Stefan Strasser wrote:
michael toksvig schrieb:
BOOST_FOREACH(pair<Key, Value> &p, some_map)
does not work, since the preprocessor just can't get over that first comma
this could be worked around with C99 variadic macros if available, something like:
#define _BOOST_FOREACH2(a1,a2,...) BOOST_FOREACH2(a1,a2) #define _BOOST_FOREACH3(a1,a2,a3,...) BOOST_FOREACH3(a1,a2,a3) // ...
#define _BOOST_FOREACH(a1,a2,a3,a4,a5,a6,a7,a8,a9,nr,...) _BOOST_FOREACH##nr(a1,a2,a3,a4,a5,a6,a7,a8,a9)
#define BOOST_FOREACH(...) _BOOST_FOREACH(__VA_ARGS__,9,8,7,6,5,4,3,2,1)
Clever, but there is an ambiguity here. What is the relations of the arguments in BOOST_FOREACH2(a1,a2,a3)? Do a1 and a2 together make up the loop variable declaration, or do a2 and a3 together form the collection expression: case 1: BOOST_FOREACH( std::pair<int,int> p, some_map ) case 2: BOOST_FOREACH( int i, some_trait<this,that>::some_list() ) I suppose you could say that case 2 can always be handled with a set of parens, as in: BOOST_FOREACH( int i, (some_trait<this,that>::some_list()) ) Therefore, all macro args but the last should be interpreted as part of the loop variable declaration. Anyway, I haven't heard of a major C++ compiler supporting C99 varargs. Is there one? I don't think we have a config macro for detecting it. Michael Toksvig's original point is a good one though. There should be a pitfalls section in the doc that warns people of this. I'll put it on my list of ToDo's. -- Eric Niebler Boost Consulting www.boost-consulting.com