Use unique_ PTR forward declaration? [repetition] - forward declaration with unique_ ptr? [duplicate]

problem ：

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• Is std::unique_ptr<T> required to know the full definition of T? If you need std :: unique_ptr <T> To know T Complete definition ？8 answers8 Answer

I have found it useful to use forward declaration of classes in combination with std::unique_ptr as in the code below. I found it in std::unique_ptr Using forward declarations of classes is useful , This is shown in the following code .It compiles and works with GCC, but the whole thing seem kind of strange, and I wonder if this is standard behaviour (ie required by the standard)? It compiles and works with GCC Working together , But the whole thing seems a little strange , I want to know if this is standard behavior （ I.e. standard requirements ）？Since B isn't a complete type when I declare the unique_ptr . Because when I declare unique_ptr when B Not a complete type .

A.hppA.hpp

#include <memory>class B;class A {    std::unique_ptr<B> myptr;    // B::~B() can't be seen from herepublic:    ~A();};

A.cppA.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). I suspect this has something to do with destructors （ So you need to call unique_ptr<B> Destructor of ） Is in a specific compilation unit （A.cpp） As defined in .

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Solution ：

Reference resources ： https://stackoom.com/en/question/uHlM