C++从重载的复制赋值运算符中调用默认的复制赋值操作符

Question

我想使用复制分配运算符的默认功能，但可以作为操作的一部分执行一些附加任务。因此，基本形式如下所示：

class Test
{
    void operator=(Test& that)
    {
        *this = that; //do the default copy operation
        this->foo()  //perform some other tasks
    }
};

这可以通过创建一个copy（）函数很容易地完成，但最好保持"="操作的干净性。

Answer 1

您可以使用一个基本实现类并从子类委托给基本operator=

// Fill this class with your implementation.
class ClsImpl {
  // some complicated class
 protected:
  ~ClsImpl() = default;
};
class Cls : public ClsImpl {
 public:
  Cls& operator=(const Cls& other) {
    if (this == &other) { return *this; }
    // assign using the base class's operator=
    ClsImpl::operator=(other); // default operator= in ClsImpl
    this->foo(); // perform some other task
    return *this;
  }
};

Answer 2

一种方法是从派生类中再次派生，以提供复制后逻辑。

#include <iostream>

// a wrapper class to provide custom copy actions
template<class Base>
struct CopyActions : Base
{
    using base_class = Base;
    using CopyActions::base_class::base_class;
    // copy operator will call the base and then perform custom action
    CopyActions& operator=(const CopyActions& r) {
        base_class::operator=(r);
        onCustomCopy(r, *this);
        return *this;
    }
};
// a class to notify us when a copy takes place, without having to write
// custom copy operators
struct copy_sentinel
{
    copy_sentinel operator=(const copy_sentinel&) {
        std::cout << "copying " << name << 'n';
        return *this;
    }
    const char* name;
};

int test_count = 0;
// a model base class
struct MyBase
{
    int base_count = test_count++;
    copy_sentinel base_s { "MyBase" };
};
// a model derived class containing only logic
struct MyDerived : MyBase
{
    int derived_count = test_count++;
    copy_sentinel base_s { "MyDerived" };
};
// a custom copy action (free function)
void onCustomCopy(const MyDerived& from, MyDerived& to)
{
    std::cout << "custom copy actionn";
}
// our derived class with custom copy actions
using SuperDerived = CopyActions<MyDerived>;
// test
int main()
{
    SuperDerived a;  // 0, 1
    SuperDerived b;  // 2, 3
    // prove initial values
    std::cout << a.base_count << ", " << a.derived_count << std::endl;
    // perform copy and report actions
    a = b;
    // prove a copy occurred
    std::cout << a.base_count << ", " << a.derived_count << std::endl;
}

预期结果：

0, 1
copying MyBase
copying MyDerived
custom copy action
2, 3