如何自动添加文字定义，基于单个用户定义的文字？ [英] How to automatically add literal definitions, based on a single user-defined literal?

问题描述

C ++ 11 提供用户定义的文字。我刚刚开始与他们一起玩，这让我想知道是否可以自动添加所有 SI乘法器到单个字面量我定义？

C++11 offers user-defined literals. I've just started to play around with them, which made me wonder whether it would be possible to automatically add all SI multipliers to a single literal I define?

例如，如果我定义

Length operator "" _m(long double m) {
    return Length(m); // Length in meters
}

其中长度是一些 Units 基类的子类，我想有一个自动 添加的机制 a href =http://www.boost.org/doc/libs/1_53_0/libs/utility/operators.htm =nofollow> boost运算符）所有文本的SI乘数，返回 Length ：

where Length is a subclass of some Units base class, I would like to have a mechanism to automatically add (in the same spirit as boost operators) SI multipliers for all literals that return a Length:

// these are added automatically when defining the literal "_m": 
                                         // Length in:
Length operator "" _Ym(long double Ym);  // Yottameters
Length operator "" _Zm(long double Zm);  // Zetameters
...                                      // ...
...                                      // ...
Length operator "" _km(long double km);  // kilometers
Length operator "" _mm(long double mm);  // millimeters
...                                      // ...       
...                                      // ...
Length operator "" _zm(long double zm);  // zeptometers
Length operator "" _ym(long double ym);  // yoctometers

就我所见，除了一些宏魔法，自动执行此操作，因为所有用户定义的文本都需要显式的定义。

As far as I could see, aside from perhaps some macro magic, there is no way to do this automatically since all user-defined literals need an explicit definition.

..或者我可以忽略某些东西吗？

..or am I overlooking something?

推荐答案

我不认为有一种方法可以做到你所要求的没有奇怪的宏。这是我可以得到的：

I do not think there is a way to do exactly what you are asking for without "bizarre macros". This is as far as I could get:

template<typename T, T (*op)(long double)>
struct SI
{
    // ...
    constexpr static T micro = op (.000001);
    constexpr static T milli = op (.001);
    constexpr static T kilo = op (1000);
    constexpr static T mega = op (1000000);
    // ...
};

struct Length
{
    constexpr Length(long double d) : _d(d) { }
    constexpr operator long double() { return _d; }
    long double _d;
};

constexpr Length operator "" _m(long double m) {
    return Length(m);
}

typedef SI<Length, ::operator "" _m> SI_Length;

int main()
{
    constexpr Length l = 3 * SI_Length::kilo;
    static_assert(l == 3000, "error");
}

如果允许使用奇怪的宏，则应执行以下操作：

If bizarre macros are allowed, then something like the following should do the job:

#define DEFINE_SI_MULTIPLIERS(T, unit) \
    constexpr T operator "" _u ## unit(long double m) \
    { return ::operator "" _ ## unit(0.000001 * m); } \
    constexpr T operator "" _m ## unit(long double m) \
    { return ::operator "" _ ## unit(0.001 * m); } \
    constexpr T operator "" _k ## unit(long double m) \
    { return ::operator "" _ ## unit(1000 * m); } \
    // ...

DEFINE_SI_MULTIPLIERS(Length, m)

int main()
{
    constexpr Length l = 3.0_km;
    static_assert(l == 3000, "error");
}

这篇关于如何自动添加文字定义，基于单个用户定义的文字？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！