Revisiting writing a "recalloc" function in C++

ejspeiro · 04-08-2013, 11:14 PM

Hello guys,

in this thread, we discussed an actually solved the issue of how to use the memset function to implement a "recalloc" function, that is, a function that would increase the size of a previously allocated array, but clearing the new positions to 0.

I ported the code to a C++ class, which I compile using the -Werror flag, thus getting the following output:

Code:

mtk_dok_sparse_matrix.cc: In member function ‘int MTK_DOKSparseMatrix::add_value(int, int, double)’:
mtk_dok_sparse_matrix.cc:106:39: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
mtk_dok_sparse_matrix.cc:113:37: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
mtk_dok_sparse_matrix.cc:122:37: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]

It was working in my C code, so I would like to revisit this issue and discuss the portability of the approach implemented in the aforementioned original thread (this one).

If the code in the previous thread is not enough, I will gladly post the class and the related method yielding the issue, but I think it should be enough.

Thanks in advanced!

\m/

NevemTeve · 04-09-2013, 02:24 AM

Typecast 'void *' to 'char *' (or 'inptr_t') before calculating.

JohnGraham · 04-09-2013, 10:38 AM

In both C and C++, sizeof(void) is undefined, and so pointer arithmetic a void* is impossible. Although GCC provides an extension that treats sizeof(void) as 1, I believe C++ is more strict in saying that this is not allowed.

ejspeiro · 04-09-2013, 09:16 PM

Quote:

Originally Posted by NevemTeve

Typecast 'void *' to 'char *' (or 'inptr_t') before calculating.

Wow! So simple! And when read next to:

Quote:

Originally Posted by JohnGraham

In both C and C++, sizeof(void) is undefined, and so pointer arithmetic a void* is impossible. Although GCC provides an extension that treats sizeof(void) as 1, I believe C++ is more strict in saying that this is not allowed.

It all makes sense together! My assumption is that

Code:

sizeof(char) = sizeof(void) // In C.

I will try the code, and hopefully it will work!