问题描述

在 C++17 中，algorithm 标头中的许多函数现在可以采用执行策略.例如，我可以定义和调用这样的函数:

In C++17, a number of functions in the algorithm header now can take an execution policy. I can for example define and call a function like this:

template <class ExecutionPolicy>
void f1(const std::vector<std::string>& vec, const std::string& elem, ExecutionPolicy&& policy) {
    const auto it = std::find(
        std::forward<ExecutionPolicy>(policy),
        vec.cbegin(), vec.cend(), elem
    );
}

std::vector<std::string> vec;
f1(vec, "test", std::execution::seq);

但是我还没有找到在运行时使用不同策略的好方法.例如，当我想根据某些输入文件使用不同的策略时.

However I haven't found a good way to use different policies at runtime. For example when I want to use a different policy depending on some input file.

我尝试了各种变体，但最终问题总是出在不同类型的 std::execution::seq、std::execution::par和 std::execution::par_unseq.

I toyed around with variants, but in the end the problem was always the different types of std::execution::seq, std::execution::par and std::execution::par_unseq.

一个有效但繁琐的解决方案如下所示:

A working but cumbersome solution would look like this:

void f2(const std::vector<std::string>& vec, const std::string& elem, const int policy) {
    const auto it = [&]() {
        if (policy == 0) {
            return std::find(
                std::execution::seq,
                vec.cbegin(), vec.cend(), elem
            );
        }
        else if (policy == 1) {
            return std::find(
                std::execution::par,
                vec.cbegin(), vec.cend(), elem
            );
        }
        else{
            return std::find(
                std::execution::par_unseq,
                vec.cbegin(), vec.cend(), elem
            );
        }
    };
}

f2(vec, "test", 0);

有没有我忽略的更优雅的解决方案?

Is there any more elegant solution I'm overlooking?

也许我应该更精确.假设目标是将策略保存在一个变量中，该变量可以具有三个策略中的任何一个.该变量应该是函数的参数.

edit: maybe I should be more precise. Let's say the goal is to save the policy in a variable that can have either of the three policies. That variable should be a parameter to the function.

推荐答案

这里的标准方法是将类型的选择与类型的使用分开:后者采用由前者的非模板函数(或模板参数较少的函数模板)多次实例化的函数模板的形式.

The standard approach here is to separate the selection of a type from the use of the type: the latter takes the form of a function template instantiated several times by the former non-template function (or function template with fewer template parameters).

为了避免在这两层之间复制普通参数，请使用通用 lambda 作为模板.为避免重复选择逻辑，请制作一个函数模板，使用适当的策略调用任何 lambda:

To avoid duplicating the normal parameters between these two layers, use a generic lambda as the template. To avoid duplicating the selection logic, make a function template that calls whatever lambda with the appropriate policy:

enum Policy {seq,par,par_unseq};

template<class F>
auto maybe_parallel(F f,Policy p) {
  switch(p) {
  case seq: return f(std::execution::seq);
  case par: return f(std::execution::par);
  default: return f(std::execution::par_unseq);
  }
}

auto f2(const std::vector<std::string>& vec,
        const std::string& elem,Policy p) {
  return maybe_parallel
    ([&](auto &pol) {return std::find(pol,vec.begin(),vec.end(),elem);},p);
}

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