const class can extend lifetime of const member ref of temporary?

Question

struct  A {

// something

~A() { std::cout <<  "A destruct!\n"; }

};

class  refWrapper {

public:

refWrapper(const  A&  a) : a_(a) {}

~refWrapper() { std::cout <<  "refWrapper destruct!\n"; }

private:

const A& a_;

};

void  func(const  refWrapper&  ref  =  A()) {

// why ~A after ~refWrapper

// Rather than after refWrapper constructor complete

}

Please use the code layout tool to format your question correctly. And it's helpful to elaborate on your question with more than just a code example. — paddy, Apr 28 '21 at 10:37
do you have code that you compiled and executed to get output different from what you expected? If so, please include a [mcve] (where is your `main`?) and actual and expected output in the question — 463035818_is_not_an_ai, Apr 28 '21 at 10:40
What is the question? Does this answer the question? https://stackoverflow.com/questions/39718268/why-do-const-references-extend-the-lifetime-of-rvalues — ypnos, Apr 28 '21 at 10:42
@ypnos: notice that there are 2 temporaries: a `refWrapper` which is bound to `ref`, and `A()`. — Jarod42, Apr 28 '21 at 10:49
outside of function call, behavior is different [Demo](http://coliru.stacked-crooked.com/a/cb17509322c6caf7). — Jarod42, Apr 28 '21 at 10:50

score 1 · Accepted Answer · answered Apr 28 '21 at 12:35

With default arguments, the call

func();

is equivalent to

func(A()); // so func(refWrapper(A()));

So,

temporary A is created first (destroyed at end of full expression)
temporary refWrapper is created second (bound to parameter reference)
temporary refWrapper destroyed.
temporary A destroyed.

Notice that there is an exception for lifetime extension or parameter:

A temporary object bound to a reference parameter in a function call ([expr.call]) persists until the completion of the full-expression containing the call.

So refWrapper is destroyed at end of full expression, and not at the end of func call (which is the same moment in given example though). So destruction should be done in reverse order of construction.

const class can extend lifetime of const member ref of temporary?

1 Answers1