[Boost-users] Re: enable_if and constructors

From: Angus Leeming (angus.leeming_at_[hidden])
Date: 2005-04-05 16:38:10

David Abrahams wrote:
>>     // Only the specializations (below) of this template will compile.
>>     template <int N>
>>     static foo
>>     set() { return invalid_value; }
> 
> Without a definition for invalid_value, this is invalid code, and on a
> conforming compiler, compilation fails at the point it is parsed.

Thanks again. g++ 3.3 appears to happy differentiate between "valid" and
"invalid" enum values with the code below, so allow me to rephrase my
earlier question: which of the two techniques we've discussed would you
use? I assume that one has technical merits over the other?

Regards,
Angus

#include <boost/static_assert.hpp>
#include <iostream>

class foo {
public:
    enum state {
        state1,
        state2,
        state3
    };

    // Only the specializations (below) of this template will compile.
    template <int N>
    static foo
    set() { BOOST_STATIC_ASSERT( N == state2 ); }

private:
    foo(state)
    {
        std::cout << "foo" << std::endl;
    }
};

template <>
foo foo::set<foo::state2>() { return foo::state2; }

template <>
foo foo::set<foo::state3>() { return foo::state3; }

int main()
{
    // Compiles, as expected.
    foo f1 = foo::set<foo::state2>();
    foo f2(foo::set<foo::state2>());
    // Fail to compile, as expected.
  foo f3 = foo::set<foo::state1>();
//  foo f4(foo::set<foo::state1>());

    return 0;
}