无法调用构造函数（模板模板参数）

Question

不知何故，我无法创建D 的实例。我不知道为什么。模板模板参数（Allocator_class）似乎是问题所在。

#include <vector>

template <template<typename T> class Allocator_class>
class T_b {
public:
    template<typename T> using A = Allocator_class<T>;
};

template< template <typename T> class A>
class C {
public:
    C() { }
};

template<typename Ttb>
class D {
public:
    template<typename T> using A = typename Ttb::template A<T>;
    typedef C<A> Data;

    D(C<A> &data) : _data(data) {}
private:
    Data &_data;
};

int main() {
    typedef T_b<std::allocator> Ttb;    
    C<std::allocator> b;
    D<Ttb>c(b);
}

来自 Clang 的错误：

test5.cpp:29:8: error: no matching constructor for initialization of 'D<Ttb>'
      (aka 'D<T_b<std::allocator> >')
        D<Ttb>c(b);
              ^ ~
test5.cpp:16:7: note: candidate constructor (the implicit copy constructor) not viable: no known
      conversion from 'C<std::allocator>' to 'const D<T_b<std::allocator> >' for 1st argument
class D {
      ^
test5.cpp:16:7: note: candidate constructor (the implicit move constructor) not viable: no known
      conversion from 'C<std::allocator>' to 'D<T_b<std::allocator> >' for 1st argument
class D {
      ^
test5.cpp:21:5: note: candidate constructor not viable: no known conversion from 'C<std::allocator>'
      to 'C<A> &' for 1st argument
    D(C<A> &data) : _data(data) {}
    ^

Answer 1

我无法解释为什么您的代码会出错。

但是，如果您想要一个解决方案...使用类专业化和模板模板模板参数...。

#include <vector>

template <template<typename T> class Allocator_class>
class T_b
 {
   public:
      template<typename T> using A = Allocator_class<T>;
 };

template <template <typename T> class A>
class C
 {
   public:
      C() { }
 };

template <typename>
class D;

template <template <template <typename> class> class X,
          template <typename> class A>
class D<X<A>>
 {
   public:
      using Data = C<A>;

      D (Data & data) : _data(data) {}

   private:
      Data & _data;
 };

int main()
 {
    typedef T_b<std::allocator> Ttb;    

    C<std::allocator> b;

    D<Ttb> c(b);
 }