How do I show the value of a #define at compile-time?

Jim_6160 0
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I am trying to figure out what version of Boost my code thinks it's using. I want to do something like this:

#error BOOST_VERSION

but the preprocessor does not expand BOOST_VERSION.

I know I could print it out at run-time from the program, and I know I could look at the output of the preprocessor to find the answer. I feel like having a way of doing this during compilation could be useful.

boost-preprocessor c-preprocessor macros
14 Answers

In Microsoft C/C++, you can use the built-in _CRT_STRINGIZE() to print constants. Many of my stdafx.h files contain some combination of these:

#pragma message("_MSC_VER      is " _CRT_STRINGIZE(_MSC_VER))
#pragma message("_MFC_VER      is " _CRT_STRINGIZE(_MFC_VER))
#pragma message("_ATL_VER      is " _CRT_STRINGIZE(_ATL_VER))
#pragma message("WINVER        is " _CRT_STRINGIZE(WINVER))
#pragma message("_WIN32_WINNT  is " _CRT_STRINGIZE(_WIN32_WINNT))
#pragma message("_WIN32_IE     is " _CRT_STRINGIZE(_WIN32_IE))
#pragma message("NTDDI_VERSION is " _CRT_STRINGIZE(NTDDI_VERSION))

and outputs something like this:

_MSC_VER      is 1915
_MFC_VER      is 0x0E00
_ATL_VER      is 0x0E00
WINVER        is 0x0600
_WIN32_WINNT  is 0x0600
_WIN32_IE     is 0x0700
NTDDI_VERSION is 0x06000000
user_1580240 
#define a <::BOOST_VERSION>
#include a
MSVC2015: fatal error C1083: Cannot open include file: '::106200': No such file or directory

Pros:

  • Works on builtin macroses
  • Works even if preprocess to file is enabled, even if invalid tokens are present:
#define a <::'*/`#>
#include a
MSVC2015: fatal error C1083: Cannot open include file: '::'*/`#': No such file or directory
GCC4.x: warning: missing terminating ' character [-Winvalid-pp-token]
#define a <::'*/`#>

Cons:

  • Sometime fails because of invalid characters in the include file path. Can be fixed by change a prefix (see update section below).

Update:

For GCC 4.7.x and lower the output throws the error:

error: #include expects "FILENAME" or <FILENAME>

To fix that you can change the prefix:

#define a <.__cplusplus>
#include a

fatal error: .201103L: No such file or directory
user_26721250

Instead of #error, try redefining the macro, just before it is being used. Compilation will fail and compiler will provide the current value it thinks applies to the macro.

#define BOOST_VERSION blah

user_16060020
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