将用户输入转换为C ++中的类型时，if / else语句过多 [英] Too many if/else statements in converting user inputs to types in C++

问题描述

我有一个带有3个模板参数的模板类。

I have a template class with 3 template arguments.

template <class T, class U, class Y>
class MyClass {};

我想通过CLI参数（例如）从用户那里获取输入。 float driver-x load

I wanna get input from users by CLI arguments, something like ./cli float driver-x load

• 第一个arg可以是 float 或 double


• 第二个arg是驱动程序名称： driver-x ， driver-y ，...


• 第三个参数与操作类型有关：加载，卸载，...

• 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, ...

如果要基于用户输入创建 MyClass 的新实例，则必须定义许多 if / else 语句。因为用户输入的是 string ，所以我必须为其准备条件。
因此，将是这样的：

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......

据我所知，无法存储

因此，有什么方法可以改善 if / else 语句？像这样的东西：

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.....;

还是其他方式？

I我正在寻找改善这些 if / else 语句的方法。

I'm looking for a way to improve these if/else statements.

推荐答案

您可以构建一些机制来为您执行此操作，将其提取到函数调用中。

You can build some machinery to do this for you, extracting it into a function call.

例如，在这里，我构建一个包含字符串和类型的元组，然后我对照所有这些字符串检查了传递的字符串：

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);
}

然后您可以在lambda内递归调用它。

And then you can call this recursively inside the lambda.

您还可以创建一个包含键元组和值元组的版本，以使用多个参数调用一个函数：

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>{});
}

看上去几乎相同，但通话水平更高。

Which looks almost the same, but there's one more level of calls.

它会像这样使用：

#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)
    );
}

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