Extend object lifetime/scope from a `if constexpr` branch

Question

Say we have the following code

struct MyClass
{
    MyClass() = delete;  // or MyClass() { }
    MyClass(int) { }
    void func() { }
};

int main()
{
    if constexpr (std::is_default_constructible_v<MyClass>) {
        MyClass myObj;
    } else {
        MyClass myObj(10);
    }

    myObj.func();  // Error
}

Here I am using if constexpr to determine whether the class is default-constructible (or not), and then create an object accordingly. In a way, I naively thought this would simplify the different branches down to just the one that's true, i.e.

    if constexpr (true) {
        /* instruction branch 1 */
    } else if constexpr (false) {
        /* instruction branch 2 */
    }

simply becomes

    /* instruction branch 1 */

But in reality, it is probably more like this

    {
        /* instruction branch 1 */
    }

But then the question becomes (going to back to the the very first example), how can I can I keep myObj in scope outside the { ... }?

Answer 1

First, your code won't work. if constexpr really needs its condition to be dependent.

I'll fix it.

template<class MyClass>
void func() {
  MyClass myObj = []{
    if constexpr (std::is_default_constructible_v<MyClass>) {
      return MyClass{};
    } else {
      return MyClass(10);
    }
  }();
  myObj.func();
}

now

int main() {
  func<MyClass>();
}

solves your problem.

Note that under c++17 rules, no copies or moves of MyClass occur in the above code.