Choose function to apply based on the validity of an expression

Question

The problem is the following, in C++14:

• Let\'s have two functions FV&& valid_f, FI&& invalid_f, and argu

Answer 1

First, a homebrew version of C++2a's is_detected:

#include 
#include 
#include 
#include 

namespace details {
  templateusing void_t=void;
  templateclass Z, class=void, class...Ts>
  struct can_apply:std::false_type{};
  templateclass Z, class...Ts>
  struct can_apply>, Ts...>:std::true_type{};
}
templateclass Z, class...Ts>
using can_apply = typename details::can_apply::type;

As it happens, std::result_of_t is the trait we want to test.

template
using can_call = can_apply< std::result_of_t, Sig >;

now can_call< Some(Sig,Goes,Here) > is true_type iff the expression you want can be called.

Now we write some compile-time if dispatch machinery.

template
using index_t=std::integral_constant;
template
constexpr index_t index_v{};

constexpr inline index_t<0> dispatch_index() { return {}; }
template =0
>
constexpr index_t<0> dispatch_index( B0, Bs... ) { return {}; }
template =0
>
constexpr auto dispatch_index( B0, Bs... ) { 
  return index_v< 1 + dispatch_index( Bs{}...) >;
}

template
auto dispatch( Bs... ) {
  using I = decltype(dispatch_index( Bs{}... ));
  return [](auto&&...args){
    return std::get( std::make_tuple(decltype(args)(args)..., [](auto&&...){}) );
  };
}

dispatch( SomeBools... ) returns a lambda. The first of the SomeBools which is compile-time truthy (has a ::value that evaluates to true in a boolean context) determines what the returned lambda does. Call that the dispatch index.

It returns the dispatch_index'd argument to the next call, and an empty lambda if that is one-past-the-end of the list.

template 
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
  dispatch(
    can_call{},
    can_call{}
  )(
    [&](auto&& valid_f, auto&&)->decltype(auto) {
      return decltype(valid_f)(valid_f)(std::forward(args)...);
    },
    [&](auto&&, auto&& invalid_f)->decltype(auto) {
      return decltype(invalid_f)(valid_f)(std::forward(args)...);
    }
  )(
    valid_f, invalid_f
  );
}

and done, live example.

We could make this generic to enable nary version. First index_over:

template
auto index_over( std::index_sequence ){
  return [](auto&&f)->decltype(auto){
    return decltype(f)(f)( std::integral_constant{}... );
  };
}
template
auto index_over(std::integral_constant ={}){
  return index_over(std::make_index_sequence{} );
}

Then auto_dispatch:

template
auto auto_dispatch( Fs&&... fs ) {
  auto indexer =  index_over();
  auto helper = [&](auto I)->decltype(auto){ 
    return std::get( std::forward_as_tuple( decltype(fs)(fs)... ) );
  };
  return indexer
  (
    [helper](auto...Is){
      auto fs_tuple = std::forward_as_tuple( helper(Is)... );
      return [fs_tuple](auto&&...args) {
        auto dispatcher = dispatch(can_call{}...);
        auto&& f0 = dispatcher(std::get(fs_tuple)...);
        std::forward(f0)(decltype(args)(args)...);
      };
    }
  );
}

with test code:

auto a = [](int x){ std::cout << x << "\n"; };
auto b = [](std::string y){ std::cout << y << "\n";  };
struct Foo {};
auto c = [](Foo){ std::cout << "Foo\n";  };
int main() {
  auto_dispatch(a, c)( 7 );
  auto_dispatch(a, c)( Foo{} );
  auto_dispatch(a, b, c)( Foo{} );
  auto_dispatch(a, b, c)( "hello world" );
}

Live example