Naming Conflict in C++: How to access a struct member called "class"

Question

I came across a naming problem while working with the xlib library:

I'm using a struct which has a member called "class". I assume this library is mostly used in plain C programs. So there's no problem.

But I'm programming in C++ and here the name "class" is a keyword and cannot be used to denote variables. So, if I'm accessing the struct via

myvariable = mystruct->class;

I'm getting the error:

expected unqualified-id before ‘class’

Given that I cannot change the struct itself, how can I access this struct member despite the naming conflict?

Answer 1

Given that I cannot change the struct itself, how can I access this struct member despite the naming conflict?

Maybe you can rename it using a #define, something like

#define class xclass
#include "header.h"
#undef class

// ...

myvariable = mystruct->xclass;

Answer 2

class is a keyword in C++. You cannot use it as a variable.

If you want to still access it than you can code that part in C and then compile it with c compiler:

typedef struct foo {
    bar class;
} foo;

bar *getClassPtr(foo *p) { return &(p->class); }

Include that part in your C++ code using,

extern "C" {
   bar *getClassPtr(foo *);
}
bar &getClass(foo &s) { return *getClassPtr(&s); }

You might also want const versions.

You still can't include the struct definition in your C++ code, so you may have to wrap the other members of foo in the same way. Unless link-time optimization can inline getClassPtr, there's some overhead in the call, compared with accessing the struct member directly from C++. Normally this will be negligible, but it's worth knowing about.

You may want to find info about extern "C".

Answer 3

You say that you're using XLib. I can only find two places in my Xlib.h where class is used as a structure member: Visual and XWindowAttributes. In both cases, the offending member is wrapped like this:

#if defined(__cplusplus) || defined(c_plusplus)
    int c_class;
#else
    int class;
#endif

Similar hackery appears in XColormapEvent to take care of the new member.

So you should be fine unless your C++ compiler isn't defining any of the necessary macros; but that would also break the usual extern "C" { ... } wrappers as well so the problem is most likely elsewhere. If you're using a struct that isn't part of the standard XLib then you should apply the above hack by hand and have a stern discussion with the library's author (and if that's you then angrily talk to yourself for a bit and we'll pretend not to listen).

If you are having trouble with the XLib structs, then try using the C++ version of the member names:

myvariable = mystruct->c_class;
mynew      = ev->c_new;

Answer 4

Class is a reserved keyword in C++ and cannot be used as a variable name. You will have to rename the variable.