如何初始化div_t对象? [英] How can I Initialize a div_t Object?

问题描述

因此，从 div 函数似乎是实现定义的.

So the order of the members returned from the div functions seems to be implementation defined.

quot是1 ^st 成员还是rem?

让我们说我正在做这样的事情:

Let's say that I'm doing something like this:

generate(begin(digits), end(digits), [i = div_t{ quot, 0 }]() mutable {
    i = div(i.quot, 10);
    return i.rem;
})

当然，这里的问题是我不知道我是在lambda捕获中初始化了i.quot还是i.rem的.用div(quot, 1)初始化i是唯一的跨平台方法吗?

Of course the problem here is that I don't know if I initialized i.quot or i.rem in my lambda capture. Is intializing i with div(quot, 1) the only cross platform way to do this?

推荐答案

我认为VS解决方法可能如下所示:

I think the VS workaround could look like this:

#include <cstdlib>
#include <type_traits>

template<class T>
struct DTMaker {
 using D = decltype(div(T{}, T{}));
 static constexpr D dt = D{0,1};
 static constexpr auto quot = dt.quot;
};

template <class T, typename std::enable_if<DTMaker<T>::quot == 0>::type* = nullptr>
typename DTMaker<T>::D make_div(const T &quot, const T& rem) { return {quot, rem}; }

template <class T, typename std::enable_if<DTMaker<T>::quot == 1>::type* = nullptr>
typename DTMaker<T>::D make_div(const T &quot, const T &rem) { return {rem, qout}; }

int main() {
   div_t d_t = make_div(1, 2);
}

[实时演示]

如果您使用的是c ++ 17，则还可以尝试使用结构化绑定，constexpr函数和SFINAE重载来检测首先在结构中声明哪个字段:

If you are using c++17 you could also try to use structured binding, constexpr function and SFINAE overloading to detect which field is declared first in the structure:

#include <cstdlib>
#include <algorithm>
#include <iterator>

constexpr bool first_quot() {
    auto [x, y] = std::div_t{1, 0};
    (void)y;
    return x;
}

template <bool B = first_quot()>
std::enable_if_t<B, std::div_t> foo() {
    int quot = 1;
    int rem = 0;
    return {quot, rem};
}

template <bool B = first_quot()>
std::enable_if_t<!B, std::div_t> foo() {
    int quot = 1;
    int rem = 0;
    return {rem, quot};
}

int main() {
    foo();
}

[实时演示]

或更简单的使用constexpr:

Or even simpler use if constexpr:

#include <cstdlib>
#include <algorithm>
#include <iterator>

constexpr bool first_quot() {
    auto [x, y] = std::div_t{1, 0};
    (void)y;
    return x;
}

std::div_t foo() {
    int quot = 1;
    int rem = 0;
    if constexpr(first_quot())
        return {quot, rem};
    else
        return {rem, quot};
}

int main() {
    foo();
}

[实时演示]

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