在编译时获取当前月份索引 [英] Get current month index in compile time

问题描述

我试图将 __ DATE __ 的格式 Mmm 转换为编译时的数字。我需要它为gcc和MS VC 2012。

我的尝试：

  template< char C0，char C1，char C2> 
 struct month {}; 
 
模板<> struct month< 'J'，'a'，'n'> {static const unsigned id = 1; }; 
模板<> struct month< 'S'，'e'，'p'> {static const unsigned id = 9; }; 
 
 static const char c0 = __DATE __ [0]; 
 static const char c1 = __DATE __ [1]; 
 static const char c2 = __DATE __ [2]; 
 
 static const unsigned currId = month< c0，c1，c2> :: id; // this gives error 
 static const unsigned currId2 = month<'S'，'e'，'p'> :: id; //这是罚款
  

在MS VC中提供

错误C2970：'月'：模板参数'C0'：'C0'：涉及与内部连接的对象表达式不能被用作非类型参数

但在 ideone 上可以正常工作。

$

编辑是否有办法使这项工作成为跨平台/编译器？

我需要一个编译时常所以的类似问题没有真正的帮助。我需要例如减去两个日期（目前在一些代码为准），并给编译时错误时，这两个相差足够大。

解决方案

首先：你确定在编译时需要这个吗？如果运行时是可以接受的，很容易： http://www.keil.com/support/ docs / 1102.htm

---

但是摆脱某些事情，让我们有一些编译时间的乐趣！

你在这里使用模板，但你真的不需要。你可以使用庞大的doom表达式：

  static const char c0 = __DATE __ [0] 
 static const char c1 = __DATE __ [1]; 
 static const char c2 = __DATE __ [2]; 
 static const unsigned int month =（
 c0 =='J'// Jan Jun Jul 
？（c1 =='a'？1：（c2 =='n'？6 ：7））
：c0 =='F'？2 
：c0 =='M'// Mar May 
？（c2 =='r'？3：5）
：c0 =='A'// 4月8月
？（c1 =='p'？4：8）
：c0 =='S'？9 
：c0 =='O'？10 
：c0 =='N'？11 
：12 
）; 
  

免责声明：我只是写在头顶。它现在的工作，但谁知道，也许我三月错

在，如果你想获得更多的乐趣*我们可以对一些字符使用算术事实：

  static const char c0 = __DATE __ [0]; 
 static const char c1 = __DATE __ [1]; 
 static const char c2 = __DATE __ [2]; 
 static const unsigned int month =（
 c0 =='J'// Jan Jun Jul 
？（c1 =='a'？1：（c2 =='n'？6 ：7））
：c0 =='M'// Mar May 
？（3 +（c2 =='y'）* 2）
：c0 =='A' / Apr Aug 
？（4 +（c1 =='u'）* 4）
：c0 =='S'？9 
：c0 <='F'？ - （c0  - 'D'）* 5）// Feb，Dec 
：（11 +'N'-c0）// Oct，Nov 
）; 
  

_{*：byfun我的意思是： / p>}

由于这些是const，你可以使用它与模板。例如，假设我们已经有了在十一月从而结束了合同的工作，我们要相信我们会以高速率得到带回在几天后一切都结束了：

  #include< iostream> 
 using namespace std; 
 
 static const unsigned int month = ...; 
 
 template< int n> class mm {
 public：
 static int v; 
}; 
 
模板<> int mm 9 :: v = 3; // still employ 
 template<> int mm 10 :: v = 2; //让我们不要太可疑
 template<> int mm 11：v = 1; // patience ... 
 // 12月没有价值 - 繁荣！我们在钱！圣诞节的时候！ 
 
 int main（）{
 std :: cout< mm< month> :: v; 
 return 0; 
} 
  

最后，如果你不想乱丢全局范围，应使用constexpr功能：

 静态constexpr INT得到月（无效）{
为const char C0 = __DATE __ [0] ; 
 const char c1 = __DATE __ [1]; 
 const char c2 = __DATE __ [2]; 
 return（
 c0 =='J'// Jan Jun Jul 
？（c1 =='a'？1：（c2 =='n'？6：7））
：c0 =='F'？2 
：c0 =='M'// Mar May 
？（c2 =='r'？3：5）
：c0 =='A'// 4月8月
？（c1 =='p'？4：8）
：c0 =='S'？9 
：c0 ==' ？10 
：c0 =='N'？11 
：12 
）; 
} 
 
 ... 
 
 std :: cout< mm< getMonth（）> :: v; 
  

I'm trying to convert month from __DATE__ in format Mmm to number in compile time. I need it for gcc and MS VC 2012.

my attempt:

template <char C0, char C1, char C2>
struct month{};

template<> struct month < 'J', 'a', 'n'> { static const unsigned id = 1; };
template<> struct month < 'S', 'e', 'p'> { static const unsigned id = 9; };

static const char c0 = __DATE__[0];
static const char c1 = __DATE__[1];
static const char c2 = __DATE__[2];

static const unsigned currId = month<c0, c1, c2>::id;      //this gives error
static const unsigned currId2 = month<'S', 'e', 'p'>::id;  //this is fine

gives in MS VC

error C2970: 'month' : template parameter 'C0' : 'c0' : an expression involving objects with internal linkage cannot be used as a non-type argument

but on ideone works fine.

Is there a way to make this work cross platform/compiler?

edit

I need a compile-time constant So answers in similar question doesn't really help. I need to for example subtract two dates (current and some date in code) and give compile time error when difference between those two is large enough.

解决方案

First: are you sure you need this at compile-time? If run-time is acceptable it's easy: http://www.keil.com/support/docs/1102.htm

---

But moving away from what's sane, let's have some compile-time fun!

You're using templates here but you really don't need to. You can use a massive expression of doom instead:

static const char c0 = __DATE__[0];
static const char c1 = __DATE__[1];
static const char c2 = __DATE__[2];
static const unsigned int month = (
    c0 == 'J' // Jan Jun Jul
        ? (c1 == 'a' ? 1 : (c2 == 'n' ? 6 : 7))
    : c0 == 'F' ? 2
    : c0 == 'M' // Mar May
        ? (c2 == 'r' ? 3 : 5)
    : c0 == 'A' // Apr Aug
        ? (c1 == 'p' ? 4 : 8)
    : c0 == 'S' ? 9
    : c0 == 'O' ? 10
    : c0 == 'N' ? 11
    : 12
);

Disclaimer: I just wrote that off the top of my head. It works now, but who knows, maybe I got March wrong.

In fact if you want to get even more fun* we can use arithmetic on some characters:

static const char c0 = __DATE__[0];
static const char c1 = __DATE__[1];
static const char c2 = __DATE__[2];
static const unsigned int month = (
    c0 == 'J' // Jan Jun Jul
        ? (c1 == 'a' ? 1 : (c2 == 'n' ? 6 : 7))
    : c0 == 'M' // Mar May
        ? (3 + (c2 == 'y') * 2)
    : c0 == 'A' // Apr Aug
        ? (4 + (c1 == 'u') * 4)
    : c0 == 'S' ? 9
    : c0 <= 'F' ? (12 - (c0 - 'D') * 5) // Feb, Dec
    : (11 + 'N' - c0) // Oct, Nov
);

_{*: by "fun" I mean: hated by other developers}

Since these are const, you can then use it with templates. For example, suppose we've got a contract job which ends in November, and we want to be sure we'll get brought back in for a few days at a high rate once it's over:

#include <iostream>
using namespace std;

static const unsigned int month = ...;

template <int n> class mm {
public:
    static int v;
};

template<> int mm<9>::v=3; // still employed
template<> int mm<10>::v=2; // let's not be too suspicious
template<> int mm<11>::v=1; // patience...
// no value for December - boom! we're in the money! Just in time for Christmas!

int main() {
    std::cout << mm<month>::v;
    return 0;
}

Finally, if you don't want to be littering the global scope, you should use a constexpr function:

static constexpr int getMonth( void ) {
    const char c0 = __DATE__[0];
    const char c1 = __DATE__[1];
    const char c2 = __DATE__[2];
    return (
        c0 == 'J' // Jan Jun Jul
            ? (c1 == 'a' ? 1 : (c2 == 'n' ? 6 : 7))
        : c0 == 'F' ? 2
        : c0 == 'M' // Mar May
            ? (c2 == 'r' ? 3 : 5)
        : c0 == 'A' // Apr Aug
            ? (c1 == 'p' ? 4 : 8)
        : c0 == 'S' ? 9
        : c0 == 'O' ? 10
        : c0 == 'N' ? 11
        : 12
    );
}

...

std::cout << mm<getMonth()>::v;

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