Re: Explanation for rvalue/lvalue

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Tue, 25 Mar 2008 16:41:45 +0100

Message-ID:

<13ui7djalqitt75@corp.supernews.com>

* Victor Bazarov:

Christian Meier wrote:

I'm using "lint" and got a message which I do not understand.
When I "lint" the following code I get the message "Initializing a
non-const reference C &' with a non-lvalue".

class C {
public:
static C GetTemporaryInstance() { return C(); }
C& operator<<(int) { return *this; }
private:
C() {}
};

int main()
{
C::GetTemporaryInstance() << 5;
}

1. Can you explain me the lint message? Where am I initializing a C
reference?

You're returning a reference to non-const from operator<<.

That seems to be what lint triggers on.

However, the initializer expression there /is/ an lvalue.

So it must be that lint takes into account the usage in main.

2. Is this code legal c++ or is it not allowed to call the operator<<
of a temporary object like the return value of
C::GetTemporaryInstance()?

I am uncertain whether the code is fully legal; a non-const reference
is bound to the temporary object which is not allowed. But this is
a common idiom... Begs the question about the validity of the idiom,
doesn't it?

I can't see anything invalid.

Cheers,

- Alf

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