Ouch: return values as lvalue

[Lionello Lunesu]

"Andrei Alexandrescu (See Website For Email)" 
<SeeWebsiteForEmail at erdani.org> wrote in message 
news:45BF7C8B.3080506 at erdani.org...
> Lionello Lunesu wrote:
>> Consider this home-made const:
>>
>> struct Task {
>>   char[] ID;
>> }
>> private Task _CurrentTask; //mutable
>> public Task CurrentTask() { return _CurrentTask; } //const
>>
>> public void StopTask() {
>>   CurrentTask.ID = null;
>> }
>>
>> Notice the bug? That last line should read "_CurrentTask.ID = null;"
>>
>> Isn't there something the compiler can do to help me catch these bugs?
>
> const will take care of it. The code above fetches a member of an rvalue, 
> which is an lvalue.

I've been thinking about this some more. Why exactly is a member of an 
rvalue, an lvalue?

struct S { int i; }
//foo returns a struct, which is placed on stack/register, somewhere anyway
S foo();
foo() = myS; // not allowed
foo().i = 2; // suddenly allowed! Why?

// func returns an int, which is also placed somewhere,
// physically, stack/register doesn't matter
int func();
func() = 2;   // I can't write this... why?

What exactly is the difference between setting the whole thing, or only one 
of its members? In the case of S above, there's no real difference, yet they 
behave differently?

L.