[SOLVED] How to enforce copy elision, why it won't work with deleted copy constructor?

How to enforce copy elision, why it won't work with deleted copy constructor?

I have an uncopiable class. Copying this would be problematic. I want to guarantee that it won't be ever copied, so I made its copy constructor deleted:

class A {
  public:
    A();
    A(const A&) = delete;
};

A fun() {
  return A();
};

int main() {
  A a = fun();
};

Unfortunately, g++ won't compile this on the reason:

t.cc: In function ‘A fun()’:
t.cc:8:12: error: use of deleted function ‘A::A(const A&)’
   return A();
            ^
t.cc:4:5: note: declared here
     A(const A&) = delete;
     ^
t.cc: In function ‘int main()’:
t.cc:12:13: error: use of deleted function ‘A::A(const A&)’
   A a = fun();
             ^
t.cc:4:5: note: declared here
     A(const A&) = delete;
     ^

But this is a very clear situation where copy elision should be used, so the copy constructor shouldn't be ever called. Why is it so?

Solution

Until C++17 copy elision is an optimization the compiler is not required to do, so classes must be copyable since the compiler might want to copy (even if it actually does not). In C++17 copy elision will be guaranteed in many cases and then classes won't need copy ctors.