Using new with decltype

Question

T *t; //T is an implementation detail
t = new T; //want to avoid naming T to allow for flexibility
t = new decltype(*t); //error: cannot use 'new' to allocate a reference
t = new std::remove_reference<decltype(*t)>::type(); //clunky

This answers why decltype(*t) returns T & and not T.

I can put my last line into a macro, but that seems suboptimal. Is there a better solution than what I have so far? Does this belong on Code Review?

Answer 1

If they're on the same line, you can use auto to only name T once:

auto t = new T;

Otherwise, you could create a small function template:

template <class T>
void do_new(T * &p) {
  p = new T;
}


// Usage:
int main()
{
  T *t;
  do_new(t);
}

As @MadScienceDreams pointed out, you can extend this to allow non-default constructors:

template <class T, class... Arg>
void do_new(T * &p, Arg &&... arg) {
  p = new T(std::forward<Arg>(arg)...);
}


// Usage:
int main()
{
  T *t;
  do_new(t);
  std::string *s;
  do_new(s, "Abc");
}

Answer 2

std::remove_pointer<decltype(t)>::type is more expressive/clear.

You can also use a local typedef if this is repeated several times, or would make a certain line grow excessively long/complicated.