使用可变参数模板创建一个简单的调度程序

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

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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