How to check (via the preprocessor) if a C source file is being compiled as C++ code

The question title should say it all, but here's an example of what sort of thing I'm looking for:

#ifndef THE_IDENTIFIER_THAT_WOULD_INDICATE_BEING_COMPILED_AS_CPLUSPLUS

/*
 * Example of something that would matter.
 */
typedef enum _bool bool;
enum _bool { false, true };

#endif

What is the identifier? It's bugging me severely, as I know I've seen code that does this before.

I'm using GCC, by the way.

(I'm surprised I couldn't find a duplicate somewhere on SO. If someone else can find one, fe开发者_运维技巧el free to redirect me and close this as a dupe.)

#ifndef __cplusplus

If I remember correctly.

The identifier is __cplusplus

#ifdef __cplusplus
#error NO C++ PLEASE
#endif

#ifdef __cplusplus

with a few really ancient compilers (early versions of cfront and a couple of ports) it was c_pluplus, IIRC.

#ifdef __cplusplus

#ifdef __cplusplus

I think the file extension matters too, if the C++ compiler is given a .c file it will compile it as C code. i have nothing to back this up though.

The identifier you are looking for is __cplusplus, which can be used like this:

#ifdef __cplusplus
// Code being compiled as C++.
#endif