Guaranteed copy elision

[Yubin Ruan <ablacktshirt@gmail.com>]

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Hi,

As mentioned in cppreference:
https://en.cppreference.com/w/cpp/language/copy_elision

it is guaranteed in c++17 that copy elision must be applied for some cases
like

    SomeBigObject SomeBigObject::Factory(...) {
        SomeBigObject local;
         ...
         return local;
    }

(examples taken from https://abseil.io/tips/11 )

but not for cases like

    SomeBigObject SomeBigObject::Factory(...) {
        SomeBigObject local1;
        SomeBigObject local2;
         ...

         if (cond_1) {
             return local1;
         } else {
             return local2;
         }
    }

For a c++ user, it is somewhat difficult to be 100% sure that copy elision
/ NVO is applied to the functions' return value above.

To be sure that a object would not be copied, we usually write something
like

         SomeBigObject obj;
         func(&obj);

while in most of the cases a one-liner like

         SomeBigObject obj = func();

would suffice.

Is there any language facility to help us guarantee that at compile time
(such as some kind of static_assert() ) so that we can be confident writing
those one-liner ?

I know that marking the copy constructor deleted would do the good, but
copy is needed in some cases.

Thanks
Yubin