为了正确处理对象复制，经验法则是三法则。在 C++11 中，移动语义是一回事，所以它是五法则。但是，在这里和互联网上的讨论中，我还看到了对四规则（半）的引用，它是五规则和复制和交换成语的组合。
那么究竟什么是四法则（半法则）？需要实现哪些功能，每个功能的主体应该是什么样的？哪个功能是一半？与五法则相比，这种方法有什么缺点或警告吗？
这是一个类似于我当前代码的参考实现。如果这是不正确的，正确的实现会是什么样子？
//I understand that in this example, I could just use `std::unique_ptr`.
//Just assume it's a more complex resource.
#include <utility>
class Foo {
public:
//We must have a default constructor so we can swap during copy construction.
//It need not be useful, but it should be swappable and deconstructable.
//It can be private, if it's not truly a valid state for the object.
Foo() : resource(nullptr) {}
//Normal constructor, acquire resource
Foo(int value) : resource(new int(value)) {}
//Copy constructor
Foo(Foo const& other) {
//Copy the resource here.
resource = new int(*other.resource);
}
//Move constructor
//Delegates to default constructor to put us in safe state.
Foo(Foo&& other) : Foo() {
swap(other);
}
//Assignment
Foo& operator=(Foo other) {
swap(other);
return *this;
}
//Destructor
~Foo() {
//Free the resource here.
//We must handle the default state that can appear from the copy ctor.
//(The if is not technically needed here. `delete nullptr` is safe.)
if (resource != nullptr) delete resource;
}
//Swap
void swap(Foo& other) {
using std::swap;
//Swap the resource between instances here.
swap(resource, other.resource);
}
//Swap for ADL
friend void swap(Foo& left, Foo& right) {
left.swap(right);
}
private:
int* resource;
};