可变参数模板的编译时“字符串”操作

Question

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 (eg '(((c ^ 0x12) ^ 0x55) + 1)' where c is the character).

A shove in the right direction would be greatly appreciated.

Thanks all.

Answer 1

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;
};

Answer 2

If I understand what you want to do correctly (actually create an array at compile time) I think variadic templates aren't enough and you'll have to wait for constexpr .

If however you don't need an actual array and can instead compromise on using something akin to tuple 's get<I> then it's possible (you can then build a char array at runtime).