Pointing Two Dimensional Pointer to return value

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

Quote:

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

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

Quote:

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

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

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... :p)


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

Code:

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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.

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

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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.

Quote:

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

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

Quote:

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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.

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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.

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

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

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.)