Question

The following code gives compilation errors on both MSVC++ 2012 CTP (with C+11 support) and Intel C++ XE 13.0:

template <typename F, typename... Args>
    void apply(F f, std::tuple<Args...>& args) {
       // doesn't do much yet
}

bool f1(char c) {
    return c == 'c';
}

int main(int argc, char* argv[]) {
    auto t = std::make_tuple('c');
    apply(f1, t);
return 0;
}

VS2012 error is:

error C2243: 'type cast' : 
conversion from 'std::tuple<char,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil> *'
 to 'std::tuple<std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil> &' exists,
 but is inaccessible

Error on Intel C++ XE 13.0 is:

error : no instance of function template "apply" matches the argument list

What am I missing? Are there really errors here, or do I just have two bad compilers?

UPDATE: Same (or similar) results when I use boost::tuple instead of std::tuple on both compilers.

ADDENDUM: Thanks for all the cross-checks in the comments. I’ve sent bug reports to both of these fine corporations.

Answer 1

That is likely a VC11 bug in the implementation of the Standard Library. Although the CTP supports variadic templates, to the best of my knowledge the Standard Library was not rewritten to use them and adopts some machinery to simulate variadic templates. This is likely at the origin of your problem.