switch语句的std :: pair？ [英] switch statement for an std::pair?

问题描述

我想切换两个整数的可能值，或者在另一个情况下切换两个bool。为了讨论起见，假设我已经做了

I want to switch over possible values of two integers, or in another case two bools. For the sake of discussion, suppose I've done

auto mypair = std::make_pair(foo, bar);

如何实现

switch(mypair) {
case make_pair(true, false): cout << "true and false"; break;
case make_pair(false, true)  cout << "false and true"; break;
case default: cout << "something else";
}

（C ++ 14/17也相关，如果这有帮助）？

with C++11? (C++14/17 also relevant if that helps)?

推荐答案

C ++的switch语句没有模式匹配的许多其他语言。

C++'s switch statement doesn't have the pattern matching power of many other languages. You'll need to take a slightly different approach.

这里有一种可能性：

pair_switch(my_pair,
    std::make_tuple(true, false, []{ std::cout << "true and false"; }),
    std::make_tuple(false, true, []{ std::cout << "false and true"; }));

您提供 std :: pair< bool，bool> 和一组case作为 std :: tuples ，其中前两个元素匹配你传递的对，第三个元素是一个函数调用

You supply a std::pair<bool,bool> and a set of cases as std::tuples, where the first two elements match the pair you pass in and the third element is a function to call for that case.

实现有几个模板技巧，但应该非常实用：

The implementation has a few template tricks, but should be pretty usable:

template <typename... Ts>
void pair_switch(std::pair<bool,bool> pair, Ts&&... ts) {
    //A table for the cases
    std::array<std::function<void()>, 4> table {};

    //Fill in the cases
    (void)std::initializer_list<int> {
        (table[std::get<0>(ts)*2 + std::get<1>(ts)] = std::get<2>(ts), 0)...
    };

    //Get the function to call out of the table
    auto& func = table[pair.first*2 + pair.second];

    //If there is a function there, call it
    if (func) {
        func();   
    //Otherwise, throw an exception
    } else {
        throw std::runtime_error("No such case");   
    }
}

现场演示

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