Question

While trying to compile wxWidgets-2.9.1 from source with c++0x flags using gcc-4.6. I came across an error

narrowing conversion of '128' from 'int' to 'char' inside { } [-fpermissive]
in the file src/gtk/dcclient.cpp. The error comes from the following files:

1. src/gtk/bdiag.xbm
2. src/gtk/cdiag.xbm
3. src/gtk/fdiag.xbm
4. src/gtk/horiz.xbm
5. src/gtk/verti.xbm
6. src/gtk/cross.xbm

This is a known bug.
http://trac.wxwidgets.org/ticket/12575
So I did as required and the program is compiling okay.

Basically, there are two kinds of fix that the diff file has

//in the file dcclient.h

1. hatches[i] = gdk_bitmap_create_from_data(NULL, bdiag_bits, bdiag_width, bdiag_height);
   hatches[i] = gdk_bitmap_create_from_data(NULL, reinterpret_cast< const char* >(bdiag_bits), bdiag_width, bdiag_height);

   //in the file bdiag.xbm and similar fixes in all the *.xbm files

2. static char bdiag_bits[] = {

   static unsigned char bdiag_bits[] = {
   0x80, 0x80, 0x40, 0x40, 0x20, 0x20, 0x10, 0x10, 0x08, 0x08, 0x04, 0x04,
   0x02, 0x02, 0x01, 0x01, 0x80, 0x80, 0x40, 0x40, 0x20, 0x20, 0x10, 0x10,
   0x08, 0x08, 0x04, 0x04, 0x02, 0x02, 0x01, 0x01};

I understand the second fix but I could not understand the first one. Why do we need to do a reinterpret_cast< const char* > The function gdk_bitmap_create_from_data is declared like this:

typedef char   gchar;//in some other header file

GdkBitmap* gdk_bitmap_create_from_data (GdkDrawable *drawable, const gchar *data, gint width, gint height);

while a few lines later in the same file dcclient.cpp the following call to the gdk_bitmap_create_from_data doesn’t give any error.

char* data = new char[data_size];
//...
GdkPixmap* pixmap = gdk_bitmap_create_from_data(mask, data, dst_w, dst_h);

Now here no typecast is required. Why do we need to do a reinterpret_cast on static unsigned char*?

Answer 1

unsigned char, signed char and char (also known as ‘plain char’) are three different types. There is no conversion between unsigned char* and char*.