Issue using QString

[prophetjohn]

Fortunately this is the newbie forum because I feel dumb all of a sudden. I did a search. My problem is that calling toStdString() on a QString crashes my program resulting in an access violation

The following code compiles and runs, but then crashes:

void q_string_to_std_string()
{
    QString herp("derp");
    herp.toStdString();
}

void q_string_to_std_string()
{
    QString herp("derp");
    herp.toStdString();
}

To copy to clipboard, switch view to plain text mode 

Any help is appreciated.

[ChrisW67]

Yes, compiles, runs, does nothing (i.e. discards the result of QString::toStdString()). Your crash is somewhere else. Run your program in a debugger and see what the backtrace looks like.

[prophetjohn]

No, that is the whole program. I pinpointed the issue, so I wrote some trivial program and sure enough. To be more clear, this prints "hi" one time and then crashes.

int main()
{
    QString herp("derp");
    std::cout << "hi\n";
    herp.toStdString();
    std::cout << "hi\n";
    return 0;
}

int main()
{
    QString herp("derp");
    std::cout << "hi\n";
    herp.toStdString();
    std::cout << "hi\n";
    return 0;
}

To copy to clipboard, switch view to plain text mode 

To reiterate, if I comment out the one line with the call to toStdString() or replace it with toASCII() ot something, there is no crash.

[fpersson]

There shouldn't be any problem with that code sample, can you post the error messanges?

//Fredrik

[ChrisW67]

Quoting myself:

Run your program in a debugger and see what the backtrace looks like.

Your code runs just fine here on 32-bit Linux, Qt 4.7.2, GCC 4.4.5. Without your error message, backtrace, or any details about which OS/compiler/Qt version we can only guess at how this is broken.

Does it behave differently if you don't ignore the return value from toStdString() ?

[SixDegrees]

If your Qt installation was configured without support for STL or other C++ template classes, this function may case problems.

What do you mean "but then crashes"? Provide the error messages produced.

[prophetjohn]

Here is what is displayed in the console during the crash

'qtt.exe': Loaded 'C:\Users\Josh\Documents\Visual Studio 2008\Projects\qtt\Debug\qtt.exe', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ntdll.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\kernel32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\KernelBase.dll'
'qtt.exe': Loaded 'C:\Windows\system\QtCore4.dll', Binary was not built with debug information.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\user32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\gdi32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\lpk.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\usp10.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\msvcrt.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\advapi32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\sechost.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\rpcrt4.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\sspicli.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\cryptbase.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ole32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ws2_32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\nsi.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.crt_1fc8b3b9a1e18e3b_9.0.30729.6161_none_50934f2ebcb7eb57\msvcp90.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.crt_1fc8b3b9a1e18e3b_9.0.30729.6161_none_50934f2ebcb7eb57\msvcr90.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.debugcrt_1fc8b3b9a1e18e3b_9.0.21022.8_none_96748342450f6aa2\msvcr90d.dll', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.debugcrt_1fc8b3b9a1e18e3b_9.0.21022.8_none_96748342450f6aa2\msvcp90d.dll', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\imm32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\msctf.dll'
First-chance exception at 0x683631ea (msvcr90d.dll) in qtt.exe: 0xC0000005: Access violation reading location 0xcccccbf4.
Unhandled exception at 0x683631ea (msvcr90d.dll) in qtt.exe: 0xC0000005: Access violation reading location 0xcccccbf4.

'qtt.exe': Loaded 'C:\Users\Josh\Documents\Visual Studio 2008\Projects\qtt\Debug\qtt.exe', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ntdll.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\kernel32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\KernelBase.dll'
'qtt.exe': Loaded 'C:\Windows\system\QtCore4.dll', Binary was not built with debug information.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\user32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\gdi32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\lpk.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\usp10.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\msvcrt.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\advapi32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\sechost.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\rpcrt4.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\sspicli.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\cryptbase.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ole32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\ws2_32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\nsi.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.crt_1fc8b3b9a1e18e3b_9.0.30729.6161_none_50934f2ebcb7eb57\msvcp90.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.crt_1fc8b3b9a1e18e3b_9.0.30729.6161_none_50934f2ebcb7eb57\msvcr90.dll'
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.debugcrt_1fc8b3b9a1e18e3b_9.0.21022.8_none_96748342450f6aa2\msvcr90d.dll', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.vc90.debugcrt_1fc8b3b9a1e18e3b_9.0.21022.8_none_96748342450f6aa2\msvcp90d.dll', Symbols loaded.
'qtt.exe': Loaded 'C:\Windows\SysWOW64\imm32.dll'
'qtt.exe': Loaded 'C:\Windows\SysWOW64\msctf.dll'
First-chance exception at 0x683631ea (msvcr90d.dll) in qtt.exe: 0xC0000005: Access violation reading location 0xcccccbf4.
Unhandled exception at 0x683631ea (msvcr90d.dll) in qtt.exe: 0xC0000005: Access violation reading location 0xcccccbf4.

To copy to clipboard, switch view to plain text mode 

I am compiling with VC++ 2008 on Window 7 x64 Ultimate. I was initially using library version 4.4.1 which I compiled from source and suspected that may be the problem, but this is happening after I updated and am using the libs and includes from the SDK version 4.7.4

It doesn't behave different if I capture the the output of the function. I removed the assignment suspecting and error could be happening there, but it doesn't look like that's the case.

Also, when it crashes, VS opens the file dbgdel.cpp and points at the following line

_ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));

_ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));

To copy to clipboard, switch view to plain text mode 

saying that it is the next to execute.

I'm fairly new to VS because most of my previous programming was done in command line Linux, so I'm not terribly familiar with the debugger, but if more info is needed, I will try to find it. Thanks.

edit: clicking around, it looks like the problem occurs during a delete call, but that's as much as I can tell.

[ChrisW67]

If your Qt installation was configured without support for STL or other C++ template classes, this function may case problems.

Surely the code would fail to compile if that were the case. QString::toStdString() would not be present after the pre-processor removed it (QT_NO_STL).

[prophetjohn]

I've essentially worked around this issue using only c-strings a QStrings and no std::strings, but if anyone else has ideas, I'd love to find out what my issue is. My only idea is that it has something to do with me having both Qt 4.4.1 and 4.7.3 on this machine, but I've updated all my environmental variables and lib/src/include directories

[ChrisW67]

Try building:

#include <string>
#include <iostream>
 
int main(void) 
{
    std::string s("Hello", 5);  // This is what Qt does in toStdString()
    std::cout << s << std::endl;
    return 0;
}

#include <string>
#include <iostream>

int main(void) 
{
    std::string s("Hello", 5);  // This is what Qt does in toStdString()
    std::cout << s << std::endl;
    return 0;
}

To copy to clipboard, switch view to plain text mode 

If that fails when run then the problem has nothing to do with Qt.

[prophetjohn]

Try building:

#include <string>
#include <iostream>
 
int main(void) 
{
    std::string s("Hello", 5);  // This is what Qt does in toStdString()
    std::cout << s << std::endl;
    return 0;
}

#include <string>
#include <iostream>

int main(void) 
{
    std::string s("Hello", 5);  // This is what Qt does in toStdString()
    std::cout << s << std::endl;
    return 0;
}

To copy to clipboard, switch view to plain text mode 

If that fails when run then the problem has nothing to do with Qt.

This executes without any problem.

[mvuori]

I don't see why crashing of a statement that does nothing, and makes no sense, is worth all this discussion.
herp.toStdString(); should obviously be removed.

Now, if this would crash, it would be worth solving, as it actually does something with the conversion:
std::string srt = herp.toStdString();