On Tue, Oct 28, 2025 at 4:41 PM Lipódi László via Boost-users <boost-users@lists.boost.org> wrote:
Hi!
According to https://www.boost.org/doc/libs/latest/doc/html/boost_asio/reference/read_unt...
"The default implementation of the is_match_condition type trait evaluates to true for function pointers and function objects with a result_type typedef. It must be specialised for other user-defined function objects."
On Tuesday, October 28th, 2025 at 15:31, Klebsch, Mario via Boost-users <boost-users@lists.boost.org> wrote:
Hello,
I want to use boost::asio::read_until() and pass a functor as match condition. But I don't get it to compile.
Here is my source code:
#include <boost/asio.hpp>
using namespace boost::asio; using iterator = buffers_iteratorstreambuf::const_buffers_type;
struct completion_functor { std::pair<iterator, bool> operator()(iterator begin, iterator end) {
return {}; } }; completion_functor functor; static_assert(is_match_condition<completion_functor>::value, "completion_functor must be match condition");
static_assert(is_match_condition<decltype(functor)>::value, "functor must be match condition");
std::pair<iterator, bool> completion_func(iterator begin, iterator end) {
return functor(begin, end); } static_assert(is_match_condition<decltype(completion_func)>::value, "completion_func must be match condition");
auto lambda = [](iterator begin, iterator end) -> std::pair<iterator, bool> {
return {}; }; static_assert(is_match_condition<decltype(lambda)>::value, "lambda must be match condition");
std::pair<iterator, bool> completion_func2(iterator begin, iterator end) {
return lambda(begin, end); } static_assert(is_match_condition<decltype(completion_func2)>::value, "completion_func2 must be match condition");
int main(int argc, char *argv[]) { io_context ctx; ip::tcp::socket s{ctx}; streambuf buffer;
read_until(s, buffer, '\n'); read_until(s, buffer, completion_func); read_until(s, buffer, completion_func2); // the following calls do not compile // read_until(s, buffer, functor); // read_until(s, buffer, lambda); }
It seems, that boost::asio::is_match_condition<> does neither accept my lammda-function nor my functor. The only completion handler, that I get accepted, is a classic, free-standing function (and static class member functions, too).
But in that function, calling the lambda or the functor does not seem to be a problem for the compiler.
The documentation of read_until describes match_condition as "The function object to be called to determine whether a match exists."
I would expect lambda functions and functors to be accepted as a function object.
Do I have an error in the declaration of lambda or completion_functor::operator()?
Does anybody now, how to get this working?
I tried boost_1_73_0 and boost_1_88_0 with g++ (Gentoo 14.3.1_p20250801 p4) 14.3.1 20250801 and clang version 20.1.8
Thanks in advance,
Klebsch Mario Funktion | R&D
Tel: +49 (0) 531 38 701 718 Raum: Braunschweig, E20
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Hi there, According to the example in the docs here - https://www.boost.org/doc/libs/latest/doc/html/boost_asio/reference/read_unt... You need to specialize the class template - boost::asio::is_match_condition for your functor types. Adding this to your example code makes it work - https://godbolt.org/z/7sTsv5hKK ``` namespace boost::asio { template <> struct is_match_condition<completion_functor> : public std::true_type {}; template <> struct is_match_condition<decltype(lambda)> : public std::true_type {}; } ``` Pavel.