I have a template class with 3 template arguments.
template <class T, class U, class Y>
class MyClass {};
I wanna get input from users by CLI arguments, something like ./cli float driver-x load
- The first arg can be
float or double
- The second arg is a driver name:
driver-x, driver-y, ...
- The third argument is about the action type:
load, unload, ...
If I want to create a new instance of MyClass based on user inputs, I have to define many if/else statements. Because a user inputs are string and I have to prepare a condition on them.
So, it will be something like this:
if (data_type == "float")
if (driver == "driver-x")
if (action == "load")
MyClass<float, DriverX, Load> t;
t......
As far as I know, it's impossible to store a type in a variable in C++.
So, is there any way exists to improve the if/else statements? Something like:
if (data_type == "float")
//
if (driver == "driver-x")
//
if (action == "load")
//
MyClass<......> t;
t.....;
Or any other way?
I'm looking for a way to improve these if/else statements.
You can build some machinery to do this for you, extracting it into a function call.
For example, here I build a tuple which contains strings and types, then I check a passed string against all of them:
#include <string_view>
#include <cstddef>
#include <tuple>
#include <utility>
#include <type_traits>
template<class T>
struct mapped_type {
const std::string_view key;
using type = T;
explicit constexpr operator bool() const noexcept {
return true;
}
};
namespace detail {
template<class K, class F, class M, std::size_t I>
constexpr void lookup_impl(const K& key, F&& f, M&& m, std::integral_constant<std::size_t, I>) {
using tuple_t = typename std::remove_cv<typename std::remove_reference<M>::type>::type;
if constexpr (I < std::tuple_size<tuple_t>::value) {
const auto& mapping = std::get<I>(m);
if (mapping.key == key) {
std::forward<F>(f)(mapping);
return;
}
lookup_impl(key, std::forward<F>(f), std::forward<M>(m), std::integral_constant<std::size_t, I + 1>{});
} else {
std::forward<F>(f)(std::false_type{});
}
}
}
// Calls `f` with the first value from `m` that matches the key
// or `std::false_type{}` if no key matches.
template<class K, class F, class M>
constexpr void lookup(const K& key, F&& f, M&& m) {
detail::lookup_impl(key, std::forward<F>(f), std::forward<M>(m), std::integral_constant<std::size_t, 0>{});
}
// This is our mapping for the first argument
inline constexpr auto data_type_map = std::make_tuple(
mapped_type<float>{ "float" },
mapped_type<double>{ "double" }
);
// Example usage
#include <iostream>
int main() {
const char* s = "float";
lookup(s, [](const auto& arg) {
if constexpr (!arg) {
std::cout << "Invalid type\n";
} else {
using type = typename std::remove_cv<typename std::remove_reference<decltype(arg)>::type>::type::type;
std::cout << "Got type: " << typeid(type).name() << '\n';
}
}, data_type_map);
}
And then you can call this recursively inside the lambda.
You could also create a version that takes a tuple of keys and a tuple of values to call one function with many arguments:
#include <string_view>
#include <tuple>
#include <utility>
#include <type_traits>
template<class T>
struct mapped_type {
const std::string_view key;
using type = T;
explicit constexpr operator bool() const noexcept {
return true;
}
};
namespace detail {
template<class K, class F, class M, std::size_t I>
constexpr void lookup_impl(F&& f, const K& key, M&& m, std::integral_constant<std::size_t, I>) {
using tuple_t = typename std::remove_cv<typename std::remove_reference<M>::type>::type;
if constexpr (I < std::tuple_size<tuple_t>::value) {
const auto& mapping = std::get<I>(m);
if (mapping.key == key) {
std::forward<F>(f)(mapping);
return;
}
lookup_impl(std::forward<F>(f), key, std::forward<M>(m), std::integral_constant<std::size_t, I + 1>{});
} else {
std::forward<F>(f)(std::false_type{});
}
}
template<class F, class K, class M, std::size_t I>
constexpr void multilookup_impl(F&& f, const K& keys, M&& mappings, std::integral_constant<std::size_t, I>) {
constexpr std::size_t size = std::tuple_size<typename std::remove_cv<typename std::remove_reference<K>::type>::type>::value;
if constexpr (I >= size) {
std::forward<F>(f)();
} else {
lookup_impl([&](const auto& current_lookup) {
multilookup_impl(
[&](const auto&... args) { std::forward<F>(f)(current_lookup, args...); },
keys, mappings, std::integral_constant<std::size_t, I + 1>{}
);
}, std::get<I>(keys), std::get<I>(mappings), std::integral_constant<std::size_t, 0>{});
}
}
}
template<class F, class K, class M>
constexpr void lookup(F&& f, const K& keys, M&& mappings) {
using map_tuple_t = typename std::remove_cv<typename std::remove_reference<M>::type>::type;
using key_tuple_t = typename std::remove_cv<typename std::remove_reference<K>::type>::type;
constexpr std::size_t size = std::tuple_size<key_tuple_t>::value;
static_assert(size == std::tuple_size<map_tuple_t>::value, "Wrong number of keys for given number of maps");
detail::multilookup_impl(std::forward<F>(f), keys, mappings, std::integral_constant<std::size_t, 0>{});
}
Which looks almost the same, but there's one more level of calls.
It would be used like this:
#include <iostream>
inline constexpr auto data_type_map = std::make_tuple(
mapped_type<float>{ "float" },
mapped_type<double>{ "double" }
);
inline constexpr auto driver_type_map = std::make_tuple(
mapped_type<DriverX>{ "driver-x" },
mapped_type<DriverY>{ "driver-y" }
);
inline constexpr auto action_type_map = std::make_tuple(
mapped_type<Load>{ "load" },
mapped_type<Unload>{ "unload" }
);
int main() {
const char* a = "float";
const char* b = "driver-x";
const char* c = "load";
lookup([](const auto& data, const auto& driver, const auto& action) {
if constexpr (!data) {
std::cout << "Could not parse data!\n";
} else if constexpr (!driver) {
std::cout << "Could not parse driver!\n";
} else if constexpr (!action) {
std::cout << "Could not parse action!\n";
} else {
using data_type = typename std::remove_cv<typename std::remove_reference<decltype(data)>::type>::type::type;
using driver_type = typename std::remove_cv<typename std::remove_reference<decltype(driver)>::type>::type::type;
using action_type = typename std::remove_cv<typename std::remove_reference<decltype(action)>::type>::type::type;
MyClass<data_type, driver_type, action_type> t;
std::cout << "Constructed a " << typeid(decltype(t)).name() << '\n';
}
},
std::array<const char*, 3>{ a, b, c },
std::forward_as_tuple(data_type_map, driver_type_map, action_type_map)
);
}
Here's my take
template<typename T>
struct proxy { // or std::type_identity
using type = T;
};
template<typename... Ts>
using choice_of = std::variant<proxy<Ts>...>;
template<typename T, typename>
using type_const_t = T;
template<typename T, typename... Ts>
std::optional<choice_of<T, Ts...>> choose(std::string const &choice, std::string const &head, type_const_t<std::string const&, Ts>... tail) noexcept {
if(choice == head) return proxy<T>{};
else if constexpr(sizeof...(Ts) == 0) return std::nullopt;
else if(auto rec = choose<Ts...>(choice, tail...)) return std::visit(
[](auto rec) -> choice_of<T, Ts...> { return rec; },
*rec);
else return std::nullopt;
}
auto data_choice = choose<float, double>(data_type, "float", "double");
auto driver_choice = choose<DriverX, DriverY>(driver, "driver-x", "driver-y");
auto action_choice = choose<Load, Unload>(action, "load", "unload");
std::visit([](auto data_type_p, auto driver_p, auto action_p) {
auto t = MyClass<typename decltype(data_type_p)::type, typename decltype(driver_p)::type, typename decltype(action_p)::type>{};
// do stuff with t
}, data_choice.value(), driver_choice.value(), action_choice.value());
Complete example on Godbolt
I think you are looking for something like X-macros:
#define YOUR_TABLE \
X(float, DriverX, "driver-x", Load) \
X(int, DriverY, "driver-y", action2) \
X(int, DriverY, "driver-y", action3)
#define X(data_type, driver, driverName, action) if((0 == strcmp(#data_type,argv[1])) \
&& (0 == strcmp(driverName,argv[2])) && (0 == strcmp(#action,argv[3])))\
{ \
MyClass<data_type, driver, action> t; \
t.... \
}
YOUR_TABLE
#undef X
Prepare your puke-bag, here is a far-from-elegant solution but
simple enough to be easily adapted.
The main drawback I see is that all the remaining of the application
that needs to work with the created instance must stand in a
lambda-closure (this solution does not return this instance).
Every possible argument is considered only once in a
dedicated function (not X times Y times Z if/else).
/**
g++ -std=c++17 -o prog_cpp prog_cpp.cpp \
-pedantic -Wall -Wextra -Wconversion -Wno-sign-conversion \
-g -O0 -UNDEBUG -fsanitize=address,undefined
**/
#include <iostream>
#include <string>
#include <stdexcept>
//----------------------------------------------------------------------------
struct DriverX { auto show() const { return "DriverX"; } };
struct DriverY { auto show() const { return "DriverY"; } };
struct Load { auto show() const { return "Load"; } };
struct Unload { auto show() const { return "UnLoad"; } };
template<typename RealType,
typename DriverType,
typename ActionType>
struct MyClass
{
RealType real{};
DriverType driver{};
ActionType action{};
auto show() const
{
return std::to_string(sizeof(real))+" bytes real, "+
driver.show()+", "+action.show();
}
};
//----------------------------------------------------------------------------
template<typename RealType,
typename DriverType,
typename DoEverythingFunction>
void
with_MyClass_3(const std::string &action,
DoEverythingFunction fnct)
{
if(action=="load")
{
return fnct(MyClass<RealType, DriverType, Load>{});
}
if(action=="unload")
{
return fnct(MyClass<RealType, DriverType, Unload>{});
}
throw std::runtime_error{"unexpected action: "+action};
}
template<typename RealType,
typename DoEverythingFunction>
void
with_MyClass_2(const std::string &driver,
const std::string &action,
DoEverythingFunction fnct)
{
if(driver=="driver-x")
{
return with_MyClass_3<RealType, DriverX>(action, fnct);
}
if(driver=="driver-y")
{
return with_MyClass_3<RealType, DriverY>(action, fnct);
}
throw std::runtime_error{"unexpected driver: "+driver};
}
template<typename DoEverythingFunction>
void
with_MyClass(const std::string &real,
const std::string &driver,
const std::string &action,
DoEverythingFunction fnct)
{
if(real=="float")
{
return with_MyClass_2<float>(driver, action, fnct);
}
if(real=="double")
{
return with_MyClass_2<double>(driver, action, fnct);
}
throw std::runtime_error{"unexpected real: "+real};
}
//----------------------------------------------------------------------------
int
main(int argc,
char **argv)
{
std::cout << "~~~~ hardcoded types ~~~~\n";
const MyClass<float, DriverX, Load> mc1;
std::cout << "mc1: " << mc1.show() << '\n';
const MyClass<double, DriverY, Unload> mc2;
std::cout << "mc2: " << mc2.show() << '\n';
std::cout << "\n~~~~ many types ~~~~\n";
for(const auto &real: {"float", "double", "int"})
{
for(const auto &driver: {"driver-x", "driver-y", "driver-z"})
{
for(const auto &action: {"load", "unload", "sleep"})
{
try
{
with_MyClass(real, driver, action,
[&](const auto &mc)
{
std::cout << "working with: " << mc.show() << '\n';
});
}
catch(const std::exception &e)
{
std::cerr << "!!! " << e.what() << " !!!\n";
}
}
}
}
if(argc>3)
{
std::cout << "\n~~~~ from command line ~~~~\n";
try
{
with_MyClass(argv[1], argv[2], argv[3],
[&](const auto &mc)
{
std::cout << "working with: " << mc.show() << '\n';
});
}
catch(const std::exception &e)
{
std::cerr << "!!! " << e.what() << " !!!\n";
}
}
return 0;
}
