Any ideas on how to turn this into a warning?

Question

I'm using this is_enum function to check if a variable is an enum or not. (See error below)

#include <boost/type_traits/is_enum.hpp>
#include <boost/static_assert.hpp>

template<typename T>
void is_enum(T)
{
    BOOST_STATIC_ASSERT(boost::is_enum<T>::value == true);
}

int main()
{
    char c = 'a';
    is_enum(c);
    return 0;
}

This gives me the following error:

-*- mode: compilation; default-directory: "/home/epronk/enums/" -*-
Compilation started at Thu Nov 10 21:20:05

g++ -I /home/epronk/src/boost_1_47_0/ q.cpp
q.cpp: In function ‘void is_enum(T) [with T = char]’:
q.cpp:13:   instantiated from here
q.cpp:7: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’ 
q.cpp:7: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’ 

Compilation exited abnormally with code 1 at Thu Nov 10 21:20:05

(not sure why g++ (Debian 4.4.5-8) 4.4.5 give me the same error twice)

Is it possible to change this function so it becomes a warning?

For a char you can try to assign 256 to it which results in a overflow error.

edited

Some context: I want to find switch statements like this one.

#define switch(arg) \
is_enums(arg); \
switch(arg)

int main()
{
    char c = Red;

    switch(c)
    {
    case Banana: // No warning
        break;
    case Red:
        break;
    case Green:
        break;
    case Blue:
        break;
    }
    return 0;
}

Answer 1

EDIT: Try BOOST_STATIC_WARNING http://www.boost.org/doc/libs/1_46_1/libs/serialization/doc/static_warning.html; the code below is my hand-hacked version of doing something similar.

Something like this:

#include <boost/type_traits/is_enum.hpp>
#include <boost/utility.hpp>
#include <boost/static_assert.hpp>

enum AB { A, B };

template<typename T>
typename boost::enable_if_c< boost::is_enum<T>::value,
                             void >::type is_enum(T) {
}

template<typename T>
typename boost::enable_if_c< !boost::is_enum<T>::value,
                             void >::type is_enum(T) {
    int NOT_AN_ENUMERATION = 1;
}

int main()
{
    char c = 'a';
    is_enum(c);
    is_enum(A);
    is_enum(B);
    return 0;
}

Will issue a warning about the unused variable if you get your compiler in the right state. With gcc and '-Wall', I get this sort of thing:

thing.cpp: In function 'typename boost::enable_if_c<(! boost::is_enum::value), void>::type is_enum(T) [with T = char]':
thing.cpp:21:   instantiated from here
thing.cpp:15: warning: unused variable 'NOT_AN_ENUMERATION'