On Sat, Apr 14, 2012 at 5:00 PM, Kevin Wu Won <exclipy@gmail.com> wrote:
[...] I came up with a solution where the handler functions can be specified inline with C++11 lambdas. Here's an example:
boost::variant< int, std::string > u("hello world"); int result = match(u, [](int i) -> int { return i; }, [](const std::string & str) -> int { return str.length(); });
The `match` function accepts the variant followed by the handler functions. The functions can be specified in any order. It will fail to compile if the functions do not match the types of the variant, if the return types are not all the same, or if a non-unary function is supplied.
This is C++11 only because it's quite pointless without lambda functions. I've tested it with gcc 4.7. It doesn't work on gcc 4.6, which can't handle the variadic templates.
It should be possible to implement the same feature on gcc 4.6. Take a look at https://github.com/gpderetta/Experiments/blob/scheduler/match.hpp (test: https://github.com/gpderetta/Experiments/blob/scheduler/match_test.cc) which, given a set of monomorphic function objects create a polymorphic function object (of arbitrary arity). It can be used with boost variant like this: boost::variant< int, std::string > u("hello world"); int result = boost::apply_visitor(u, match( [](int i) { return i; }, [](const std::string & str) { return str.length(); })); The 'match' function (interestingly we used the same name :) ) works fine on gcc 4.6; the implementation is surprisingly simple and it is completely indipendeng from boost.variant. If you are interested feel free to use the code and/or incorporate it in your library (it is under the boost license). HTH, -- gpd