Question

I have the following code that is failing to compile:

#include <iostream>
#include "boost/mpl/set.hpp"
#include "boost/mpl/at.hpp"
#include "boost/type_traits/is_same.hpp"

struct TypeSet {

    typedef boost::mpl::set<int, float> typeset;

    template<typename T>
    static bool hasType()
    {
        using namespace boost;
        using namespace boost::mpl;
        return is_same< at< typeset, T >::type, T >::value; // <-- ERROR IS HERE
    }  
};


int main(int argc, const char * argv[])
{
    bool hasInt = TypeSet::hasType<int>();
    std::cout << (hasInt ? "set contains int" : "set does not contain int") << std::endl;    
    return 0;
}

The code is being compiled with the Apple LLVM clang 4.1 compiler and boost 1.5.2 and the error is “Template argument for type parameter must be a type” – basically the compiler is complaining that boost::mpl::at is not returning a type. The offending code is taken pretty much verbatim from the boost documentation so I’m at a loss as to what’s wrong with this (and as far as I can tell boost::mpl::at does return a type).

Answer 1

You need

typename at< typeset, T >::type

since it depends on template parameter T. So you have to tell the compiler that type is a type in this context.