What is difference using enum class variables in main and other callable in c++?

Question

1

#include<type_traits>
#include<iostream>
using namespace std;

enum class Widget;
using UType = underlying_type_t<Widget>;
UType getWidget();

int main() {
    cout<<getWidget();
    cout<<static_cast<int>(Widget::a);
    return 0;
}

UType getWidget() {
    return static_cast<UType<Widget>>(Widget::a);
}

enum class Widget {
    a, b, c
};

2

#include<iostream>
#include<type_traits>
using namespace std;

template<typename T>
using UType = underlying_type_t<T>;

enum class Widget;

template<typename T>
UType<T> getWidget();

int main() {
    cout<<getWidget<Widget>();
    //cout<<static_cast<int>(Widget::a);
    return 0;
}

template<typename T>
UType<T> getWidget() {
    return static_cast<UType<Widget>>(Widget::a);
}


enum class Widget {
    a, b, c
};

When I used Widget::a in main, compiler failed because of "Undefined identifier Error". But, when I called GetWidget, this code was compiled. What is difference in two situations?

---2 is working if I use getWidget(). but 1 isn't. ---compiler : visual studio 2019 iso c++14 standard ---error message :

1. c2027 : use of undefined type 'Widget'

2. C2065 : 'a' : undeclared identifier

Answer 1

You need to define enum class Widget before you can use any of the enumerators (Widget::a). You have the definition below everything else, where it invisible to everything else and therefore useless. The declaration is visible, but a mere declaration is not enough to use the enumerators. It is only enough to use the name of the enumeration itself (Widget).

MSVC does not detect the error in getWidget because its handling of templates is not standards compliant. Its implementation of the two-phase lookup is incomplete and is not enabled by default. If you enable it with a compiler switch, it is still incomplete. It seems to be broken for qualified names, which are not looked up in the first phase. That is, if you write x::y in a template, MSVC looks up x in the first phase (and complains if it is not defined), but ignores y. Other compilers do not have this bug.

If you make getWidget a non-template, which it should have been in the first place, MSVC will complain.