CODEWITHSUNDEEP
c++classc++11overloadingenable-if
xunzhang
xunzhang

Reputation: 2936

How to use enable_if in a class obj?

The code below is OK:

template <class T>
std::enable_if<std::is_atomic<T>::value, bool>
foo(T t) { return true; }

template <class T>
std::enable_if<tmp::is_sequence<T>::value, bool>
foo(T t) { return false; }

int main(void){
  foo(1);  // return true
  auto std::vector<int> a{2};
  foo(a);  // return false
}

But when I use a class to bundle them, it can not be compiled:

template <class T>
class test {
public:

std::enable_if<std::is_atomic<T>::value, bool>
foo(T t) { return true; }

std::enable_if<tmp::is_sequence<T>::value, bool>
foo(T t) { return false; }
};

int main(...) {
  test<int> obj;
  obj.foo(1);
  test<std::vector<int>> obj2;
  std::vector<int> tmp{2};
  obj2.foo(tmp);
}

clang++ print:

error: functions that differ only in their return type cannot be overloaded

So I write something to cheat to the compiler(add an S in second foo):

template <class S>
std::enable_if<tmp::is_sequence<T>::value, bool>
foo(T t) { return false; }

It still can not work:

error: no type named 'type' in 'std::enable_if<false, bool>'

How can I make it work in a class?

Upvotes: 0

Views: 416

Answers (3)

SergV
SergV

Reputation: 1277

You forgot to add ::type after enable_if: (see enable_if)

template <class T> std::enable_if<std::is_atomic<T>::value, bool>::type
foo(T t) { return true; }

Upvotes: 1

ice-phoenix
ice-phoenix

Reputation: 361

If you really want to do what you're doing, the classic idiom is to introduce a fake SFINAE argument with a default value:

bool foo(T t, std::enable_if<..., void*> = nullptr) { ... }

Upvotes: 0

ForEveR
ForEveR

Reputation: 55897

Both member-functions should have different template parameter (following will work ok)

template <class T>
class test {
public:

template<typename U>
typename std::enable_if<std::is_atomic<U>::value, bool>::type
foo(U t) { return true; }

template<typename U>
typename std::enable_if<tmp::is_sequence<U>::value, bool>::type
foo(U t) { return false; }

};

Upvotes: 0

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