ByteArray manipulation issue

Joseph22121

What is wrong with my code and how to fix it?

#include <QByteArray>

const char* getString()

{

QByteArray ba("Sample output");

return ba.constData();

}

int main(int argc, char *argv[])

{

const char *str = getString();

printf("%s\n", str);

return 0;

}

J.Hilk

@Joseph22121

https://doc.qt.io/qt-5/qbytearray.html#constData

The pointer remains valid as long as the byte array isn't reallocated or destroyed.

the byte array is destroyed as soon as the code reaches } therefore the returned pointer becomes invalid

Joseph22121

Thanks for your quick comments. I am beginner to Qt APIs. How to fix this code? Please advise me here.

J.Hilk

@Joseph22121 said in ByteArray manipulation issue:

Thanks for your quick comments. I am beginner to Qt APIs. How to fix this code? Please advise me here.

well, thats not really a Qt issue but a C++ one

you have to make your data persistent.

make the byte array a member variable
make it static //bad idea
memcpy it to heap allocated char array
return the byte array instead of a pointer to the internal data
ask yourself do you really want/need printf or could you simply use something else, maybe QDebug

Joseph22121

@J-Hilk Do you mean to say to dynamically allocate the memory using new operator in c++?

J.Hilk

@Joseph22121 this is clearly a broken down example of your actual issue. And as long as I don't know your real situation, I can not recommend a solution for your.

You could simply new the QByteArray, yes, but that will cause for this simple function:

const char* getString()

{

 QByteArray *ba(newQByteArray("Sample output"));

return ba->constData();

}

a memory leak

why do you go the way over the byte array anyway?

const char* getString()
{
    return "Sample output";
}

int main()

{

    const char *str = getString();

    printf("%s\n", str);

    delete str;
    return 0;

}

Joseph22121

@J-Hilk said in ByteArray manipulation issue:

QByteArray *ba(newQByteArray("Sample output"));

return ba->constData();

Thanks for your quick response. Here QByteArray class provides an array of bytes.QByteArray can embed '\0' bytes. QByteArray makes a deep copy of the const char * data, so you can modify it later without experiencing side effects.
Do you mean to use this way to avoid memory leak issue?

#include <QByteArray>

const char* getString()

{

QByteArray *ba(newQByteArray("Sample output"));

return ba->constData();

}

int main(int argc, char *argv[])

{

const char *str = getString();

printf("%s\n", str);
delete str;

return 0;

}

jsulm

@Joseph22121 said in ByteArray manipulation issue:

QByteArray makes a deep copy of the const char * data

No, it does not! Read https://doc.qt.io/qt-5/qbytearray.html#constData
"The pointer remains valid as long as the byte array isn't reallocated or destroyed" - and this is exactly what is happening here!

Joseph22121

so how to fix the issue here?

Below is what I thought to do. Is it correct?

const char* getString()
{
QByteArray ba("Sample output"); // creates byte array of data
return strdup(ba.constdata()); // this will do the copy string to heap allocated memory
}

JonB

@Joseph22121
I guess you could, but why are you doing any of this with QByteArray? Your goal seems to be to return a char * pointing to "Sample output", you can do that without going anywhere near Qt classes or the complications you are introducing.

Also

const char *str = getString();
delete str;

I don't think/know that you can delete a const char *. In any case, you must not delete something allocated via strdup(), which uses malloc().

Joseph22121

Do you mean this code is fine? I have avoided using QByteArray here.

const char* getString()
{
return "Sample output";
}

JonB

@Joseph22121
Sure! If you just want to return some constant string, for whatever reason. Certainly so far you have nothing which needs a QByteArray, unless your real code wants to do something different from what you show.

Joseph22121

Can I conclude the fix for the code as below?

#include <QByteArray>

const char* getString()
{
return "Sample output";
}

int main()

{

const char *str = getString();

printf("%s\n", str);

return 0;

}

Joseph22121

I think using QByteArray object ba is not required here. Have modified the code as follows. Please review it.
#include <QByteArray>

const char* getstring()
{
static const char *const MY_STRING = "Sample output"; // its a const string in static memory
return MY_STRING;
}

int main()

{

const char *str = getString();

printf("%s\n", str);

return 0;

}

jsulm

@Joseph22121 Yes, static variable is one of the solutions suggested by @J-Hilk ...

JonB

@Joseph22121 said in ByteArray manipulation issue:

const char* getstring()
{
static const char *const MY_STRING = "Sample output"; // its a const string in static memory
return MY_STRING;
}

You are welcome to write it like this if it makes you happy/reads clearer. But just be aware this does nothing different from plain:

const char* getstring()
{
    return "Sample output";   // this is also a const string in static memory!
}

Joseph22121

@JonB Thanks everyone for your inputs and suggestions. This thread can be closed.

Christian Ehrlicher

@Joseph22121 said in ByteArray manipulation issue:

This thread can be closed.

You can do this with the Topic Tools in your first post.