unpredictable runtime behavior while converting QString to const char* - c++

Question

I have these following codes set up-
class ID3{
const char* fileName;
TagLib::FileRef *file;
public:
ID3(const char *);
QImage* artwork();
}
ID3::ID3(const char* fileNameStr){
this->fileName = fileNameStr;
this->file = new TagLib::FileRef(fileNameStr);
qDebug()<<fileNameStr; //OUTPUT 2
}
QImage* ID3::artwork(){
QString str = QString::fromLocal8Bit(this->fileName);
qDebug()<<str; //OUTPUT 3
//MORE CODES-------
}
const char * QstrTocChar(QString str){
QByteArray ba = str.toLocal8Bit();
qDebug()<<ba.constData(); //OUTPUT 1
return ba.constData();
}
int main(int argc, char *argv[]){
.
.
.
QString fileName = "C:/Qt/Qt5.0.2/Projects/taglib_test/music files/Muse_-_Madness.mp3";
file = new ID3(QstrTocChar(fileName));
QImage *image = file->artwork();
}
Now when I run the program, I get these strange outputs
OUTPUT 1
C:/Qt/Qt5.0.2/Projects/taglib_test/music files/Muse_-_Madness.mp3
OUTPUT 2
????p???e'2/
OUTPUT 3
"°í³àpµ˜Æe'2/"
Not sure about OUTPUT 2 but I expect OUTPUT 3 to be same as OUTPUT 1. I am a Qt newbie. Would really appreciate advice/help in understanding, these strange character encoding issues and how to get OUTPUT 3 fixed.
Thanks!

Answer 1

ba.constantData() is returning a pointer to data which will be invalid when QstrToChar finishes executing (the 8-bit converted QByteArray), when QstrToChar completes, all you have left is free'd junk.
What if you just did:
file = new ID3(fileName.toLocal8Bit().constData());
in your main routine?
Actually, you still probably need to keep your own copy of this data in your private ID3 char *, since it can go away with the destruction of these temporaries.
Your code should be this, instead:
class ID3{
std::string fileName;
std::smart_ptr<TagLib::FileRef> file;
public:
ID3(std::string);
QImage* artwork();
}
ID3::ID3(std::string fileNameStr) {
this->fileName = fileNameStr;
this->file.reset(new TagLib::FileRef(fileNameStr));
qDebug()<<fileNameStr; //OUTPUT 2
}
QImage* ID3::artwork(){
QString str = QString::fromLocal8Bit(this->fileName);
qDebug()<<str; //OUTPUT 3
//MORE CODES-------
}
std::string QstrToCppString(QString str){
QByteArray ba = str.toLocal8Bit();
qDebug()<<ba.constData(); //OUTPUT 1
return std::string(ba.constData());
}
int main(int argc, char *argv[]){
.
.
.
QString fileName = "C:/Qt/Qt5.0.2/Projects/taglib_test/music files/Muse_-_Madness.mp3";
file = new ID3(QstrToCppString(fileName));
QImage *image = file->artwork();
}
Notice that I've wrapped your TagLib::FileRef in a smart_ptr as well, since you are new-ing it, you'll need to manage the memory. An alternative would be to write a proper destructor for your ID3 class. You're definitely leaking these currently (unless you just didn't share your destructor code).