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

Question

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.

Answer 1

You could write a program that prints out BOOST_VERSION and compile and run it as part of your build system. Otherwise, I think you're out of luck.

Answer 2

As far as I know '#error' only will print strings, in fact you don't even need to use quotes.

Have you tried writing various purposefully incorrect code using "BOOST_VERSION"? Perhaps something like "blah[BOOST_VERSION] = foo;" will tell you something like "string literal 1.2.1 cannot be used as an array address". It won't be a pretty error message, but at least it'll show you the relevant value. You can play around until you find a compile error that does tell you the value.

Answer 3

Are you looking for

#if BOOST_VERSION != "1.2"
#error "Bad version"
#endif

Not great if BOOST_VERSION is a string, like I've assumed, but there may also be individual integers defined for the major, minor and revision numbers.

Answer 4

You could also preprocess the source file and see what the preprocessor value evaluates to.

Answer 5

Looking at the output of the preprocessor is the closest thing to the answer you ask for.

I know you've excluded that (and other ways), but I'm not sure why. You have a specific enough problem to solve, but you have not explained why any of the "normal" methods don't work well for you.

Answer 6

BOOST_VERSION is defined in the boost header file version.hpp.

Answer 7

If you are using Visual C++, you can use #pragma message:

#include <boost/preprocessor/stringize.hpp>
#pragma message("BOOST_VERSION=" BOOST_PP_STRINGIZE(BOOST_VERSION))

Edit: Thanks to LB for link

Apparently, the GCC equivalent is (not tested):

#pragma message "BOOST_VERSION=" BOOST_PP_STRINGIZE(BOOST_VERSION)

Answer 8

Take a look at the Boost documentation as well, regarding how you are using the macro:

In reference to BOOST_VERSION, from http://www.boost.org/doc/libs/1_37_0/libs/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.boost_helper_macros:

Describes the boost version number in XXYYZZ format such that: (BOOST_VERSION % 100) is the sub-minor version, ((BOOST_VERSION / 100) % 1000) is the minor version, and (BOOST_VERSION / 100000) is the major version.

Answer 9

And I was thinking that I know C very well...