Date: Tue, 30 Apr 2013 10:48:52 +0200
From: Vladimir Marek <Vladimir.Marek@Oracle.COM>
To: Kim Minh Kaplan <kimminh.kaplan+nomuch@afnic.fr>
Subject: Re: [PATCH] don't store temporary value returned from c_str()
Message-ID: <20130430084852.GA19599@pub.cz.oracle.com>
Mail-Followup-To: Kim Minh Kaplan <kimminh.kaplan+nomuch@afnic.fr>,
	Tomi Ollila <tomi.ollila@iki.fi>, notmuch@notmuchmail.org,
	Vladimir Marek <vlmarek@volny.cz>
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	<m2sj2cjp2b.fsf@guru.guru-group.fi>
	<20130427101134.GI10394@pub.cz.oracle.com>
	<8761z4h7ih.fsf@kaki.tech.prive.nic.fr>
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Cc: Tomi Ollila <tomi.ollila@iki.fi>, notmuch@notmuchmail.org,
	Vladimir Marek <vlmarek@volny.cz>
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Hi,

Thank you, I found it eventually too. But I wrote little test program
(attached) which confused me. I haven't had much time to take a look
into it since weekend.

The idea is to have temporary object where I can detect whether
destructor was called.

I thought that

printf ("%s\n", s.c_str());
will print "test"

and

x=s.c_str();
printf ("%s\n", x);

will print "destroyed"

On my machine both prints "destroyed".


I still believe my fix is correct, but I'm not at the position to be
able to defend it at the moment :)

Thank you
-- 
	Vlad

> The january 2012 working draft: 
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
> 
>     12.2 Temporary objects [class.temporary]
> 
>     1 Temporaries of class type are created in various contexts: binding a
>     reference to a prvalue (8.5.3), returning a prvalue (6.6.3) […]
> 
>     3 When an implementation introduces a temporary object of a class that
>     has a non-trivial constructor (12.1, 12.8), it shall ensure that a
>     constructor is called for the temporary object. Similarly, the
>     destructor shall be called for a temporary with a non-trivial destructor
>     (12.4). Temporary objects are destroyed as the last step in evaluating
>     the full-expression (1.9) that (lexically) contains the point where they
>     were created.

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#include <stdio.h>
#include <vector>
#include <string.h>

using namespace std;

class array {
	vector<char> impl;

	public:

	array(int size):impl(size) { }
	array(array &in):impl(in.impl) { }

	char*
	operator[](size_t i) {
		return &impl[i];
	};

	~array() {
		strcpy(&impl[0], "destroyed");
	}
};

class str {
	array tmp;

	public:

	str(const char *input):tmp(100) {
		strcpy(tmp[0], input);
	};

	const char* c_str() {
		return (array(tmp))[0];
	};
};


int
main (int argc, char **argv)
{
	str s("test");
	const char *x;
	printf ("%s\n", s.c_str());
	x=s.c_str();
	printf ("%s\n", x);
	return 0;
}

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