Virtual assignment operator

不应该使用virtual assignment operator

在下面文章中提出了这个观点:

1、sonarsource Assignment operators should not be "virtual"

2、C++ Core Guidelines C.60 - Make copy assignment non-virtual, take the parameter by const&, and return by non-const&

3、C++ Core Guidelines C.63 - Make move assignment non-virtual, take the parameter by &&, and return by non-const &

sonarsource Assignment operators should not be "virtual"

C++ does not support polymorphic copy or move assignment operators.

For example, the signature of a copy assignment operator on a "Base" class would be Base& operator=(const Base& other). And on a "Derived" class that extends "Base", it would be Derived& operator=(const Derived& other).

NOTE:

1、上述说法是片面的，还是有办法实现virtual assignment operator的，下面的

Because these are two entirely different method signatures, the second method does not override the first, and adding virtual to the "Base" signature does not change which method is called.

NOTE:

1、这段话让我想到了C++仅仅支持Covariant return type，并不支持 Covariant function parameter

It is possible to add an operator= override in a derived class, but doing so is an indication that you may need to reexamine your application architecture.

Noncompliant Code Example

NOTE: Noncompliant

class Base {
public:
  virtual Base& operator=(const Base& other); // Noncompliant
};

class Derived : public Base {
public:
  Derived& operator=(const Derived& other);
};

NOTE: 测试程序如下:

class Base
{
public:
  virtual Base& operator=(const Base &other) // Noncompliant
  {
      return *this;
  }
};

class Derived: public Base
{
public:
  Derived& operator=(const Derived &other)
  {
      return *this;
  }
};
int main()
{

}
// g++ -Wall -pedantic test.cpp && ./a.out

Compliant Solution

NOTE:

1、"Compliant"的意思是:"顺从的；服从的；应允的"

class Base {
protected:
  Base& operator=(const Base& other); // not virtual
};

class Derived : public Base {
public:
  Derived& operator=(const Derived& other);
};

NOTE: 测试程序如下:

class Base
{
public:
  Base& operator=(const Base &other) // not virtual
  {
      return *this;
  }
};

class Derived: public Base
{
public:
  Derived& operator=(const Derived &other)
  {
      return *this;
  }
};
int main()
{

}
// g++ -Wall -pedantic test.cpp && ./a.out

See

1、C++ Core Guidelines C.60 - Make copy assignment non-virtual, take the parameter by const&, and return by non-const&

2、C++ Core Guidelines C.63 - Make move assignment non-virtual, take the parameter by &&, and return by non-const &

实现 virtual assignment operator

如果实在要使用virtual assignment operator，那如何实现呢？本节讨论这个。

在 stackoverflow What is object slicing? # A 中，给出了virtual assignment operator的实现，这篇文章收录在了Object-slicing 章节。

TODO root.cer When to use virtual assignment