I'm trying to create two overloads of the function that takes a handler as the argument:
template <typename Handler>
void foo (Handler h);
The first overload should be called if handler takes boost::asio::yield_context as its parameter,
template <class Handler>
void foo (Handler h,
enable_if_t<is_same<result_of_t<Handler (yield_context)>, void>::value>* = 0);
and second one should be called if handler takes boost::function as its parameter.
using func_type = boost::function<void(int,int)>;
template <typename Handler>
void foo (Handler h,
enable_if_t<is_same<result_of_t<Handler (func_type)>, void>::value>* = 0);
Unfortunately that does not work:
main.cpp:22:3: error: call to 'foo' is ambiguous
foo ([] (func_type f) {});
^~~
main.cpp:11:6: note: candidate function [with Handler = (lambda at main.cpp:22:8)]
void foo (Handler h,
^
main.cpp:16:6: note: candidate function [with Handler = (lambda at main.cpp:22:8)]
void foo (Handler h,
^
1 error generated.
Interesting, but the code works fine with std::function:
using func_type = boost::function<void (int,int)>;
As far as I understand that's because boost::function has excessive overloads for all possible calling operators, and this confuses result_of checks.
Anyway, is it possible to create the "foo" overloads that could "distinguish" between handlers taking yield_context and handlers taking boost::function as its parameters?
Coliru code: http://coliru.stacked-crooked.com/a/18344cd1b8364466
The standard library seems to have adopted a "moniker" argument approach:
unique_lock<mutex> lk(std::defer_lock, mx);
or
std::pair<Foo, Foo> p1(t, t);
std::pair<Foo, Foo> p2(std::piecewise_construct, t, t);
or
std::async(std::launch::deferred, foo, argument1);
std::async(std::launch::async, foo, argument1);
etc.
The pattern is that
struct defer_lock_t final { }; // for exposition
constexpr defer_lock_t defer_lock;
These "monikers" have "unique" detectable types that are guaranteed not confusable/convertible etc. with your actual argument types.