Calling toLatin1().data() via a wrapper function c_str()

Anestis_Papadopulos

Hi guys....

This is probably very basic but I can't figure it out atm. I need to have the char* data of my QString object returned and I shall use an "alias" function named "c_str()" for that. My class which derives from QString is called AquaString. When I call "obj.toLatin1().data();" (with "AquaString obj;") then the result is there. But when I call it via the alias function "obj.c_str();" then the result is broken. It has to do with the return of a temporary object or so. My question is, how do I do it right?

Here are the files to test this:
aquastring.hpp:

#ifndef AQUASTRING_HPP
#define AQUASTRING_HPP

#include <QString>

class AquaString : public QString
{
public:
	AquaString(const AquaString& other);
	AquaString(const QString& str);
	
	char* c_str();
	const char* c_str() const;
};

#endif // AQUASTRING_HPP

aquastring.cpp:

#include "aquastring.hpp"

#include <QDebug>

AquaString::AquaString(const AquaString& other)
{
	if (*this != other) {
		*this = other;
	}
}

AquaString::AquaString(const QString& str) : QString(str)
{
	// uses the QString constructor.
}

char* AquaString::c_str()
{
	return toLatin1().data();
}

const char* AquaString::c_str() const
{
	return toLatin1().constData();
}

// EOF

main.cpp:

#include <QDebug>

#include "aquastring.hpp"

int main(int argc, char **argv)
{
	Q_UNUSED(argc);
	Q_UNUSED(argv);
	
	const QString tmp("From QString");
	AquaString str(tmp);
	
	qDebug()  << "=================================:";
	qDebug()  << "QString                          :" << tmp;
	qDebug()  << "QString.toLatin1()               :" << tmp.toLatin1();
	qDebug()  << "QString.toLatin1().data()        :" << tmp.toLatin1().data();
	qDebug()  << "QString.toLatin1().constData()   :" << tmp.toLatin1().constData();
	qDebug()  << "=================================:";
	qDebug()  << "AquaString                       :" << str;
	qDebug()  << "AquaString.toLatin1()            :" << str.toLatin1();
	qDebug()  << "AquaString.toLatin1().data()     :" << str.toLatin1().data();
	qDebug()  << "AquaString.toLatin1().constData():" << str.toLatin1().constData();
	qDebug()  << "AquaString.c_str()               :" << str.c_str();
	
	return 0;
}

and the result looks like:

=================================: 
QString                          : "From QString" 
QString.toLatin1()               : "From QString" 
QString.toLatin1().data()        : From QString 
QString.toLatin1().constData()   : From QString 
=================================: 
AquaString                       : "From QString" 
AquaString.toLatin1()            : "From QString" 
AquaString.toLatin1().data()     : From QString 
AquaString.toLatin1().constData(): From QString 
AquaString.c_str()               : ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ: 
=================================:

Christian Ehrlicher

@Anestis_Papadopulos said in Calling toLatin1().data() via a wrapper function c_str():

char* AquaString::c_str()
{
return toLatin1().data();
}

You create a temporary and return a dangling pointer.

Anestis_Papadopulos

Hi Christian. Thx for the quick answer.

I have a working version of the c_str() function and it looks like:

char* AquaString::c_str()
{
	unsigned int len = length()+1;
	char* ret = new char[len];
	memcpy(ret, toLatin1().data(), len);
	ret[len] = '\0';
	return ret;
}

The point is that I would rather call the QString functions somehow "direkt" instead of making copies via memcpy or similar functions. Is there a way to do that or am I just losing time here? I think the later is the case but maybe there is some kind of trick/call that I am not aware of... Thx for your time anyway :)

Christian Ehrlicher

C++ basics - in your second example the temporary is deleted after the usage. Simply said: Temporaries are deleted when the ; is processed.