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

Question

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, feel free to redirect me and close this as a dupe.)

It's __cplusplus. The macro is defined by C++ compilers. Duplicated from: stackoverflow.com/questions/3858308/… — wkl
– wkl, Commented Nov 10, 2010 at 21:27
btw: C99 added a native boolean type called _Bool, aliased to bool if you include <stdbool.h> — Christoph
– Christoph, Commented Nov 10, 2010 at 21:49
See also Preprocessor directive to test if this is C or C++ to learn about extern "C" and some ways to handle. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 23, 2012 at 6:43
I find it amusing that 6 answers and a comment that states the answer all hit enter at the same time: Nov 10 '10 21:27 — Adrian
– Adrian, Commented Feb 1, 2019 at 21:30

frast · Accepted Answer · 2010-11-10 21:27:11Z

18

#ifndef __cplusplus

If I remember correctly.

answered Nov 10, 2010 at 21:27

frast

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2 Comments

Anya Shenanigans Over a year ago

it's typically used in the header file guards #ifdef __cplusplus\n extern "C" { \n#endif

user325117 Over a year ago

+1 You remember correctly. Should be #ifndef for what William wants.

pmg · Accepted Answer · 2024-12-17 11:48:24Z

8

The identifier is __cplusplus

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

C23 6.10.10.1p3

The implementation shall not predefine the macro __cplusplus, nor shall it define it in any standard header."

C++11 16.8.1

The following macro names shall be defined by the implementation:

__cplusplus : The name __cplusplus is defined to the value 201103L when compiling a C++ translation unit.

edited Dec 17, 2024 at 11:48

answered Nov 10, 2010 at 21:28

pmg

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Comments

Jerry Coffin · Accepted Answer · 2010-11-10 21:27:32Z

3

#ifdef __cplusplus

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

answered Nov 10, 2010 at 21:27

Jerry Coffin

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nmichaels · Accepted Answer · 2010-11-10 21:27:35Z

2

#ifdef __cplusplus

answered Nov 10, 2010 at 21:27

nmichaels

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James · Accepted Answer · 2010-11-10 21:28:09Z

2

#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.

answered Nov 10, 2010 at 21:28

James

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Richard J. Ross III · Accepted Answer · 2011-12-23 14:46:28Z

1

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

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

edited Dec 23, 2011 at 14:46

Richard J. Ross III

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answered Nov 10, 2010 at 21:27

alecov

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