Copy/Move Constructor not invoked

Vinoth Rajendran4

Hi All,

Can some one help me understand, why either copy/move constructor is not called in this scenario.

class Hello
{
public:
    Hello() = default;
    Hello(const Hello&)
    {
        cout << "Copy ctor";
    }

    Hello(Hello&&)
   { 
       cout << "Move ctor";
   }

};

int main()
{
    Hello a = Hello();
    return 0;
}

Does compiler does any optimization is this scenario ?

JonB

@Vinoth-Rajendran4
I'm going to stick my neck, because I'm not a C++ expert like some the people here.

But if you were expecting it to create a default for Hello a and then assign (copy/move) to it from = Hello(), I don't think it does that. I think C++ says this initialize-and-assign just create and initalizes with a Hello().

Vinoth Rajendran4

@JonB , I was expecting Hello() will call default constructor, followed by move constructor for object a creation.

JonB

@Vinoth-Rajendran4
Yes. and I'm saying I believe C++ says create-and-assign is not treated as create followed by assign, it's just one operation. But I don't have a reference for you, so wait till someone other says so...! :)

To allay your worries about optimization, I would do something with a just to make sure.

Vinoth Rajendran4

@JonB . Thanks for your input.

kshegunov

@JonB said in Copy/Move Constructor not invoked:

But I don't have a reference for you, so wait till someone other says so...!

https://en.cppreference.com/w/cpp/language/direct_initialization

If T is a class type,
- if the initializer is a prvalue expression whose type is the same class as T (ignoring cv-qualification), the initializer expression itself, rather than a temporary materialized from it, is used to initialize the destination object.
  (Before C++17, the compiler may elide the construction from the prvalue temporary in this case, but the appropriate constructor must still be accessible: see copy elision)

JonB

@kshegunov
Thank you!

And, IIRC some issue over this, which C++ version says T t = T(); does the same as T t(T()); ? Have they always does behaved the same?

kshegunov

@JonB said in Copy/Move Constructor not invoked:

And, IIRC some issue over this, which C++ version says T t = T(); does the same as T t(T()); ?

c++17

Have they always does behaved the same?

Yes for a very long time. Before codified into the standards the compilers just used to elide the copy (which they're free to do).

JonB

@kshegunov said in Copy/Move Constructor not invoked:

Before C++17, the compiler may elide the construction from the prvalue temporary in this case [...]

I'm still not understanding what this is saying. What is it that <= C++17 is allowed to do which >=C++20 is not?

kshegunov

It says that before c++17 the compiler may direct initialize from the/a temporary but is not mandated to (although they used to do it). After c++17 it's not optional anymore.