Get type with value_type in pertect forward

Question

I create a function which perfect forward into this function:

template<typename T>
void push_back_3(T&& container) {
  using containter_val_type = typename T::value_type;
  containter_val_type val = 3;
  container.emplace_back(val);
}

The function can accept container input, such as std::vector, std::list...

I know above std container has implement type-trait value_type .

So I just use std::vector<T>::value_type or std::list<T>::value_type ,

I'll get type T.

then

I demo program:

int main(int argc, char const *argv[]) {
  std::vector<int> ivec{1,2,3,4};
  push_back_3(ivec);
  for (std::vector<int>::iterator iter = ivec.begin(); 
        iter != ivec.end(); 
        ++iter) 
  {
    std::cout << *iter << "\n";
  }

  return 0;
}

But I got error message:

> $ clang++ t.cc -o t                                                               
t.cc:11:40: error: type 'std::vector<int, std::allocator<int> > &' cannot be used prior to '::' because it has no members
  using containter_val_type = typename T::value_type;
                                       ^
t.cc:18:3: note: in instantiation of function template specialization 'push_back_3<std::vector<int, std::allocator<int> > &>' requested here
  push_back_3(ivec);
  ^
1 error generated.

I don't know why I got above error.

Answer 1

T is deduced to be lvalue reference, you have to remove reference by:

typename std::decay_t<T>::value_type;

Forwarding reference for lvalue (this is such case, because ivec is named object) returns T&. There is no way to get value_type for reference type.

Demo