缺少表达方式SFINAE的解决方法 [英] Workaround for lack of expression SFINAE

问题描述

我试图为 std :: tuple 中的每个值调用一个函数，当然没有办法迭代一个元组，因此我求助于使用迭代元组

I'm trying to call a function for each value in a std::tuple, of course there is no way to iterate a tuple and so I've resorted to using the template techniques discussed in iterate over tuple

但是，我使用的是Visual Studio 2013，它不支持表达式SFINAE，因此该代码将不起作用。我曾尝试根据常数（例如5、4、3、2、1、0）对模板进行部分专业化处理，但没有成功。我当然不是模板专家，所以我希望有人可以帮助我。我的表达式SFINAE代码如下。

However, I'm using Visual Studio 2013 and it does not support expression SFINAE, so that code won't work. I've tried to partially specialize the templates based on a constant numbers (e.g 5, 4, 3, 2, 1, 0), but haven't had any success. I'm certainly no template expert, and so I was hoping somebody could help me out. My expression SFINAE code is below.

#include <iostream>
#include <tuple>

using namespace std;

struct ArgPush {
    void push(bool x) {}
    void push(int x) {}
    void push(double x) {}
    void push(const char* x) {}
    void push(const std::string& x) {}

    template<std::size_t I = 0, typename... Tp>
    inline typename std::enable_if<I == sizeof...(Tp), void>::type
        push_tuple(const std::tuple<Tp...>& t)
    { }

    template<std::size_t I = 0, typename... Tp>
    inline typename std::enable_if<I < sizeof...(Tp), void>::type
        push_tuple(const std::tuple<Tp...>& t)
    {
        push(std::get<I>(t));
        push_tuple<I + 1, Tp...>(t);
    }
};

int main() {
    ArgPush().push_tuple(std::make_tuple(1,2,3,4));
    ArgPush().push_tuple(std::make_tuple("hello", "msvc makes me sad", 4, true));
    return 0;
}

推荐答案

MSVC不喜欢在 enable_if 中进行相等比较。因此，将它们从那里移到帮助器模板。然后，您的代码在VS2013上编译。

MSVC doesn't like the equality comparisons being made within the enable_if. So move these out from there to a helper template. Then your code compiles on VS2013.

template<std::size_t I, typename... Tp>
struct comparator
{
    static const bool value = (I < sizeof...(Tp));
};

template<std::size_t I = 0, typename... Tp>
inline typename std::enable_if<!comparator<I, Tp...>::value>::type
    push_tuple(const std::tuple<Tp...>& t)
{ }

template<std::size_t I = 0, typename... Tp>
inline typename std::enable_if<comparator<I, Tp...>::value>::type
    push_tuple(const std::tuple<Tp...>& t)
{
    push(std::get<I>(t));
    push_tuple<I + 1, Tp...>(t);
}

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