Re: a question on execution order

"Victor Bazarov" <>
Wed, 23 Jan 2008 14:31:30 -0500
Igor Tandetnik wrote:

Victor Bazarov <> wrote:

Mycroft Holmes wrote:

struct tricky
 int& x;
 ~tricky() { x = 1234; }

int f()
int result = 6789;
tricky t = { result };
return result;

even if my above conjecture is correct, what if the compiler applies
NVRO and turns 'f' to something like:

void f(int& result)
result = 6789;
tricky t = { result };

I don't think NRVO has anything to do with the behaviour of 'f'.

Well, the standard 12.8/15 says that, under NRVO, the name of the
local variable being returned and the name of the variable at the
call site the result is being assigned to are treated as aliases
referring to the same object. This object is destroyed at the latter
of the two times the two objects would be destroyed had NRVO not been
Without NRVO, a copy is made by the return statement, and then ~tricky
destructor acts on the original. But with NRVO, copies are elided,
there's just one object, and it appears that ~tricky destructor does
affect this object. There's nothing in the standard that says NRVO is
disallowed if some other object's destructor may have a side-effect
that affects the return value.

Since the return is by value, a temporary of type 'int' is copy-
constructed [from 'result'], so the return statement will cause
the function to return 6789.

It is true that NRVO is only applicable to objects of class type, so
having an 'int' return value clouds the issue somewhat. But consider:

struct S { int x; }

S f() {
   S local = {6789};
   tricky t = {local.x};
   return local;

S global = f();
assert(global.x == 6789);

Again, according to the standard, if NRVO is applied, global and local
are two aliases for the same object. It appears that ~tricky destrutor
has no choice but to set x==1234 inside that object, which would then
trigger the assert.

I guess I missed the part that allowed the local object to be treated
as an alias of the object initialised by the return value from the
function. Musing on the class/non-class, would then this be better:

    S global = { f_original() };
    assert(global.x == 6789 };

('f_original' is the one with 'int' return type) ?

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