使用可变参数模板创建一个简单的调度程序 [英] Create a simple dispatcher using variadic templates

问题描述

我想使用可变参数模板来帮助解决使用va-args的问题。基本上，我想调用一个单数函数，将一个命令与一个变量参数列表一起传递给函数，然后将参数分配给另一个函数。



我已经使用try和true（但不是类型安全）va_list实现了这个。这是我使用可变参数模板进行的尝试。下面的例子没有编译，因为你会很快发现原因...

I would like to use variadic templates to help solve an issue using va-args. Basically, I want to call a singular function, pass into the function a "command" along with a variable list of arguments, then dispatch the arguments to another function.

I've implemented this using tried and true (but not type safe) va_list. Here's an attempt I made at doing this using variadic templates. The example doesn't compile below as you will quickly find out why...

#include <iostream>

    using namespace std;
    typedef enum cmd_t
    {
        CMD_ZERO,
        CMD_ONE,
        CMD_TWO,
    } COMMANDS;


    int cmd0(double a, double b, double c)
    {
        cout << "cmd0  " << a << ", " << b << ", " << c << endl;
        return 0;
    }

    int cmd1(int a, int b, int c)
    {
        cout << "cmd1  " << a << ", " << b << ", " << c << endl;
        return 1;
    }

    template<typename... args>
    int DispatchCommand(COMMANDS cmd, Args... args)
    {
        int stat = 0;
        switch (cmd)
        {
        case CMD_ZERO:
            cmd0(args...);
            break;
        case CMD_ONE:
            cmd1(args...);
            break;
        default:
            stat = -1;
            break;
        }
        return stat;
    }

    int main()
    {
        int stat;
        stat = DispatchCommand(CMD_ZERO, 1, 3.141, 4);
        stat = DispatchCommand(CMD_ONE, 5, 6, 7);
        stat = DispatchCommand(CMD_TWO, 5, 6, 7, 8, 9);

        system("pause");
        return 0;
    }

有没有人知道如何修改此函数以正确使用可变参数模板？

Does anyone have an idea on how I can modify this function to use variadic templates correctly?

推荐答案

我认为你在这里遇到的问题是，你得到的错误是cmd0不带5个参数？



这是因为DispatchCommand（CMD_TWO，5,6,7,8,9）创建的函数正在创建期望调用cmd0的代码（和cmd1）包含所有5个参数，如果cmd == CMD_ZERO。请记住，模板是编译时，而不是运行时。



如果static_if可用，那么这可能是一个解决方案，而不是切换条件。但是，我不相信它是由任何编译器实现的，并且争论仍然激烈（AFAIK）关于它的使用/支持/标准化。



然而，另一个解决方案是使用部分专业的模板类（模板函数不能部分专门化），虽然这看起来像是你想要实现的代码的大量代码:) ...

I presume the problem you are having here is that you are getting an error along the lines of cmd0 does not take 5 arguments?

This is because the function created as a result of the DispatchCommand(CMD_TWO, 5, 6, 7, 8, 9) is creating code that expects to call cmd0 (and cmd1) with all 5 arguments, if the cmd == CMD_ZERO. Remember that templates are compile time, not runtime.

If static_if was available, then that might be a solution, instead of the switch conditions. However, I don't believe it is implemented by any compiler and debate still rages (AFAIK) about it's use / support / standardisation.

However, another solution would be with partially specialized template classes (template functions cannot be partially specialized), although this seems like an aweful lot of code for what you are trying to achieve :) ...

template<int>
class Dispatcher
{
public:
    template <typename... Args>
    static int exec(Args... args) {
        return -1;
    }
};

template<>
class Dispatcher<CMD_ZERO>
{
public:
    template <typename... Args>
    static int exec(Args... args) {
        cmd0(args...);
        return 0;
    }
};

template<>
class Dispatcher<CMD_ONE>
{
public:
    template <typename... Args>
    static int exec(Args... args) {
        cmd1(args...);
        return 0;
    }
};

template <int _Cmd, typename... Args >
int DispatchCommand(Args... args)
{
    return Dispatcher<_Cmd>::exec(args...);
}


int main()
{
    int stat;
    stat = DispatchCommand<CMD_ZERO>(1, 3.141, 4);
    stat = DispatchCommand<CMD_ONE>(5, 6, 7);
    stat = DispatchCommand<CMD_TWO>(5, 6, 7, 8, 9);

    system("pause");
    return 0;
}

当然，这只适用于DispatchCommand的模板参数是编译时常量.....我希望这有一些帮助吗？

Of course, this only works if the template parameters to DispatchCommand are compile time constants..... I hope this is of some help ?

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