Library redefines NULL

Question

I'm working with a library that redefines NULL. It causes some problems with other parts of my program. I'm not sure what I can do about it. Any idea? My program's in C++, the library's in C.

#ifdef NULL
#undef NULL
#endif

/**
 * NULL define.
 */
#define NULL    ((void *) 0)

Oh, and it produces these errors:

Generic.h:67: error: default argument for parameter of type 'LCD::LCDBase*' has type 'void*'
Generic.cpp: In constructor 'LCD::Generic::Generic(std::string, Json::Value*, int, LCD::LCDBase*)':
Generic.cpp:44: error: invalid conversion from 'void*' to 'QObject*'
Generic.cpp:44: error:   initializing argument 2 of 'LCD::LCDWrapper::LCDWrapper(LCD::LCDInterface*, QObject*)'
Generic.cpp: In member function 'void LCD::Generic::BuildLayouts()':
Generic.cpp:202: error: invalid conversion from 'void*' to 'LCD::Widget*'
Generic.cpp: In member function 'void LCD::Generic::AddWidget(std::string, unsigned int, unsigned int, std::string)':
Generic.cpp:459: error: invalid conversion from 'void*' to 'LCD::Widget*'
scons: *** [Generic.o] Error 1

Here's the first one:

Generic(std::string name, Json::Value *config, int type, LCDBase *lcd = NULL);

Edit: Ok, casting explicitly works, but how do I cast for a function pointer?

Answer 1

Can you rebuild the library without that define? That's what I'd try first. NULL is a pretty standard macro, and should be assumed to be defined everywhere.

Right now, your problem is that C++ doesn't allow automatic casts from void * to other pointer types like C does.

From C++ Reference:

In C++, NULL expands either to 0 or 0L.

If that doesn't work, just do a global replace in the library: NULL to LIBDEFINEDNULL or something. That way you'll keep the library code intact and avoid the macro collision.

Answer 2

Do you have access to that libraries source? If so, I think a search and replace on their code is in order. (Replace their NULL with LIBNAME_NULL or something similar.) If that's simply not an option then I would recommend using 0 in your code instead of NULL.

I'm curious, though: What problems is that causing? They're not changing the value of null, only the default casting.

Answer 3

The most generic approach would be to wrap up the offending includes and store and restore the macros previous definition. This, however, is compiler-dependent.
This is how you could do it with VC:

#pragma push_macro("NULL")
#include <offendinglib.h>
#pragma pop_macro("NULL")

Alternatively, set the macro to what you need it to be afterwards:

#include <offendinglib.h>
#undef NULL
#define NULL 0

Answer 4

Yes, you do just need to cast appropriately:

Generic(std::string name, Json::Value *config, int type,
    LCDBase *lcd = (LCDBase *)NULL);

Answer 5

One of your comments says you've considered redefining it yourself, but you don't know what to redefine it to.

A lot of implementations will define NULL like this:

#undef NULL

#ifdef __cplusplus
#define NULL 0
#else
#define NULL ((void*)0)
#endif

This is because in C, it makes sense to have it as a void pointer, because it's a pointer type that can be implicitly cast to other types.

C++ doesn't allow this (which is causing your problems), but using 0 instead of NULL works.

I think in all recent versions, GCC will actually define it to __null, which is a non-portable extension.