Copy constructor for a class with unique_ptr

对一个有unique_ptr类型成员变量的class，它的copy constructor要如何写？总的来说，需要deep copy，关于此，在 Smart-pointers\Guideline-deep-copy 章节进行了讨论。

stackoverflow Copy constructor for a class with unique_ptr

How do I implement a copy constructor for a class that has a unique_ptr member variable? I am only considering C++11.

A

Since the unique_ptr can not be shared, you need to either deep-copy its content or convert the unique_ptr to a shared_ptr.

NOTE: 下面code，使用的是deep copy

#include <memory>
class A
{
    std::unique_ptr<int> up_;

public:
    A(int i) :
                    up_(new int(i))
    {
    }
    A(const A &a) :
                    up_(new int(*a.up_))
    {
    }
};

int main()
{
    A a(42);
    A b = a;
}
// g++ --std=c++11 test.cpp

You can, as NPE mentioned, use a move-ctor instead of a copy-ctor but that would result in different semantics of your class. A move-ctor would need to make the member as moveable explicitly via std::move:

A( A&& a ) : up_( std::move( a.up_ ) ) {}

Having a complete set of the necessary operators also leads to

A& operator=( const A& a )
{
   up_.reset( new int( *a.up_ ) );
   return *this,
}

A& operator=( A&& a )
{
   up_ = std::move( a.up_ );
   return *this,
}

NOTE: 完整如下:

#include <memory>
class A
{
  std::unique_ptr<int> up_;

public:
  A(int i) :
                  up_(new int(i))
  {
  }
  A(const A &a) :
                  up_(new int(*a.up_))
  {
  }
  A& operator=(const A &a)
  {
      up_.reset(new int(*a.up_));
      return *this;
  }

  A& operator=(A &&a)
  {
      up_ = std::move(a.up_);
      return *this;
  }
};

int main()
{
  A a(42);
  A b = a;
}
// g++ --std=c++11 test.cpp

If you want to use your class in a std::vector, you basically have to decide if the vector shall be the unique owner of an object, in which case it would be sufficient to make the class moveable, but not copyable. If you leave out the copy-ctor and copy-assignment, the compiler will guide your way on how to use a std::vector with move-only types.

使用std::make_unique

具体的例子参见 wikipedia Opaque pointer，它他收录在 C++\Idiom\OOP\Pointer-to-implementation-PImpl\Opaque-data-type\Opaque-pointer 章节中。