How to type cast or convert from QString to char *

kishore_hemmady

structurechar.fileName = stuct.fileNameQString.toStdString().c_str();
here stuct.fileNameQString is a QString type and
structurechar.fileName is a character pointer.
please help me in resolving this.

SGaist

Hi,

What will you do with your structurechar ?

What you are currently doing is using a pointer to a char array build from a temporary std::string object which means that your fileName is pointing to a memory location that's not valid anymore by the end of the line.

If you want to use that structure later on, you have to properly allocate fileName and copy the data inside it.

kishore_hemmady

@SGaist
I am connecting my QT code to a standard C++ code,
here my stuct.fileNameQString contains a file path which is of QString ,
while assigning this data to the other module code like to the structure
structurechar.fileName which is of type char * (char pointer) i am getting an cast error.

SGaist

My point stays: properly allocate fileName and copy the stuff you need in there.

mvuori

The point is very valid one, but doesn't explain the cast error.
Whenever there is an error, it is important to see the error message exactly, not just a hint that there was one.

kishore_hemmady

@SGaist
Still i am facing the same problem even after allocating the memory for the structure.
void MainWindow::on_clicked()
{

 structptr = ( mystruct*)malloc(sizeof(mystruct));
if (structptr==NULL) {
    qDebug()<<"ERROR!";

}
else
    structptr->word=GUI_videostruct.videoFileName.toStdString().c_str();;
    qDebug()<<"the file name is"<<*structptr;
    free(structptr);

}
where strcutptr is the structure obj of mystruct * type
error: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
structptr->word=GUI_videostruct.videoFileName.toStdString().c_str();;
^

jsulm

@kishore_hemmady What you are doing is not what @SGaist suggested. He suggested to copy the STRING.
So, copy what GUI_videostruct.videoFileName.toStdString().c_str() returns to your allocated string of same size...

kishore_hemmady

@jsulm
I am not getting it,
can you help me by the snippet please.

jsulm

@kishore_hemmady

size_t size = strlen(stuct.fileNameQString.toStdString().c_str());
structurechar.fileName = malloc(size);
strncpy(structurechar.fileName, stuct.fileNameQString.toStdString().c_str(), size);

JonB

@kishore_hemmady
I believe you want to know why you get the compilation error you see?

c_str() returns a const char*. Note the const. This C-string is considered immutable, i.e. it must not be updated.

You do not show your definition of mystruct.word, but I'm guessing that is char *, not const char *? So it is mutable.

Hence the error messge.

Is that what you wanted to know about the error?

kishore_hemmady

@JonB
@jsulm
@SGaist

Thank you.The issue is solved

SR__

@kishore_hemmady
Have you lookedd at https://wiki.qt.io/Technical_FAQ#How_can_I_convert_a_QString_to_char.2A_and_vice_versa.3F ?

Do not forget to allocate space for the final 0 of the C-string btw.