char and initializer lists

问题:

I'd like to pass some numeric byte values via an initializer list a variadic template into an array. Is that possible?

template < int N > struct a {
  char s[N];
  template < typename ... A >
  a (A ... _a) : s {_a...} {}
};

int main () {
  // g++-4.5: error: narrowing conversion of »_a#0« from »int« to »char« inside { }
  a < 3 > x { 1, 2, 3 };
}

What I can think of is

• to use octal representation, '\001' etc., or
• to cast every single value.

But both is not satisfying.

回答1:

NOTE: All of this is unnecessary unless you have added functionality to the class so it's no longer an aggregate. (For example, other constructors, private members, a base class, etc.) The immediate way to fix the code in the question is simply to remove the constructor. So, let's assume there's something more to it.

I've seen some people trying to do things like this. It seems ugly, dealing with conversion semantics and trying to artificially re-create the functionality of a usual function call.

Here is a strategy to create an array class that simply has the right constructor in the first place.

Template aliasing would put the icing on the cake by hiding the ::type ugliness, but it's not in GCC yet.

template< typename ... NT >
struct var_ctor_array {
    enum { size_e = 0 }; // only used for zero size case
};

template< typename T, typename ... NT >
struct var_ctor_array< T, NT ... > {
    enum { size_e = 1 + sizeof...( NT ) };

    T st[ size_e ];

    var_ctor_array( T elem0, NT ... elemN )
        : st { elem0, elemN ... } {}
};

template< typename T, size_t N, typename ... NT >
struct gen_var_ctor_array {
    typedef typename gen_var_ctor_array< T, N-1, T, NT ... >::type type;
};

template< typename T, typename ... NT >
struct gen_var_ctor_array< T, 0, NT ... > {
    typedef var_ctor_array< NT ... > type;
};

int main() { // usage
    gen_var_ctor_array< char, 5 >::type five( 1, 2, 3, 4, 5 );
}

回答2:

You don't need any complicated code

template < int N > struct a {
  char s[N];
  template < typename ... A >
  a (A ... _a) : s {static_cast<char>(_a)...} {}
};

回答3:

You're not actually using initializer lists. The constructor receives a variadic template and you initialize x with uniform initialization.

The only problem is I don't know of an elegant way of initializing an array with an initializer_list, AFAIK std::array should have a constructor that accepts initializer_list but it doesn't seem to be supported by g++ yet.

#include <utility>
template < int N > struct a {
    char s[N];

    a (std::initializer_list<char> list) {
        if (N != list.size()) 
            throw "list wrong size";

        int i = 0;
        const char* p = list.begin();
        while(p != list.end())
            s[i++] = *p++;
    }
};