Template Argument Type Deduction Fails with C++11 <type_traits>

Question

I'm trying to understand how to use C++(11) <type_traits>.

Here's my trivial test program

#include <type_traits>

template<class U, class S>
inline U add(typename std::enable_if<std::is_unsigned<U>::value,U>::type a,
             typename std::enable_if<std::is_signed  <S>::value,S>::type b)
{
    return a + b;
}

int main(int argc, const char * argv[], const char * envp[])
{
    unsigned int ui;
    int i;
    auto a = add(ui, i);
    return 0;
}

When compiled with GCC 4.8.1 it errors as

/home/per/f.cpp: In function ‘int main(int, const char**, const char**)’:
/home/per/f.cpp:15:23: error: no matching function for call to ‘add(unsigned int&, int&)’
     auto a = add(ui, i);
                       ^
/home/per/f.cpp:15:23: note: candidate is:
/home/per/f.cpp:5:10: note: template<class U, class S> U add(typename std::enable_if<std::is_unsigned<U>::value, U>::type, typename std::enable_if<std::is_signed<S>::value, S>::type)
 inline U add(typename std::enable_if<std::is_unsigned<U>::value,U>::type a,
          ^
/home/per/f.cpp:5:10: note:   template argument deduction/substitution failed:
/home/per/f.cpp:15:23: note:   couldn't deduce template parameter ‘U’
     auto a = add(ui, i);
                       ^

I have no clue why GCC can't deduce the template parameter U. Anybody knows what information my code is missing, that is how I write a program in C++11 that takes a unsigned integral type as first argument and signed integral type as second?

Answer 1

typename std::enable_if<std::is_unsigned<U>::value,U>::type is not a deducible context. In order to deduce U from this the compiler would need the ability to apply the reverse operation of std::enable_if. It doesn't look too hard, that's true, but that's because you are talking about a simple thing like enable_if. It would be impossible to require this of every trait, so C++ just plays it cool and does not make any kind of weird rule exceptions: it's not deducible in general, it's not deducible in this one.

You can do it this way instead:

template<class U, class S,
         EnableIf<std::is_unsigned<U>, std::is_signed<S>>...>
U add(U a, S b)

Or in compilers that don't support that style properly you can just add an extra defaulted argument:

template<class U, class S>
U add(U a, S b,
      typename std::enable_if<std::is_unsigned<U>::value
          && std::is_signed<S>::value,void>::type* = nullptr)

... or mess up the return type.

template<class U, class S>
typename std::enable_if<std::is_unsigned<U>::value
    && std::is_signed<S>::value,U>::type
add(U a, S b)

Answer 2

You are not giving the compiler a chance to deduce U and S. You can rewrite your function as follows, and move the SFINAE checks in the template parameter list:

template<class U, class S,
    typename std::enable_if<std::is_unsigned<U>::value &&
                            std::is_signed  <S>::value
        >::type* = nullptr>
inline U add(U a, S b)
{
    return a + b;
}

Here is a live example.

Answer 3

You first have to deduce the types before you can reason about the types!

It should be:

template <typename U, typename S>
typename std::enable_if<std::is_unsigned<U>::value &&
                        std::is_signed<S>::value>, U>::type
add(U u, S s)
{
    // ...
}

Answer 4

It's not possible to deduce a template parameter from a "nested typedef" expression. That is, it's possible to deduce U from some_template<U>, but not from some_template<U>::type.

The compiler cannot possibly enumerate all (infinite!) instantiations of some_template and see for which of them the nested typedef equals the actual argument type.