从函数返回const对局部变量的引用

Question

I have some questions on returning a reference to a local variable from a function:

class A {
public:
    A(int xx)
    : x(xx)
    {
        printf("A::A()\n");
    }
};

const A& getA1()
{
    A a(5);
    return a;
}

A& getA2()
{
    A a(5);
    return a;
}

A getA3()
{
    A a(5);
    return a;
}

int main()
{
    const A& newA1 = getA1(); //1
    A& newA2 = getA2(); //2
    A& newA3 = getA3(); //3
}

My questions are =>

1. Is the implementation of getA1() correct? I feel it is incorrect as it is returning the address of a local variable or temporary.

2. Which of the statements in main (1,2,3) will lead to undefined behavior?

3. In const A& newA1 = getA1(); does the standard guarantee that a temporary bound by a const reference will not be destroyed until the reference goes out of scope?

Answer 1

1. Is getA1() implementation correct ? I feel it is incorrect as it is returning address of local variable or temporary.

The only version of getAx() that is correct in your program is getA3() . Both of the others have undefined behaviour no matter how you use them later.

2. Which of the statements in main ( 1,2,3) will lead to undefined behavior ?

In one sense none of them. For 1 and 2 the undefined behaviour is as a result of the bodies of the functions. For the last line, newA3 should be a compile error as you cannot bind a temporary to a non const reference.

3. In const A& newA1 = getA1(); does standard guarantees that temporary bound by a const reference will not be destroyed until the reference goes out of scope?

No. The following is an example of that:

A const & newConstA3 = getA3 ();

Here, getA3() returns a temporary and the lifetime of that temporary is now bound to the object newConstA3 . In other words the temporary will exist until newConstA3 goes out of scope.

Answer 2

Q1: Yes, this is a problem, see answer to Q2.

Q2: 1 and 2 are undefined as they refer to local variables on the stack of getA1 and getA2. Those variables go out of scope and are no longer available and worse can be overwritten as the stack is constantly changing. getA3 works since a copy of the return value is created and returned to the caller.

Q3: No such guarantee exists to see answer to Q2.

Answer 3

I think the main problem is that you are not returning temporaries at all, you should

return A(5);

rather than

A a(5);
return a;

Otherwise you are returning local variable address, not temporary. And the temporary to const reference only works for temporaries.

I think its explained here: temporary to const reference

Answer 4

If you will compile this on VC6 you will get this warning

******Compiler Warning (level 1) C4172 returning address of local variable or temporary A function returns the address of a local variable or temporary object. Local variables and temporary objects are destroyed when a function returns, so the address returned is not valid.******

While testing for this problem i found interesting thing (given code is working in VC6):

 class MyClass
{
 public:
 MyClass()
 {
  objID=++cntr;
 }
MyClass& myFunc()
{
    MyClass obj;
    return obj;
}
 int objID;
 static int cntr;
};

int MyClass::cntr;

main()
{
 MyClass tseadf;
 cout<<(tseadf.myFunc()).objID<<endl;

}