C++ 使用 unique_ptr 进行前向声明？
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Forward declaration with unique_ptr?
提问by Zyx 2000
I have found it useful to use forward declaration of classes in combination with std::unique_ptras in the code below. It compiles and works with GCC, but the whole thing seem kind of strange, and I wonder if this is standard behaviour (i.e. required by the standard)? Since B isn't a complete type when I declare the unique_ptr.
我发现将类的前向声明与std::unique_ptr下面的代码结合使用很有用。它编译并与 GCC 一起工作，但整个事情看起来有点奇怪，我想知道这是否是标准行为（即标准要求）？由于 B 在我声明unique_ptr.
A.hpp
hpp
#include <memory>
class B;
class A {
std::unique_ptr<B> myptr;
// B::~B() can't be seen from here
public:
~A();
};
A.cpp
A.cpp
#include "B.hpp"
//B.hpp has to be included, otherwise it doesn't work.
A::~A() = default; // without this line, it won't compile
// however, any destructor definiton will do.
I suspect this has to do with the destructor (and therefore the need to call the destructor of unique_ptr<B>) is defined in a specific compilation unit (A.cpp).
我怀疑这与unique_ptr<B>在特定编译单元 (A.cpp) 中定义的析构函数有关（因此需要调用 的析构函数）。
回答by James Kanze
It's explicitly legal. The rule is that the types used to instantiate
a template in the standard library must be complete, unlessotherwise
specified. In the case of unique_ptr, §20.7.1/5 says “[...] The
template parameter T of unique_ptr may be an incomplete type.”
这显然是合法的。规则是用于在标准库中实例化模板的类型必须是完整的，除非另有说明。在 的情况下unique_ptr，第 20.7.1/5 节说“[...] unique_ptr 的模板参数 T 可能是不完整的类型。
There are certain operations on the pointer which require a complete
type; in particular, when the object will actually be destructed (at
least with the default deleter). In your example, for example, if
A::~A()were inline, this might cause problems. (Note that if you
don't declare the destructor yourself, it will be inline. Which
partially defeats the purpose of using std::unique_ptr.)
指针上的某些操作需要完整的类型；特别是，当对象实际上将被破坏时（至少使用默认删除器）。例如，在您的示例中，如果
A::~A()是内联的，这可能会导致问题。（请注意，如果您自己不声明析构函数，它将是内联的。这在一定程度上违背了使用 的目的std::unique_ptr。）
