Prev: ANNOUNCE: just::thread library V1.3 released
Next: "Scoping" variables the middle of their "natural" scope - RAII - is it a good practice?
From: Andrew on 15 Jan 2010 10:32
I am getting this warning from g++ 4.4.2 and yes indeed it is taking
the address of a temporary, but the temporary ought to still be alive
when it matters AFAIC. So can someone tell why what, if anything, I am
doing wrong. Here is my example:

#include <iostream>
#include <string>

class Temporary {
public:
Temporary(const std::string& str) : m_str(str) {}
std::string str() const { return m_str;}
private:
std::string m_str;
};

class Example {
public:
void doSomething(Temporary* temp)
{
std::cout << "temp str = " << temp->str() << std::endl;
}
};

int main(int argc, char* argv[])
{
Example example;
example.doSomething(&Temporary("Hello World."));

return 0;
}

Regards,

Andrew Marlow

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Alf P. Steinbach on 15 Jan 2010 11:11
* Andrew:
> I am getting this warning from g++ 4.4.2 and yes indeed it is taking
> the address of a temporary, but the temporary ought to still be alive
> when it matters AFAIC. So can someone tell why what, if anything, I am
> doing wrong. Here is my example:
>
> #include <iostream>
> #include <string>
>
> class Temporary {
> public:
> Temporary(const std::string& str) : m_str(str) {}
> std::string str() const { return m_str;}
> private:
> std::string m_str;
> };
>
> class Example {
> public:
> void doSomething(Temporary* temp)
> {
> std::cout << "temp str = " << temp->str() << std::endl;
> }
> };
>
> int main(int argc, char* argv[])
> {
> Example example;
> example.doSomething(&Temporary("Hello World."));
>
> return 0;
> }

In C++98 the built-in address operator does not apply to rvalues, hence the
warning (presumably it says something about language extension or such?).

You could do something like

template< typename T >
T& tempRef( T const& t ) { return const_cast<T&>( t ); }

But better just replace 'Temporary*' with 'Temporary const&'.


Cheers & hth.,

- Alf

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Öö Tiib on 17 Jan 2010 21:35
On Jan 16, 5:32 am, Andrew <marlow.and...(a)googlemail.com> wrote:
> I am getting this warning from g++ 4.4.2 and yes indeed it is taking
> the address of a temporary, but the temporary ought to still be alive
> when it matters AFAIC. So can someone tell why what, if anything, I am
> doing wrong. Here is my example:
>
> #include <iostream>
> #include <string>
>
> class Temporary {
> public:
> Temporary(const std::string& str) : m_str(str) {}
> std::string str() const { return m_str;}
> private:
> std::string m_str;
>
> };
>
> class Example {
> public:
> void doSomething(Temporary* temp)
> {
> std::cout << "temp str = " << temp->str() << std::endl;
> }
>
> };
>
> int main(int argc, char* argv[])
> {
> Example example;
> example.doSomething(&Temporary("Hello World."));
>
> return 0;
>
> }
>
> Regards,
>
> Andrew Marlow

You cannot use temporarily constructed object as lvalue by standard.
Taking non-const pointer to one implies that you may access your
temporary through that pointer as lvalue. Take const reference and you
will get no warnings.


--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Andrew on 17 Jan 2010 21:45
On 16 Jan, 04:11, "Alf P. Steinbach" <al...(a)start.no> wrote:
> > I am getting this warning from g++ 4.4.2 and yes indeed it is taking
> > the address of a temporary, but the temporary ought to still be alive
> > when it matters AFAIC. So can someone tell why what, if anything, I am
> > doing wrong.

> In C++98 the built-in address operator does not apply to rvalues, hence the
> warning (presumably it says something about language extension or such?).

Ah, yes, this is what I wanted to know. Thanks.

> You could do something like
>
> template< typename T >
> T& tempRef( T const& t ) { return const_cast<T&>( t ); }
>
> But better just replace 'Temporary*' with 'Temporary const&'.

Unfortunately, not. The function that takes a pointer is from outside
our project and cannot be changed. I was hoping for something where
the function signature would remain unaltered. Perhaps the only way is
to introduce a named automatic variable, which is what the original
code was trying to avoid.

-Andrew


--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

 | 
Pages: 1
Prev: ANNOUNCE: just::thread library V1.3 released
Next: "Scoping" variables the middle of their "natural" scope - RAII - is it a good practice?