The problem is the following, in C++14:
- Let's have two functions
FV&& valid_f, FI&& invalid_f, and arguments Args&&... args
- The function
apply_on_validity should apply valid_f on args if the expression std::forward<FV>(valid_f)(std::forward<Args>(args)...) is valid
- Otherwise and if
std::forward<FV>(invalid_f)(std::forward<Args>(args)...) is a valid expression, apply_on_validity should apply invalid_f on args
- Otherwise
apply_on_validity should do nothing
I guess the code should look like something like this:
template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
// Apply valid_f by default
std::forward<FV>(valid_f)(std::forward<Args>(args)...);
}
template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
// Apply invalid_f if valid_f does not work
std::forward<FV>(invalid_f)(std::forward<Args>(args)...);
}
template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
// Do nothing when neither valid_f nor invalid_f work
}
But I don't really know how to do that. Any idea?
Link to the generalization here.
Take:
template <int N> struct rank : rank<N-1> {};
template <> struct rank<0> {};
and then:
template <class FV, class FI, class... Args>
auto apply_on_validity_impl(rank<2>, FV&& valid_f, FI&& invalid_f, Args&&... args)
-> decltype(std::forward<FV>(valid_f)(std::forward<Args>(args)...), void())
{
std::forward<FV>(valid_f)(std::forward<Args>(args)...);
}
template <class FV, class FI, class... Args>
auto apply_on_validity_impl(rank<1>, FV&& valid_f, FI&& invalid_f, Args&&... args)
-> decltype(std::forward<FI>(invalid_f)(std::forward<Args>(args)...), void())
{
std::forward<FI>(invalid_f)(std::forward<Args>(args)...);
}
template <class FV, class FI, class... Args>
void apply_on_validity_impl(rank<0>, FV&& valid_f, FI&& invalid_f, Args&&... args)
{
}
template <class FV, class FI, class... Args>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
return apply_on_validity_impl(rank<2>{}, std::forward<FV>(valid_f), std::forward<FI>(invalid_f), std::forward<Args>(args)...);
}
DEMO
Piotr Skotnicki's answer is superb, but code like that makes me feel compelled to point out how much cleaner C++17 will be thanks to constexpr if and additional type traits like is_callable: Demo Demo*This version creates more warnings but is simpler
template <class FV, class FI, class... Args>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
if constexpr (std::is_callable_v<FV(Args...)>)
std::cout << "Apply valid_f by default\n";
else
{
if constexpr (std::is_callable_v<FI(Args...)>)
std::cout << "Apply invalid_f if valid_f does not work\n";
else
std::cout << "Do nothing when neither valid_f nor invalid_f work\n";
}
}
Here's an alternative answer, just for kicks. We need a static_if:
template <class T, class F> T&& static_if(std::true_type, T&& t, F&& ) { return std::forward<T>(t); }
template <class T, class F> F&& static_if(std::false_type, T&& , F&& f) { return std::forward<F>(f); }
And an is_callable. Since you're just supporting functions, we can do it as:
template <class Sig, class = void>
struct is_callable : std::false_type { };
template <class F, class... Args>
struct is_callable<F(Args...), void_t<decltype(std::declval<F>()(std::declval<Args>()...))>>
: std::true_type
{ };
And then we can construct the logic in place:
template <class FV, class FI, class... Args>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
auto noop = [](auto&&...) {};
static_if(
is_callable<FV&&(Args&&...)>{},
std::forward<FV>(valid_f),
static_if(
std::is_callable<FI&&(Args&&...)>{},
std::forward<FI>(invalid_f),
noop
)
)(std::forward<Args>(args)...);
}
First, a homebrew version of C++2a's is_detected:
#include <utility>
#include <type_traits>
#include <iostream>
#include <tuple>
namespace details {
template<class...>using void_t=void;
template<template<class...>class Z, class=void, class...Ts>
struct can_apply:std::false_type{};
template<template<class...>class Z, class...Ts>
struct can_apply<Z, void_t<Z<Ts...>>, Ts...>:std::true_type{};
}
template<template<class...>class Z, class...Ts>
using can_apply = typename details::can_apply<Z, void, Ts...>::type;
As it happens, std::result_of_t is the trait we want to test.
template<class Sig>
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<std::size_t I>
using index_t=std::integral_constant<std::size_t, I>;
template<std::size_t I>
constexpr index_t<I> index_v{};
constexpr inline index_t<0> dispatch_index() { return {}; }
template<class B0, class...Bs,
std::enable_if_t<B0::value, int> =0
>
constexpr index_t<0> dispatch_index( B0, Bs... ) { return {}; }
template<class B0, class...Bs,
std::enable_if_t<!B0::value, int> =0
>
constexpr auto dispatch_index( B0, Bs... ) {
return index_v< 1 + dispatch_index( Bs{}...) >;
}
template<class...Bs>
auto dispatch( Bs... ) {
using I = decltype(dispatch_index( Bs{}... ));
return [](auto&&...args){
return std::get<I::value>( 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 <class FV, class FI, class... Args /*, Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
dispatch(
can_call<FV(Args...)>{},
can_call<FI(Args...)>{}
)(
[&](auto&& valid_f, auto&&)->decltype(auto) {
return decltype(valid_f)(valid_f)(std::forward<Args>(args)...);
},
[&](auto&&, auto&& invalid_f)->decltype(auto) {
return decltype(invalid_f)(valid_f)(std::forward<Args>(args)...);
}
)(
valid_f, invalid_f
);
}
and done, live example.
We could make this generic to enable nary version. First index_over:
template<class=void, std::size_t...Is >
auto index_over( std::index_sequence<Is...> ){
return [](auto&&f)->decltype(auto){
return decltype(f)(f)( std::integral_constant<std::size_t, Is>{}... );
};
}
template<std::size_t N>
auto index_over(std::integral_constant<std::size_t, N> ={}){
return index_over(std::make_index_sequence<N>{} );
}
Then auto_dispatch:
template<class...Fs>
auto auto_dispatch( Fs&&... fs ) {
auto indexer = index_over<sizeof...(fs)>();
auto helper = [&](auto I)->decltype(auto){
return std::get<decltype(I)::value>( 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<Fs(decltype(args)...)>{}...);
auto&& f0 = dispatcher(std::get<decltype(Is)::value>(fs_tuple)...);
std::forward<decltype(f0)>(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
