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C++17 copy elision and object destruction


C++17 copy elision and object destruction

By : Razy
Date : November 23 2020, 04:01 AM
will help you The symmetric case where the source outlives the target is when the prvalue is a parameter:
code :
struct A {
  static int *data;
  A() {if(!refs++) data=new int(42);}
  A(const A&) {++refs;}  // not movable
  ~A() {if(!--refs) delete data;}
private:
  static int refs;
};
int A::refs,*A::data;
int* f(A) {return A::data;}
A returnA();
int returnInt() {return *f(returnA());} // ok


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Invalid object after elision of copy operation?

Invalid object after elision of copy operation?


By : tibbot
Date : March 29 2020, 07:55 AM
hope this fix your issue Because Foo f( Foo() ); is a function declaration.
I think you want: Foo f;
code :
Foo f( (Foo()) );
copy elision of temporary object

copy elision of temporary object


By : Guy Berger
Date : March 29 2020, 07:55 AM
this will help Several things are going on here.
funcElision is omitting the copy in its return because of Named Return Value Optimization (NRVO), a special rule for named return values when: "a function returns a class type by value, and the return statement's expression is the name of a non-volatile object with automatic storage duration, which isn't a function parameter, or a catch clause parameter, and which has the same type (ignoring top-level cv-qualification) as the return type of the function." In funcElisionExternal, the return value of funcElision is used as a "nameless temporary", which can specifically elide copies (and moves post-C++11). An A is then constructed and returned using Return Value Optimization (RVO) since it is also a nameless temporary. In funcElisionInternal, tmp is a named temporary, so it can only elide copies with NRVO. However, it is not the value that is being returned, and does not have the same type as the function's return signature - it is passed to A's constructor first. So it cannot use NRVO.
Can copy elision/RVO cause a copy/move from the same object

Can copy elision/RVO cause a copy/move from the same object


By : Андрей Король
Date : March 29 2020, 07:55 AM
will help you
However, with copy elision/RVO, is it possible that the copy will happen from obj to obj?
code :
SomeObject obj = copy_maybe(true, obj);
SomeObject obj = copy_maybe(false, obj);
SomeObject obj = obj;
Copy elision when creating object inside emplace()

Copy elision when creating object inside emplace()


By : jwillette
Date : March 29 2020, 07:55 AM
Does that help
My question is: is the compiler able to optimize this and skip the create and copy? Or should I really try to fix these occurrences?
Copy elision and temporary bound-by-ref object

Copy elision and temporary bound-by-ref object


By : Vladimir Plokhotnich
Date : March 29 2020, 07:55 AM
wish of those help You never know that a copy will be elided. Copy elision is never mandatory.
Therefore, either both cases are UB, or none. It depends on what StaticStuff:send does with the object you pass in. If it retains any poitner/reference to y._xRef or *y._xPtr, then dereferencing that pointer/reference after MakeX() has returned will indeed cause UB, as the original object x was one with automatic storage duration inside MakeX() and its lifetime has now ended.
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