C++ how to convert from char * to unsigned char * in function call?

Question

When building my C++ application the build fails at this line of code

if (!PyTuple_GetByte(poArgs, 0, &SourceCell.window_type))

with this error

error C2664: 'PyTuple_GetByte' : cannot convert parameter 3 from 'char *' to 'unsigned char *'

This is the called function:

bool PyTuple_GetByte(PyObject* poArgs, int pos, unsigned char* ret);

The third parameter &SourceCell.window_type is type char.

Is there a way to convert/cast the parameter inside the function call like

if (!PyTuple_GetByte(poArgs, 0, reinterpret_cast<unsigned char*>(&SourceCell.window_type)))

or do I have to deal with it in another way?

Did `if (!PyTuple_GetByte(poArgs, 0, reinterpret_cast(&SourceCell.window_type)))` not work? — NathanOliver, Feb 22 '17 at 13:15
@πάνταῥεῖ I think `static_cast` will not work here: http://stackoverflow.com/questions/10151834/why-cant-i-static-cast-between-char-and-unsigned-char as they're different types no? — EdChum, Feb 22 '17 at 13:16
The `static_cast` did not work, @πάνταῥεῖ is right. I tried that before. Currently I am trying to use a `reinterpret_cast` but wanted to know if any experts here have a more elegant solution. — Klappstuhl, Feb 22 '17 at 13:22
if you can change type of the SourceCell.window_type to be unsigned char instead of a normal char, no cast will be needed — Marian Spanik, Feb 22 '17 at 13:56
Safe way is to create an unsigned char buffer, read into it, then assign to the char array from it checking the values. — Malcolm McLean, Feb 22 '17 at 16:19
@EdChum i.e. you have to `reinterpret_cast`. c-style casts are defined in terms of some `*_cast`s — Caleth, Jun 30 '17 at 11:48

score 1 · Accepted Answer · edited Jun 30 '17 at 09:00

From the error, the signature of the PyTuple_GetByte function was expecting a third parameter of type unsigned char*, but you passed a variable of type char* at its invocation. I think you have two options here.

You can change the signature of function PyTuple_GetByte to expect a char* parameter.
You need to convert your input variable from type char* to type unsigned char*, before you can pass it into PyTuple_GetByte.

The conversion is normally like this:

unsigned char* convert_var = reinterpret_cast<unsigned char*>(&SourceCell.window_type); // (c++ way)

or

unsigned char* convert_var = (unsigned char*)(&SourceCell.window_type); // (c way)

C++ how to convert from char * to unsigned char * in function call?

1 Answers1