The differences between concepts and “old-school” (say, enable_if-based) SFINAE are purely cosmetic. There is seemingly nothing which is possible with concepts and impossible with enable_if. Concepts are just syntax sugar. They make something easier, plus they offer better error messages.
One thing is possible with concepts, but not with SFINAE
It’s extensibility.
template<typename Mutex>
struct lock_guard {
Mutex& mtx_;
lock_guard(Mutex& m): mtx_(m) {
mtx_.lock();
}
~lock_guard() {
mtx_.unlock();
}
};Now you want to make it work with MFC-style mutexes whose methods are called Lock and Unlock, capitailzed. What can you do? You can specialize individually for all specific lockable classes you need, but that’s a lot of copy-pasted code.
template<typename Mutex, typename SFINAE_Hook = void>
struct lock_guard {
Mutex& mtx_;
lock_guard(Mutex& m): mtx_(m) {
mtx_.lock();
}
~lock_guard() {
mtx_.unlock();
}
};
template <typename MFCMutex>
struct lock_guard<MFCMutex, std::enable_if_t<is_pascal_case_lockable_v<MFCMutex>>, void>> {
MFCMutex& m_mutex;
lock_guard(Mutex& m) : m_mutex(m) { m_mutex.Lock(); }
~lock_guard() { m_mutex.Unlock(); }
};Without concepts one has to think and try to envision extension points like this one. With concepts one can just do:
template <lowercase_lockable Mutex>
struct lock_guard {
Mutex& m_mutex;
lock_guard(Mutex& m) : m_mutex(m) { m_mutex.lock(); }
~lock_guard() { m_mutex.unlock(); }
};
template <typename MFCMutex> requires pascal_case_lockable<MFCMutex>
struct lock_guard<MFCMutex> { ... };For SFINAE, include all except not writed code, for concept, it’s only this except all others code! It’s the main difference I think.