How to return member that can be changed?

Question

Multi thread enviroment . The content of Foo can be multi thread.

class Foo
{
public:
   const A & getA() {return a_;} //has guard
   void setA(A newA){a         


        

Answer 1

You need to be more careful who you listen to. The only time the lifetime of something gets extended if a temporary object is bound immediately to a const-reference. For example, like so:

Foo bar() { return Foo(); }

int main()
{
    Foo const & f = bar();

    /* stuff */

} // the object referred to by f is extended till here

Your situation is nothing like that. In particular, returning a const-reference does not create a temporary object, so there's nothing here who's life gets prolonged. In particular, the following is definitely an error:

A const & bar() { Foo x; return x.getA(); }

int main()
{
    A const & a = bar(); // dangling reference; object dies upon return from Foo::getA()
}