Using lambda or functor as match_condition with boost::asio::read_until()

28 Oct 2025

      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_iterator<streambuf::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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Klebsch, Mario

Lipódi László

Pavel Vazharov

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