c++ class constructor template with std::enable_if and std::decay

Question

class DirectoryEntry; // forward declaration

template <class T>
struct isPathable { static const bool value = false; };

template<> struct isPathable<char*>
{
    static const bool value = true;
};
template<> struct isPathable<const char*>
{
    static const bool value = true;
};
template<> struct isPathable<std::string>
{
    static const bool value = true;
};
template<> struct isPathable<std::vector<char> >
{
    static const bool value = true;
};
template<> struct isPathable<std::list<char> >
{
    static const bool value = true;
};
template<> struct isPathable<DirectoryEntry>
{
    static const bool value = true;
};

class path
{
private:
    std::string m_pathname;
public:

    // constructors:
    // ------------------------
    path() noexcept {}
    path(const path &p) : m_pathname(p.m_pathname) {}

    template <class Source>
    path(Source const &source,
        std::enable_if_t<isPathable<std::decay_t<Source>> >* = 0)
    {
        // do stuff
    }
...
};

I get the following error message:

/usr/bin/c++   -I../lib -Wall -Werror -std=c++17 -g   -pthread -MD -MT app/CMakeFiles/infinityApp.dir/src/main.cpp.o -MF app/CMakeFiles/infinityApp.dir/src/main.cpp.o.d -o app/CMakeFiles/infinityApp.dir/src/main.cpp.o -c ../app/src/main.cpp

error: type/value mismatch at argument 1 in template parameter list for ‘template<bool _Cond, class _Tp> using enable_if_t = typename std::enable_if::type’

std::enable_if_t<isPathable<std::decay_t<Source>> >* = 0)
                                                  ^
note:   expected a constant of type ‘bool’, got ‘isPathable<typename std::decay<_Tp>::type>’

From the error message I see that there is a problem with the isPathable part as it doesn't pass on a bool, but I don't understand the why. Where is the problem and how should I change my code? Maybe there is a better solution to such problems?

Answer 1

template<> struct isPathable<char*>
{
    static const bool value = true;
};

You're defining a bunch of specializations in this fashion. Your specializations define a boolean member value, initialized as true. In your constructor:

/* ... */ std::enable_if_t<isPathable<std::decay_t<Source>> >* = 0)

Note that the template parameter to std::enable_if_t is a boolean value, but if you parse out what you're specifying here, you're specifying a typename as a template parameter. You obviously meant something along the line of...

/* ... */ std::enable_if_t<isPathable<std::decay_t<Source>>::value >* = 0)

A few other tweaks you can try and improve your template:

• Define your class member as a constexpr, and not merely a const.

• You can probably avoid using a dummy formal parameter to the constructor, by doing something like:

  template <class Source,
        std::enable_if_t<isPathable<std::decay_t<Source>>::value >>
  path(Source const &source)
  {
      // do stuff
  }