Pointing Two Dimensional Pointer to return value

[babu198649]

Hi
I want two dimensional pointer point to a return value.
I have done this as below

char **argv;
char *temp = QCoreApplication::applicationFilePath().toAscii(). data(); //returns char*
argv = &temp;

If temporary pointer variable(char *temp) has to be eliminated, it can be done using type casting as below.

char **argv = (char**)QCoreApplication::applicationFilePath().to Ascii().data();

Is there a clean way to do the same without using temporary pointer variable and type casting.

[caduel]

the first problem here is the temporary QByteArray... your pointer is invalid (on the next line)!
the next problem is that char** means an array of char* (in the context of main), not the address of some string (char* in C).
Your code will therefore produce doubly undetermined fun...
(sorry, but the code is just about as wrong as it can get in so few lines... )


What you want to achieve is surprisingly hard too do.
(It might be easier to pass argc/argv from main().)

QList<QByteArray> arr; // no tempory QByteArrays - the data() pointers must remain valid
std::vector<char*> v_args; // store arbitrary number of pointers in contiguous memory (like a C array)
foreach(const QString &s, QCoreApplication::arguments())
{
  arr << s.toLatin1();
  v_args.push_back( arr.back().data() );
}
 
// vector has contig. memory, thus we can use the address of the first entry
// (yes, this code assumes that the number of arguments is not zero; which is quite safe as the program name is the first arg)
char **argv = &v_args[0];

QList<QByteArray> arr; // no tempory QByteArrays - the data() pointers must remain valid
std::vector<char*> v_args; // store arbitrary number of pointers in contiguous memory (like a C array)
foreach(const QString &s, QCoreApplication::arguments())
{
  arr << s.toLatin1();
  v_args.push_back( arr.back().data() );
}

// vector has contig. memory, thus we can use the address of the first entry
// (yes, this code assumes that the number of arguments is not zero; which is quite safe as the program name is the first arg)
char **argv = &v_args[0];

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(All this is completely untested, of course. But it should work that way.)


Lesson learned: what out whenever you want to get a char* from a QString. The danger of temporaries and dangling pointers is very real.

[babu198649]

the first problem here is the temporary QByteArray... your pointer is invalid

Thanks for alarming.
I am using Third Party library(Code Sourcery VSIPL++) which uses the application file path to setup the memory allocator,But surprisingly my app works fine with the code i posted.

It might be easier to pass argc/argv from main()

The third party library is used in a function other than the main function.

[caduel]

Thanks for alarming.
I am using Third Party library(Code Sourcery VSIPL++) which uses the application file path to setup the memory allocator,But surprisingly my app works fine with the code i posted.

You're welcome.
The problem with undefined behaviour (like accessing no longer valid memory) is that it might work, until you change something totally unrelated, bring the code to a different machine, have a newer Qt version, another gcc...
So, even if you have been unlucky and got no crash: do fix it.

[babu198649]

Does this below macro(stolen from Qwt3d example ) does the same job.

#define QWT3DLOCAL8BIT(qstring) ((const char*)(qstring.toLocal8Bit()))

[caduel]

Well, an array of c-strings is a different thing than the adress of a c-string.
this will work, if you (resp. the code you are passing this to) are only interested in the first item (ie argv[0]). It is dirty anyway.

(I'd rather store argc/argv either in global variables or make it accessible via some api.)