使用可变模板的编译时'字符串'操作 [英] Compile-time 'String' Manipulation with Variadic Templates

问题描述

嘿，所有，我目前正试图编写一个编译时字符串加密（使用'字符串'和'加密'相当宽松）lib。

我到目前为止如下：

  // Cacluate在编译时的窄字符串长度
 template< char ... ArgsT> 
 struct CountArgs 
 {
 template< char ... ArgsInnerT> struct Counter; 
 
 template< char Cur，char ... Tail> 
 struct Counter< Cur，Tail ...> 
 {
 static unsigned long const Value = Counter< Tail ...> :: Value + 1; 
}; 
 
 template< char Cur> 
 struct Counter< Cur> 
 {
 static unsigned long const Value = 1; 
}; 
 
 static unsigned long const Value = Counter< ArgsT ...> :: Value; 
}; 
 
 //'在编译时加密窄字符串
 template< char ... Chars> 
 struct EncryptCharsA 
 {
 static const char Value [CountArgs< Chars ...> :: Value + 1]; 
}; 
 
 template< char ... Chars> 
 char const EncryptCharsA< Chars ...> :: Value [CountArgs< Chars ...> :: Value + 1] = 
 {
 Chars ... 
 }; 
  

但是我不知道如何对字符执行操作，数组。我只想对每个字符执行一个简单的操作（例如'（（（c ^ 0x12）^ 0x55）+ 1）'其中c是字符）。

<非常感谢。

感谢所有。

解决方案

如果你只是想一次操作一个字符，它的容易：

  template< char c& struct add_three {
 enum {value = c + 3}; 
}; 
 
 template< char ... Chars> struct EncryptCharsA {
 static const char value [sizeof ...（Chars）+ 1]; 
}; 
 
 template< char ... Chars> 
 char const EncryptCharsA< Chars ...> :: value [sizeof ...（Chars）+ 1] = {
 add_three< Chars> :: value ... 
}; 
 
 int main（）{
 std :: cout< EncryptCharsA<'A'，'B'，'C'> :: value< std :: endl; 
 //打印DEF
} 
  

注意 CountArgs 是多余的（这就是 sizeof ... ），并且这使用元素方式转换参数包中的元素。

---

要使转换依赖于前面的结果，一个选项是递归地使用字符，一次一个，逐步构建新模板：

 模板< char ... P> struct StringBuilder {
 template< char C> struct add_char {
 typedef StringBuilder< P ...，C>类型; 
}; 
 
 static const char value [sizeof ...（P）+1]; 
}; 
 
 template< char ... P> const char StringBuilder< P ...> :: value [sizeof ...（P）+1] = {
 P ... 
}; 
 
 template< class B，char ...> struct EncryptImpl; 
 
 template< class B，char Seed，char Head，char ... Tail> 
 struct EncryptImpl< B，Seed，Head，Tail ...> {
 static const char next = Head + Seed; //或任何
 typedef typename EncryptImpl< 
 typename B :: template add_char< next> :: type，
 next，Tail ... 
> :: type type; 
}; 
 
 template< class B，char Seed> struct EncryptImpl< B，Seed> {
 typedef B type; 
}; 
 
 template< char ... P> struct Encrypt {
 typedef typename EncryptImpl< StringBuilder<>，0，P ...> :: type type; 
}; 
  

Hey all, I'm currently trying to write a compile-time string encryption (using the words 'string' and 'encryption' quite loosely) lib.

What I have so far is as follows:

// Cacluate narrow string length at compile-time
template <char... ArgsT>
struct CountArgs
{
 template <char... ArgsInnerT> struct Counter;

 template <char Cur, char... Tail>
 struct Counter<Cur, Tail...>
 {
  static unsigned long const Value = Counter<Tail...>::Value + 1;
 };

 template <char Cur>
 struct Counter<Cur>
 {
  static unsigned long const Value = 1;
 };

 static unsigned long const Value = Counter<ArgsT...>::Value;
};

// 'Encrypt' narrow string at compile-time
template <char... Chars>
struct EncryptCharsA
{
 static const char Value[CountArgs<Chars...>::Value + 1];
};

template<char... Chars>
char const EncryptCharsA<Chars...>::Value[CountArgs<Chars...>::Value + 1] =
{
 Chars...
};

However I can't figure out how to perform operations on the characters as I expand them into the static array. I'd just like to execute a simple operation on each character (e.g. '(((c ^ 0x12) ^ 0x55) + 1)' where c is the character).

A shove in the right direction would be greatly appreciated.

Thanks all.

解决方案

If you just want to operate on one character at a time its easy:

template<char c> struct add_three {
    enum { value = c+3 };
};

template <char... Chars> struct EncryptCharsA {
    static const char value[sizeof...(Chars) + 1];
};

template<char... Chars>
char const EncryptCharsA<Chars...>::value[sizeof...(Chars) + 1] = {
    add_three<Chars>::value...
};

int main() {   
    std::cout << EncryptCharsA<'A','B','C'>::value << std::endl;
    // prints "DEF"
}

Note that CountArgs is redundant (that's what sizeof... is for) and that this uses element-wise transformation of the elements in a parameter-pack.

---

To make the transformation dependent on previous results, one option would be to consume the characters recursively, one at a time, and incrementally build a new template from that:

template<char... P> struct StringBuilder {
    template<char C> struct add_char {
        typedef StringBuilder<P..., C> type;
    };

    static const char value[sizeof...(P)+1];
};

template<char... P> const char StringBuilder<P...>::value[sizeof...(P)+1] = {
    P...
};

template<class B, char...> struct EncryptImpl;

template<class B, char Seed, char Head, char... Tail> 
struct EncryptImpl<B, Seed, Head, Tail...> {
    static const char next = Head + Seed; // or whatever
    typedef typename EncryptImpl<
        typename B::template add_char<next>::type,
        next, Tail...
    >::type type;
};

template<class B, char Seed> struct EncryptImpl<B, Seed> {
    typedef B type;
};

template<char... P> struct Encrypt {
    typedef typename EncryptImpl<StringBuilder<>, 0, P...>::type type;
};

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