can't test them now.

struct T {
    int t;
    T() : t(0) {}
};

int main() {
    T(i_do_not_exist);
    return 0;
}

Answer 1

T(i_do_not_exist); is an object declaration with the same meaning as T i_do_not_exist; .

N4567 § 6.8[stmt.ambig]p1

There is an ambiguity in the grammar involving expression-statement s and declaration s: An expression-statement with a function-style explicit type conversion (5.2.3) as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a ( . In those cases the statement is a declaration .

§ 8.3[dcl.meaning]p6

In a declaration TD where D has the form

( D1 )

the type of the contained declarator-id is the same as that of the contained declarator-id in the declaration

T D1

Parentheses do not alter the type of the embedded declarator-id , but they can alter the binding of complex declarators.

Answer 2

Because it defines a variable of type T:

http://coliru.stacked-crooked.com/a/d420870b1a6490d7

#include <iostream>

struct T {
    int t;
    T() : t(0) {}
};

int main() {
    T(i_do_not_exist);
    i_do_not_exist.t = 120;
    std::cout << i_do_not_exist.t;
    return 0;
}

The above example looks silly, but this syntax is allowed for a reason.

A better example is:

int func1();
namespace A
{
   void func1(int);
   struct X {
       friend int (::func1)();
   };
}

Probably other examples could be found.