我试图弄清楚是否可以将概念用作类的一种接口，而无需虚拟表的开销。我整理了一个可以工作的示例，但是我必须将我的类实例存储在由它们的共同继承而不是它们的共同概念定义的数组中。我没有看到关于概念数组的帖子中讨论过任何内容，但 g++ 6.3.0 似乎不允许这样做。错误是：
$ g++ -fconcepts -std=c++1z custom_concept.cpp
custom_concept.cpp: In function ‘int main()’:
custom_concept.cpp:37:20: error: ‘shapes’ declared as array of ‘IShape*’
IShape* shapes[2] = {&square, &rect}; // doesn't work
^
custom_concept.cpp:39:25: error: ‘shapes’ was not declared in this scope
for (IShape* shape : shapes )
^~~~~~
如果我将IShape*数组更改为Rectangle*数组（如导致第一个错误的行下方的注释行中），程序将按预期编译和运行。
为什么不允许使用概念指针数组？这可能会在未来的 c++ 版本中被允许吗？
（我的示例包括虚函数和继承，尽管我的目标是消除它们。我将它们包括在内只是为了方便Rectangle*版本工作。如果我能让IShape*版本工作，我计划删除虚函数和遗产。）
这是代码：
#include <iostream>
template <typename T>
concept bool IShape = requires (T x, T z, int y)
{
{ T() } ;
{ T(x) } ;
{ x = z } -> T& ;
{ x.countSides() } -> int ;
{ x.sideLength(y) } -> int ;
};
struct Rectangle
{
Rectangle() {};
Rectangle(const Rectangle& other) {};
Rectangle& operator=(Rectangle& other) {return *this; };
virtual std::string getName() { return "Rectangle"; }
int countSides() {return 4;}
virtual int sideLength(int side) { return (side % 2 == 0) ? 10 : 5; }
};
struct Square : public Rectangle
{
Square() {};
Square(const Square& other) {};
Square& operator=(Square& other) {return *this; };
std::string getName() override { return "Square"; }
int sideLength(int side) override { return 10; }
};
int main()
{
Square square;
Rectangle rect;
IShape* shapes[2] = {&square, &rect}; // doesn't work
// Rectangle* shapes[2] = {&square, &rect}; // works
for (IShape* shape : shapes )
{
for (int side = 0 ; side < shape->countSides() ; ++side )
{
std::cout << shape->getName() << " side=" << shape->sideLength(side) << "\n";
}
}
return 0;
};
感谢@Yakk 关于使用元组的想法。G++ 6.3.0 尚未完全实现 #include 文件以包含 C++17 标准定义的 apply()，但它在 std::experimental 中可用。（我认为它可能会添加到更高版本的 g++ 中。）这是我最终得到的结果：
#include <iostream>
#include <tuple>
#include <experimental/tuple>
template <typename T>
concept bool IShape = requires (T x, T z, int y)
{
{ T() } ;
{ x = z } -> T& ;
{ T(x) } ;
{ x.countSides() } -> int ;
{ x.sideLength(y) } -> int ;
};
struct Rectangle
{
Rectangle() {};
Rectangle(const Rectangle& other) {};
Rectangle& operator=(Rectangle& other) {return *this; };
std::string getName() { return "Rectangle"; }
int countSides() {return 4;}
int sideLength(int side) { return (side % 2 == 0) ? 10 : 5; }
};
struct Square
{
Square() {};
Square(const Square& other) {};
Square& operator=(Square& other) {return *this; };
std::string getName() { return "Square"; }
int countSides() {return 4;}
int sideLength(int side) { return 10; }
};
void print(IShape& shape)
{
for (int side = 0 ; side < shape.countSides() ; ++side )
{
std::cout << shape.getName() << " side=" << shape.sideLength(side) << "\n";
}
};
int main()
{
Square square;
Rectangle rect;
auto shapes = std::make_tuple(square, rect);
std::experimental::apply([](auto&... shape) { ((print(shape)), ...); }, shapes) ;
return 0;
};