char和初始化列表 [英] char and initializer lists

问题描述

我想通过初始化列表一个可变参数模板将一些数字字节值传递到数组。这是可能吗？

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 };
}

我可以想到的是

• 使用八进制表示法，'\001'等或


• 来投射每个值。

但两者都不令人满意。

推荐答案

注意：除非您向类添加了功能，否则这些都是不必要的，因此它不再是聚合。 （例如，其他构造函数，私有成员，基类等）解决问题中的代码的直接方法是简单地删除构造函数。所以，让我们假设有更多的东西。

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.

我见过一些人想做这样的事情。它看起来很丑陋，处理转换语义，并试图人为地重新创建一个通常的函数调用的功能。

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.

这里是一个策略创建一个数组类，正确的构造函数。

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

模板别名将通过隐藏 :: type 丑陋，但它不在GCC。

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 );
}

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