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当使用奇怪的循环模板模式时,如何实例化基类?

template-meta-programming metaprogramming templates c++11 c++

Evandro Coan · 技术社区 · 6 年前

我正在构建自己的阵列实现,其中包含几个新功能和支持操作符。我 researched a lot std::array 最后,它引起了很多问题,我决定使用组合而不是继承。

下面,我们可以看到我的一小部分 Array 使用模板元编程的自定义实现。在这个简单的版本中,有一个 std::ostream 和一个简单的 operator/ 定义:

#include <array>
#include <iostream>

template <unsigned int array_width, typename DataType, typename DerivedType>
struct Array {
  std::array<DataType, array_width> _data;

  Array() {
    for(int index = 0; index < array_width; ++index) _data[index] = 1;
  }

  DerivedType operator/(const double& data) {
    unsigned int column;
    DerivedType new_array;

    for(column = 0; column < array_width; column++) {
      new_array._data[column] = _data[column] / data;
    }
    return new_array;
  }

  friend std::ostream& operator<<( std::ostream &output, const Array &array ) {
    unsigned int column; output << "(";
    for( column=0; column < array_width; column++ ) {
      output << array._data[column];
      if( column != array_width-1 ) {
        output << ", ";
      }
    }
    output << ")"; return output;
  }
};

struct Coordinate : public Array<3, double, Coordinate> {
  typedef Array< 3, double, Coordinate > SuperClass;
  double& x;
  double& y;
  double& z;

  Coordinate() : SuperClass{}, x{this->_data[0]}, y{this->_data[1]}, z{this->_data[2]} {}
};

int main() {
  Coordinate coordinate;
  std::cout << "coordinate: " << coordinate << std::endl;

  Coordinate new_coordinate = coordinate / 10.0;
  std::cout << "new_coordinate: " << new_coordinate << std::endl;
}

但是,此实现使用 Curiously Recurring Template Pattern 有一个限制。我找不到直接实例化基类数组的方法 排列

int main() {
  Array<5, int> int_array;
  std::cout << "int_array: " << int_array << std::endl;

  Array<5, int> new_int_array = int_array / 10;
  std::cout << "new_int_array: " << new_int_array << std::endl;
}

编者说:

test.cpp: In function 'int main()':
test.cpp:45:15: error: wrong number of template arguments (2, should be 3)
   Array<5, int> int_array;
               ^
test.cpp:6:8: note: provided for 'template<unsigned int array_width, class DataType, class DerivedType> struct Array'
 struct Array {
        ^~~~~
test.cpp:48:15: error: wrong number of template arguments (2, should be 3)
   Array<5, int> new_int_array = int_array / 10;
               ^
test.cpp:6:8: note: provided for 'template<unsigned int array_width, class DataType, class DerivedType> struct Array'
 struct Array {
        ^~~~~

然后,我尝试将自己的模板类作为 struct Array 声明如下:

template <unsigned int array_width, typename DataType, typename DerivedType>
struct Array;

template <unsigned int array_width, typename DataType, typename DerivedType=Array>
struct Array {
  std::array<DataType, array_width> _data;
  // ...

但是,我发现编译器似乎不允许将模板类传递给另一个模板类,因为如果没有实例化模板类,它们不会定义类型。

test.cpp:8:77: error: invalid use of template-name 'Array' without an argument list
 template <unsigned int array_width, typename DataType, typename DerivedType=Array>
                                                                             ^~~~~
test.cpp:8:77: note: class template argument deduction is only available with -std=c++1z or -std=gnu++1z
test.cpp:6:8: note: 'template<unsigned int array_width, class DataType, class DerivedType> struct Array' declared here
 struct Array;
        ^~~~~
test.cpp: In function 'int main()':
test.cpp:48:15: error: template argument 3 is invalid
   Array<5, int> int_array;
               ^
test.cpp:51:15: error: template argument 3 is invalid
   Array<5, int> new_int_array = int_array / 10;