Casting int to enum problem

**jw232** · 01-28-2009

Code:

#include <iostream>

class Foo {
public:    
    enum Color { r, b, g };
    Color c;
    Foo(Color cc) : c(cc) {}
};

int main() {
    int i = 2;
    //Foo f(Foo::Color(i)); //function-style cast gives me an error
    //Foo f(Foo::Color(2)); //however this works
    Foo f((Foo::Color)i); //C-style cast works
    std::cout << f.c << '\n';
    std::cin.get();
}

From what I understand, you can cast an int to enum with a function-style cast or a C-style cast. However, the line using a function-style cast gives me the error "error C2228: left of '.c' must have class/struct/union" in Visual Studio. It works if I use a constant instead of i. Can someone explain to me why the function-style cast is giving me problems? Thanks.

**anon** · 01-28-2009

That's because the syntax is ambigous: where you see a constructor call and a cast, the compiler sees a function declaration, basically the same as:

Code:

Foo f(Foo::Color i);

That is: f is a function that takes a Foo::Color and returns a Foo.

The second version (with 2) works, since the literal value shows this cannot be a function declaration.

A workaround would be to use an extra pair of brackets which are not allowed in function declarations:

Code:

Foo f((Foo::Color(i)));

And yet another unambigous possibility: C++ casts:

Code:

Foo f(static_cast<Foo::Color>(i));

There are some cases where you run into this ambiguity, so you better be aware.

Edit: If that is the error message, VC++ in this case words it a bit poorly. GCC is clearer in telling you what it sees, not only what it expects to see:

16 request for member `c' in `f', which is of non-class type `Foo ()(Foo::Color)'

But you can't expect all compiler errors to be worded clearly. With experience, and if you pay attention to the messages, you'll get used to diagnosing the causes better.

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