id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
195	Add support for flags with a composite value.	jurko.gospodnetic@…	somebody	"Add support for flags constructing their value from a some fixed string part as well as one or more feature values.

Currently it is possible to define a flag that uses a fixed predefined string as its value:
{{{
toolset.flags compile.action MY_FLAG <some-feature> : --my-fixed-compiler-option ;
}}}

Or it is possible to define a flag that uses a feature's value as its own using either of the following:
{{{
toolset.flags compile.action MY_FLAG <some-feature> ;
toolset.flags compile.action MY_FLAG <condition>xxx : <feature-whose-value-we-want> ;
}}}

But it is not possible to define a flag that would take a property like <template-depth>128 and construct '-ftemplate-depth-128' as its value.

As a workaround one can make the flag take only the property's value as its own and then add whatever text to it is needed to construct a proper command-line option in the action using the tool's value.

As I see it this support would not bring all that much to Boost Build except possibly some interface completeness. The only thing I can think off is that one could prepare a flag suitable for multiple actions and then not duplicate the code for constructing the actual command-line option in each of those actions. For an example see gcc.compile.c++ & gcc.compile.c++.pch rules in Boost Build's tools/gcc.jam toolset module.

I believe the key to implementing this feature is defining the syntax used to define such flags and then modifying the toolset.handle-flag-value rule accordingly.

If & when this support is added it can be used to merge existing flags like TEMPLATE_DEPTH into more generic ones like OPTIONS where they do seem to fit in nicely and have not already been merged with them only due to this missing support."	enhancement	new	minor	M13	component1	2.0		flag composite feature property value