[Closed] Passing QString's to C external libraries
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Hi! I just noticed a behavior that is very strange when using QString's and an external C++ library. What I've done is simply to pass a QString to a function of an external library this way:
@QString qString("test");
const char* _qString = qString.toStdString().c_str();
externalFunction(_qString);@What is happening is something very very strange, my suspect is that it's somehow related to implicit sharing (but I'm not sure). The output of this piece of code I place for debugging inside externalFunction(const char* clientPath):
@fprintf(stderr, "1: %s.\n", clientPath);
std::string rootPath, leafName, fileExt;
char* _rootPath = new char[strlen(clientPath) + 1];
strcpy(_rootPath, clientPath);
fprintf(stderr, "_rootPath: %s.\n", _rootPath);
rootPath = _rootPath;
rootPath[0] = 'A';
fprintf(stderr, "1b: %s.\n", clientPath);
fprintf(stderr, "Address of clientPath: %x.\n", clientPath);
fprintf(stderr, "Address of _rootPath: %x.\n", _rootPath);
fprintf(stderr, "Address of rootPath: %x.\n", rootPath.c_str());@is:
1: /home/luca/2.jpg.
_rootPath: /home/luca/2.jpg.
1b: Ahome/luca/2.jpg.
Address of clientPath: 8751de4.
Address of _rootPath: 874fcc8.
Address of rootPath: 8751de4.Any idea why this is happening precisely and how I can avoid it? Is this related to Qt?
Thanks! -
The problem is this line: _const char* qString = qString.toStdString().c_str();, because it create a temporary std::string that will go out of scope before you can use it (so you will have a dangling pointer).
You can achieve what you want something like this:
@
void c_func(char* str) {
printf("%s\n", str);
}int main()
{
QString s("Test QString to char pointer");char *ptr = strdup(s.toLocal8Bit().data()); //duplicate the returned c-style string
c_func(ptr);
free(ptr);QString s0("This also work");
c_func(s0.toLocal8Bit().data()); //or make the temporary in your function call - this way the temp variable should be valid through the function call
return 0;
}
@ -
See the "FAQ entry":http://developer.qt.nokia.com/faq/answer/how_can_i_convert_a_qstring_to_char_and_vice_versa
Closing this thread for this reason.
