的一种方法是从派生类派生再次提供复制后的逻辑使用基本实现类和委托到基地operator=
。
#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 action\n";
}
// 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;
}
预计业绩:
0, 1
copying MyBase
copying MyDerived
custom copy action
2, 3
为什么你使用'static_cast'强制调用基类的实现,而不是仅仅通过'ClsImpl ::运算符=(其他)显式调用它;'。如果'ClsImpl'只是一种委托复制分配功能的方法,那么您应该使用私有继承来防止隐式转换并引入切片风险。 –
@CaptainObviously他们相同的事情,但省略演员是一种收益,我同意。如果私有继承不需要大量的'using'声明来公开所有基类的成员函数,那么私有继承将会非常好。 –
是的,很多'using'语句会吸引人。无论如何,使基类中的复制构造函数和复制赋值运算符都受到保护。 –