如何分支/切换多个条件？

Question

有没有办法在不编写看似乱七八糟的代码的情况下分支多个条件？可以理解C++ 11或C++ 14中的语法糖.

#include <iostream>

enum state
{
    STATE_1,
    STATE_2,
    STATE_3,
    STATE_4,
    STATE_5,
    STATE_6,
    STATE_7,
    STATE_8,
};

state f(int a, bool b, const std::string& str)
{
    // How not to:
    if (a < 0)
    {
        if (b == false)
        {
            if (str != "morning")
            {
                return STATE_1;
            }
            else
            {
                return STATE_2;
            }
        }
        else
        {
            if (str != "morning")
            {
                return STATE_3;
            }
            else
            {
                return STATE_4;
            }
        }
    }
    else // a >= 0
    {
        if (b == false)
        {
            if (str != "morning")
            {
                return STATE_5;
            }
            else
            {
                return STATE_6;
            }
        }
        else
        {
            if (str != "morning")
            {
                return STATE_7;
            }
            else
            {
                return STATE_8;
            }
        }
    }
}

int main()
{
    std::cout << "State: " << f(1, true, "morning") << std::endl;
}

Answer 1

可以在编译时在POD中嵌入一个布尔(条件结果)列表switch.

用法: main.cpp

#include <iostream> /* std::cout */
#include "mswitch.h" /* mswitch, mcase */

enum state
{
    STATE_1,
    STATE_2,
    STATE_3,
    STATE_4,
    STATE_5,
    STATE_6,
    STATE_7,
    STATE_8,
};

state f(int a, bool b, const std::string& str)
{
    mswitch(a >= 0, b == true, str == "morning")
    {
        mcase(false, false, false): return STATE_1;
        mcase(false, false, true) : return STATE_2;
        mcase(false, true, false) : return STATE_3;
        mcase(false, true, true)  : return STATE_4;
        mcase(true, false, false) : return STATE_5;
        mcase(true, false, true)  : return STATE_6;
        mcase(true, true, false)  : return STATE_7;
        mcase(true, true, true)   : return STATE_8;
    }
    return STATE_1;
}

int main()
{
    std::cout << "State: " << f(1, true, "morning") << std::endl;
}

语法糖: mswitch.h

#ifndef MSWITCH_GUARD_H
#define MSWITCH_GUARD_H

#include <initializer_list>
#include <cstddef>

namespace mswitch
{
    constexpr long long encode(long long value, size_t size) { return value << 6 | (0x3F & size); }

    class mswitch
    {
        std::initializer_list<bool> _flags;
    public:
        mswitch(std::initializer_list<bool> const& l) : _flags(l) {}
        operator long long() const
        {
            long long result = 0;
            size_t index = 0;
            for (bool b : _flags) {
                result |= b << index++;
            }
            return encode(result, _flags.size());
        }
    };

    template<bool head, bool... tail>
    struct mcase
    {
        constexpr mcase() = default;
        constexpr operator long long() const
        {
            return encode(tll(), 1+sizeof...(tail));
        }
        constexpr long long tll() const { return head | mcase<tail...>().tll() << 1; }
    };

    template<bool b>
    struct mcase<b>
    {
        constexpr mcase() = default;
        constexpr operator long long() const { return encode(tll(), 1); }
        constexpr long long tll() const { return b; }
    };
}

#define mswitch(head, ...) switch(mswitch::mswitch{head, __VA_ARGS__})
#define mcase(head, ...) case mswitch::mcase<head, __VA_ARGS__>()

#endif // MSWITCH_GUARD_H

编译 g++ -std=c++14 -O2 -Wall -pedantic main.cpp

这个怎么运作

该mswitch和mcase(如果可能的话,在编译时,使用对象只是建立constexpr功能)的布尔列表和双射switch能力long long.由于mcases具有编译时常量,因此所有switch标签实际上都是连续的编译时常量.

Answer 2

我会为此制作一个查找表:

#include <iostream>
#include <string>

enum state {
  STATE_1,
  STATE_2,
  STATE_3,
  STATE_4,
  STATE_5,
  STATE_6,
  STATE_7,
  STATE_8,
};

state f(int a, bool b, const std::string& str) {
  static const state table[2][2][2] = {
    STATE_8, // 0, 0, 0
    STATE_7, // 0, 0, 1
    STATE_6, // 0, 1, 0
    STATE_5, // 0, 1, 1
    STATE_4, // 1, 0, 0
    STATE_3, // 1, 0, 1
    STATE_2, // 1, 1, 0
    STATE_1  // 1, 1, 1
  };
  return table[a < 0][b == false][str != "morning"];
}

int main() {
  std::cout << f(1, true, "morning") << std::endl;
}

Answer 3

我同意,模式匹配非常适合那里.不幸的是,内置switch在C++中非常有限.

编译时布尔包的实现非常简单.

#include <type_traits>

namespace detail
{
    constexpr std::size_t pack_bool(std::size_t result)
    {
        return result;
    }

    template<typename T, typename... Ts>
    constexpr std::size_t pack_bool(std::size_t result, T arg, Ts... args)
    {
        static_assert(std::is_same<bool, T>::value, "boolean expected");
        return pack_bool((result << 1) | arg, args...);
    }
}

template<typename T, typename... Ts>
constexpr std::size_t pack_bool(T arg, Ts... args)
{
    static_assert(std::is_same<bool, T>::value, "boolean expected");
    return detail::pack_bool(arg, args...);
}

现在,您可以在switch声明中使用它

#include <iostream>

enum state
{
    STATE_1,
    STATE_2,
    STATE_3,
    STATE_4,
    STATE_5,
    STATE_6,
    STATE_7,
    STATE_8,
};

state f(int a, bool b, const std::string& str)
{
    switch (pack_bool(a >= 0, b == true, str == "morning"))
    {
        case pack_bool(false, false, false) : return STATE_1;
        case pack_bool(false, false, true)  : return STATE_2;
        case pack_bool(false, true,  false) : return STATE_3;
        case pack_bool(false, true,  true)  : return STATE_4;
        case pack_bool(true,  false, false) : return STATE_5;
        case pack_bool(true,  false, true)  : return STATE_6;
        case pack_bool(true,  true,  false) : return STATE_7;
        case pack_bool(true,  true,  true)  : return STATE_8;
    }
    return STATE_1;
}

int main()
{
    std::cout << "State: " << f(1, true, "morning") << std::endl;
}

Answer 4

这是我的版本：

特征：

1. 保留编译器对缺失案例的检查，并提供有关缺失案例的信息性消息。

2. 案例的编译时评估意味着零运行时开销

3. 没有宏会污染全局命名空间并随机阻止仅头文件库工作:-)

缺点：

1. 需要预定义一些样板枚举（仅一次，在我为您完成的库中）

代码：

#include <iostream>
#include <utility>
#include <sstream>
#include <string>

namespace detail{

    template<size_t N> struct boolean_value;
    template<size_t N> using boolean_value_t = typename boolean_value<N>::type;
    template<size_t N> constexpr auto to_int(boolean_value_t<N> b) { return static_cast<int>(b); };
    template<size_t N> constexpr auto to_boolean_value(int i) { return static_cast<boolean_value_t<N>>(i); };

    template<> struct boolean_value<1> {
        enum type { bit0, bit1 };
    };

    template<> struct boolean_value<2> {
        enum type { bit00, bit01, bit10, bit11 };
    };

    template<> struct boolean_value<3> {
        enum type { bit000, bit001, bit010, bit011, bit100, bit101, bit110, bit111 };
    };

    template<class...Args, size_t...Is>
    static constexpr auto make_bitfield(std::tuple<Args...> t, std::index_sequence<Is...>)
    {
#if __cplusplus > 201402L
        int accum = (0 | ... | (std::get<Is>(t) ? (1 << Is) : 0));
#else
        int accum = 0;
        using expand = int[];
        (void) expand { (std::get<Is>(t) ? accum |= (1 << Is) : 0) ... };
#endif
        return to_boolean_value<sizeof...(Is)>(accum);
    }

}

template<class...Args>
constexpr
auto mcase(Args&&...args)
{
    return detail::make_bitfield(std::make_tuple(bool(std::forward<Args>(args))...),
                                 std::index_sequence_for<Args...>());
}

// little function to defeat the optimiser, otherwise clang inlines the whole program!
auto get_result()
{
    using namespace std;

    istringstream ss("foo 2");
    auto result = tuple<string, int>();
    ss >> get<0>(result) >> get<1>(result);
    return result;
}

int main()
{
    using namespace std;
    const auto result = get_result();
    const auto& s1 = std::get<0>(result);
    const auto& v1 = std::get<1>(result);

    switch(mcase(s1 == "foo"s, v1 == 2))
    {
        case mcase(true, true):
            cout << mcase(true, true) << endl;
            break;

        case mcase(false, false):
            cout << mcase(false, false) << endl;
            break;
    }
    return 0;
}

编译器输出示例：

./mswitch.cpp:114:12: warning: enumeration values 'bit01' and 'bit10' not handled in switch [-Wswitch]
    switch(mcase(s1 == "foo"s, v1 == 2))
           ^
1 warning generated.
./mswitch.cpp:114:12: warning: enumeration values 'bit01' and 'bit10' not handled in switch [-Wswitch]
    switch(mcase(s1 == "foo"s, v1 == 2))
           ^
1 warning generated.

运行时输出：

3