The predef_check utility provides a facility for building a program that will check a given set of expressions against the definitions it detected when it was built.

predef_check programs

Even though there is only one predef_check program, there are variations for each of the languages that are detected by Predef to match the convention for sources files. For all of them one invokes with a list of expression arguments. The expressions are evaluated within the context of the particular predef_check program and if they all are true zero (0) is returned. Otherwise the index of the first false expression is returned.

The expression syntax is simple:

predef-definition [ relational-operator version-value ]

predef-definition can be any of the Predef definitions. For example BOOST_COMP_GCC.

relational-operator can be any of: >, <, >=, <=, == and !=.

version-number can be a full or partial version triplet value. If it's a partial version triple it is completed with zeros. That is x.y is equivalent to x.y.0 and x is equivalent to x.0.0.

The relations-operator and version-number can be ommited. In which case it is equivalent to:

predef-definition > 0.0.0

Using with Boost.Build

You can use the predef_check programs directly from Boost Build to configure target requirements. This is useful for controlling what gets built as part of your project based on the detailed version information available in Predef. The basic use is simple:

import path-to-predef-src/tools/check/predef
    : check require
    : predef-check predef-require ;

exe my_windows_program : windows_source.cpp
    : [ predef-require "BOOST_OS_WINDOWS" ] ;

That simple use case will skip building the my_windows_program unless one is building for Windows. Like the direct predef_check you can pass mutiple expressions using relational comparisons. For example:

import path-to-predef-src/tools/check/predef
    : check require
    : predef-check predef-require ;

lib my_special_lib : source.cpp
    : [ predef-require "BOOST_OS_WINDOWS != 0" "BOOST_OS_VMS != 0"] ;

And in that case the my_special_lib is built only when the OS is not Windows or VMS. The requires rule is a special case of the check rule. And is defined in terms of it:

rule require ( expressions + : language ? )
{
    return [ check $(expressions) : $(language) : : <build>no ] ;
}

The expression can also use explicit "and", "or" logical operators to for more complex checks:

import path-to-predef-src/tools/check/predef
    : check require
    : predef-check predef-require ;

lib my_special_lib : source.cpp
    : [ predef-require "BOOST_OS_WINDOWS" or "BOOST_OS_VMS"] ;

You can use the check rule for more control and to implement something other than control of what gets built. The definition for the check rule is:

rule check ( expressions + : language ? : true-properties * : false-properties * )

When invoked as a reuirement of a Boost Build target this rule will add the true-properties to the target if all the expressions evaluate to true. Otherwise the false-properties get added as requirements. For example you could use it to enable or disable features in your programs:

import path-to-predef-src/tools/check/predef
    : check require
    : predef-check predef-require ;

exe my_special_exe : source.cpp
    : [ predef-check "BOOST_OS_WINDOWS == 0"
        : : <define>ENABLE_WMF=0
        : <define>ENABLE_WMF=1 ] ;

For both check and require the language argument controls which variant of the predef_check program is used to check the expressions. It defaults to "c++", but can be any of: "c", "cpp", "objc", and "objcpp".