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🎨 Allow call_by_need to customize the call #410

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Merged
elbeno merged 1 commit into from
Aug 14, 2026
Merged

🎨 Allow call_by_need to customize the call #410

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66 changes: 42 additions & 24 deletions include/stdx/call_by_need.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -46,23 +46,31 @@ struct void_t {};
template <typename T>
using is_nonvoid_t = std::bool_constant<not std::is_same_v<T, void_t>>;

template <std::size_t Base, std::size_t Len, typename F, typename Args>
template <typename CallPolicy, typename F, typename... Args>
using invoke_result_t =
decltype(CallPolicy::invoke(std::declval<F>(), std::declval<Args>()...));

template <typename CallPolicy, std::size_t Base, std::size_t Len, typename F,
typename Args>
constexpr auto invoke(F &&f, Args &&args) -> decltype(auto) {
return [&]<std::size_t... Is>(
std::index_sequence<Is...>) -> decltype(auto) {
using R = std::invoke_result_t<F, decltype(get<Base + Is>(
std::forward<Args>(args)))...>;
if constexpr (std::is_void_v<R>) {
std::forward<F>(f)(get<Base + Is>(std::forward<Args>(args))...);
return void_t{};
} else {
return std::forward<F>(f)(
get<Base + Is>(std::forward<Args>(args))...);
}
}(std::make_index_sequence<Len>{});
return
[&]<std::size_t... Is>(std::index_sequence<Is...>) -> decltype(auto) {
using R = invoke_result_t<CallPolicy, F,
decltype(get<Base + Is>(
std::forward<Args>(args)))...>;
if constexpr (std::is_void_v<R>) {
CallPolicy::invoke(std::forward<F>(f),
get<Base + Is>(std::forward<Args>(args))...);
return void_t{};
} else {
return CallPolicy::invoke(
std::forward<F>(f),
get<Base + Is>(std::forward<Args>(args))...);
}
}(std::make_index_sequence<Len>{});
}

template <typename... Fs> struct by_need {
template <typename CallPolicy, typename... Fs> struct by_need {
template <typename... Args>
[[nodiscard]] consteval static auto compute_call_info_impl() {
auto results = std::array<call_info, sizeof...(Fs) + sizeof...(Args)>{};
Expand All @@ -72,8 +80,8 @@ template <typename... Fs> struct by_need {
[&]<std::size_t N, std::size_t Base, std::size_t Len>() -> bool {
return [&]<std::size_t... Is>(std::index_sequence<Is...>) -> bool {
if constexpr (requires {
typename std::invoke_result_t<
nth_t<N, Fs...>,
typename invoke_result_t<
CallPolicy, nth_t<N, Fs...>,
nth_t<Base + Is, Args...>...>;
}) {
results[result_count++] = {N, Base, Len};
Expand Down Expand Up @@ -145,15 +153,24 @@ template <typename... Fs> struct by_need {
struct safe_forward {
template <typename T> constexpr auto operator()(T &&t) -> T { return t; }
};

struct default_call_policy_t {
template <typename F, typename... Args>
constexpr static auto invoke(F &&f, Args &&...args)
-> std::invoke_result_t<F, Args...> {
return std::forward<F>(f)(std::forward<Args>(args)...);
}
};
} // namespace cbn_detail

template <tuplelike Fs, tuplelike Args>
template <typename CallPolicy = cbn_detail::default_call_policy_t, tuplelike Fs,
tuplelike Args>
constexpr auto call_by_need(Fs &&fs, Args &&args) {
constexpr auto calls =
[&]<std::size_t... Is, std::size_t... Js>(std::index_sequence<Is...>,
std::index_sequence<Js...>) {
return cbn_detail::by_need<decltype(get<Is>(
std::forward<Fs>(fs)))...>::
return cbn_detail::by_need<
CallPolicy, decltype(get<Is>(std::forward<Fs>(fs)))...>::
template compute_call_info<decltype(get<Js>(
std::forward<Args>(args)))...>();
}(std::make_index_sequence<tuple_size_v<std::remove_cvref_t<Fs>>>{},
Expand All @@ -165,11 +182,12 @@ constexpr auto call_by_need(Fs &&fs, Args &&args) {
}(std::make_index_sequence<tuple_size_v<Fs>>{});

auto ret = [&]<std::size_t... Is>(std::index_sequence<Is...>) {
return tuple<
decltype(cbn_detail::invoke<calls[Is].arg_base, calls[Is].arg_len>(
get<calls[Is].fn_idx>(std::move(new_fs)),
std::forward<Args>(args)))...>{
cbn_detail::invoke<calls[Is].arg_base, calls[Is].arg_len>(
return tuple<decltype(cbn_detail::invoke<CallPolicy, calls[Is].arg_base,
calls[Is].arg_len>(
get<calls[Is].fn_idx>(std::move(new_fs)),
std::forward<Args>(args)))...>{
cbn_detail::invoke<CallPolicy, calls[Is].arg_base,
calls[Is].arg_len>(
get<calls[Is].fn_idx>(std::move(new_fs)),
std::forward<Args>(args))...};
}(std::make_index_sequence<calls.size()>{});
Expand Down
23 changes: 23 additions & 0 deletions test/call_by_need.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -214,3 +214,26 @@ TEST_CASE("function returning reference", "[call_by_need]") {
STATIC_REQUIRE(std::is_same_v<decltype(r), stdx::tuple<int &> const>);
CHECK(get<0>(r) == 17);
}

namespace {
template <typename T> struct custom_call_policy_t {
template <typename F, typename... Args>
constexpr static auto invoke(F &&f, Args &&...args)
-> decltype(std::forward<F>(f).template operator()<T>(
std::forward<Args>(args)...)) {
return std::forward<F>(f).template operator()<T>(
std::forward<Args>(args)...);
}
};
} // namespace

TEST_CASE("custom call policy", "[call_by_need]") {
auto r = stdx::call_by_need<custom_call_policy_t<int>>(
stdx::tuple{[&]<typename T>(arg_t<0>) {
STATIC_CHECK(std::same_as<T, int>);
return 17;
}},
stdx::tuple{arg<0>});
STATIC_REQUIRE(std::is_same_v<decltype(r), stdx::tuple<int>>);
CHECK(get<0>(r) == 17);
}