将const引用返回给临时对象的成员

Question

What happens when a const reference to aa member of a temporary object is returned; What is the lifetime of that object;

Eg

struct temp
{
  T m_mine;

  static temp make()
  {
    return temp();
  }
};

T const & foo()
{
  return temp::make().m_mine;
}

What is the behavior with c++98 and c++11?

Answer 1

The constness of the object or the referee type doesn't matter in this context: it's simply a return of a reference to an object that at that time has ceased to exist. Using the reference is then Undefined Behavior.

Likewise, if you bind a member reference to const, to an object, that does not prolong the life of the referee.

Object lifetime extension is only for the case of binding a local reference to an object, and only for the cases of reference to const object or rvalue reference.

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^{The example code as it was at the time I wrote this, has several problems. Please only post real code (to the degree possible). And it should be pasted, not retyped.}

^{(Also, since there is now at least 2 answers referring to the problems of the code, it's too late to correct it without possibly changing the context of the answers and thereby invalidating them. So it's important to get the code correct in the original posting. Worth keeping in mind for next SO question.)}

Answer 2

Your code is incorrect in multiple accounts before it is even getting to compile:

• there is at least a semicolon missing after the type declaration
• the code uses the non- static member make() as if it is a static function

Once getting over that: the reference returned refers to a subobject of an object which is destroyed after the return statement is executed and before anything can get hold of it. That is, there is a stale reference return. Any access to this reference will result in undefined behavior. If you are lucky, the program crashes at this point. If you are unlucky it does something you like it to happen. For example it may "work" until the program is demonstrated to a client or an investor at which point it may decide to rather display insults.