c++ constructor malfunctioning - c++

Question

I am trying to program a qt widget application which deals with linked list of nodes, the node contains 3 char* data member and 2 int members and a "next" pointer of "node" type,
my issue is the char* members in nodes of the linked list are getting saved same as the third char* member, I tried using debugger and found out that
the length of all the 3 ints i.e. lentitl,lenpub,lenpub are initialized to same value, and all the 3 char* members are getting same value,
the node constructor is as follows
node::node(char* titl,char* auth,char* pub,int pri,int stockp)
{
int lentitl,lenauth,lenpub;
lenpub=strlen(pub);
lentitl=strlen(titl);
lenauth=strlen(auth);
title=new char[lentitl+1];
author=new char[lenauth+1];
publisher=new char[lenpub+1];
strcpy(title,titl);
strcpy(author,auth);
strcpy(publisher,pub);
price=pri;
stockposition=stockp;
next=NULL;
}
the node functions if called from another class function named "addbook",and addbook is called from mainwindow.cpp, the functions calls for addbook is as follows
void MainWindow::on_addbook_clicked()
{
char *titl,*auth,*pub;
int pri,stockp;
titl=ui->title->toPlainText().toLatin1().data();
auth=ui->author->toPlainText().toLatin1().data();
pub=ui->publisher->toPlainText().toLatin1().data();
pri=ui->price->toPlainText().toInt();
stockp=ui->stockposition->toPlainText().toInt();
p.addbook(titl,auth,pub,pri,stockp);
}
and the function call for node is as follows
void shop::addbook( char *titl, char *auth, char *pub, int pri, int stockp)
{
node *p=new node(titl,auth,pub,pri,stockp);
if(start==NULL)
{
start=p;
end=p;
}
else
{
p->next=start;
start=p;
}
}
the zip of whole project is here
and here is the link to the ouput
the screen shot of the output is
as you can see in the image,string entered at "publisher" textedit is getting set to all 3 char* of the node,
can anyone explain me why is this happening?

Answer 1

titl=ui->title->toPlainText().toLatin1().data();
This is probably the issue: toLatin1() returns a new QByteArray which owns its data. You assign the internal data pointer of that QByteArray to titl. However, the QByteArray is only a temporary variable and will get destroyed in the next line of code. When the QByteArray gets destroyed, it will free its data, which means your titl is now pointing to memory that was already freed - i.e. titl points to an invalid memory location.
Suggested solution alternatives:
Use QString instead of char* in your node class, much easier to deal with memory management issues that way
Make sure your QByteArray temporary lives as long as you need the data (until you strcpy the data):
QByteArray titl = ui->title->toPlainText().toLatin1();
QByteArray auth = ui->author->toPlainText().toLatin1();
QByteArray pub = ui->publisher->toPlainText().toLatin1();
pri=ui->price->toPlainText().toInt();
stockp=ui->stockposition->toPlainText().toInt();
p.addbook(titl.data(),auth.data(),pub.data(),pri,stockp);

Answer 2

I'm not expert of Qt but there is something that doesn't sound good to me in terms of C++. Mainly a function in your code is returning a "char * " pointer instead of a "const char*", I'm talking about these lines:
char *titl,*auth,*pub;
int pri,stockp;
titl=ui->title->toPlainText().toLatin1().data();
If the data was directly provided by the title class I expect to obtain a const char* pointer in order to do not be allowed to modify it. The only reason you can handle a char* ( or in other words a well known library is designed to return a non const pointer) is that there is a temporary object or a static buffer in the middle of the calls: toLatin1 or data.
Reading the docs of Qt: toLatin1 is returning a temporary object, a QByteArray.
QByteArray toLatin1 () const
for more information: http://qt-project.org/doc/qt-4.8/qstring.html
So simply modify each string request like:
QByteArray titlArray = ui->title->toPlainText().toLatin1();
titl=titlArray.data();
in that way each string will point to a still existing buffer when you are calling
p.addbook(titl,auth,pub,pri,stockp);