我在C ++中使用多重继承,并通过显式调用它们的基础来扩展基本方法。假设以下层次结构:
Creature
/ \
Swimmer Flier
\ /
Duck
哪个对应
class Creature
{
public:
virtual void print()
{
std::cout << "I'm a creature" << std::endl;
}
};
class Swimmer : public virtual Creature
{
public:
void print()
{
Creature::print();
std::cout << "I can swim" << std::endl;
}
};
class Flier : public virtual Creature
{
public:
void print()
{
Creature::print();
std::cout << "I can fly" << std::endl;
}
};
class Duck : public Flier, public Swimmer
{
public:
void print()
{
Flier::print();
Swimmer::print();
std::cout << "I'm a duck" << std::endl;
}
};
现在这提出了一个问题 - 调用duck的print
方法调用它各自的基本方法,所有这些方法又调用Creature::print()
方法,因此它最终被调用两次 -
I'm a creature
I can fly
I'm a creature
I can swim
I'm a duck
我想找到一种方法来确保只调用一次base方法。类似于虚拟继承的工作方式(在第一次调用时调用基础构造函数,然后在其他派生类的连续调用中仅指定指向它的指针)。
是否有一些内置的方法可以做到这一点,还是我们需要自己实现一个?
如果是这样,你会怎么做?
问题不是特定于印刷。我想知道是否有一种扩展基本方法和功能的机制,同时保持调用顺序并避免钻石问题。
我现在明白,最突出的解决方案是添加辅助方法,但我只是想知道是否有一种“更清洁”的方式。
这很可能是XY问题。但是......只是不要两次打电话。
#include <iostream>
class Creature
{
public:
virtual void identify()
{
std::cout << "I'm a creature" << std::endl;
}
};
class Swimmer : public virtual Creature
{
public:
virtual void identify() override
{
Creature::identify();
tell_ability();
std::cout << "I'm a swimmer\n";
}
virtual void tell_ability()
{
std::cout << "I can swim\n";
}
};
class Flier : public virtual Creature
{
public:
virtual void identify() override
{
Creature::identify();
tell_ability();
std::cout << "I'm a flier\n";
}
virtual void tell_ability()
{
std::cout << "I can fly\n";
}
};
class Duck : public Flier, public Swimmer
{
public:
virtual void tell_ability() override
{
Flier::tell_ability();
Swimmer::tell_ability();
}
virtual void identify() override
{
Creature::identify();
tell_ability();
std::cout << "I'm a duck\n";
}
};
int main()
{
Creature c;
c.identify();
std::cout << "------------------\n";
Swimmer s;
s.identify();
std::cout << "------------------\n";
Flier f;
f.identify();
std::cout << "------------------\n";
Duck d;
d.identify();
std::cout << "------------------\n";
}
I'm a creature
------------------
I'm a creature
I can swim
I'm a swimmer
------------------
I'm a creature
I can fly
I'm a flier
------------------
I'm a creature
I can fly
I can swim
I'm a duck
------------------
我们可以让基类跟踪属性:
#include <iostream>
#include <string>
#include <vector>
using namespace std::string_literals;
class Creature
{
public:
std::string const attribute{"I'm a creature"s};
std::vector<std::string> attributes{attribute};
virtual void print()
{
for (auto& i : attributes)
std::cout << i << std::endl;
}
};
class Swimmer : public virtual Creature
{
public:
Swimmer() { attributes.push_back(attribute); }
std::string const attribute{"I can swim"s};
};
class Flier : public virtual Creature
{
public:
Flier() { attributes.push_back(attribute); }
std::string const attribute{"I can fly"s};
};
class Duck : public Flier, public Swimmer
{
public:
Duck() { attributes.push_back(attribute); }
std::string const attribute{"I'm a duck"s};
};
int main()
{
Duck d;
d.print();
}
同样,如果它不仅仅是打印,而是函数调用,那么我们可以让基类跟踪函数:
#include <iostream>
#include <functional>
#include <vector>
class Creature
{
public:
std::vector<std::function<void()>> print_functions{[this] {Creature::print_this(); }};
virtual void print_this()
{
std::cout << "I'm a creature" << std::endl;
}
void print()
{
for (auto& f : print_functions)
f();
}
};
class Swimmer : public virtual Creature
{
public:
Swimmer() { print_functions.push_back([this] {Swimmer::print_this(); }); }
void print_this()
{
std::cout << "I can swim" << std::endl;
}
};
class Flier : public virtual Creature
{
public:
Flier() { print_functions.push_back([this] {Flier::print_this(); }); }
void print_this()
{
std::cout << "I can fly" << std::endl;
}
};
class Duck : public Flier, public Swimmer
{
public:
Duck() { print_functions.push_back([this] {Duck::print_this(); }); }
void print_this()
{
std::cout << "I'm a duck" << std::endl;
}
};
int main()
{
Duck d;
d.print();
}
一种简单的方法是创建一组辅助类,它们模仿主层次结构的继承结构,并在其构造函数中执行所有打印。
struct CreaturePrinter {
CreaturePrinter() {
std::cout << "I'm a creature\n";
}
};
struct FlierPrinter: virtual CreaturePrinter ...
struct SwimmerPrinter: virtual CreaturePrinter ...
struct DuckPrinter: FlierPrinter, SwimmerPrinter ...
然后,主层次结构中的每个print方法都会创建相应的帮助程序类。没有手动链接。
为了可维护性,您可以将每个打印机类嵌套在相应的主类中。
当然,在大多数现实世界中,您希望将对主对象的引用作为其助手的构造函数的参数传递。
您对print
方法的明确调用构成了问题的关键。
这方面的一种方法是放弃print
呼叫,并用say替换它们
void queue(std::set<std::string>& data)
并将打印消息累积到set
中。然后,层次结构中的那些函数不止一次被调用无关紧要。
然后,您可以在Creature
中使用单个方法实现集合的打印。
如果您想保留打印顺序,那么您需要将set
替换为另一个尊重插入顺序的容器并拒绝重复。
如果您想要那个中间类方法,请不要调用基类方法。最简单和最简单的方法是提取额外的方法,然后重新实现Print
很容易。
class Creature
{
public:
virtual void print()
{
std::cout << "I'm a creature" << std::endl;
}
};
class Swimmer : public virtual Creature
{
public:
void print()
{
Creature::print();
detailPrint();
}
void detailPrint()
{
std::cout << "I can swim" << std::endl;
}
};
class Flier : public virtual Creature
{
public:
void print()
{
Creature::print();
detailPrint();
}
void detailPrint()
{
std::cout << "I can fly" << std::endl;
}
};
class Duck : public Flier, public Swimmer
{
public:
void print()
{
Creature::Print();
Flier::detailPrint();
Swimmer::detailPrint();
detailPrint();
}
void detailPrint()
{
std::cout << "I'm a duck" << std::endl;
}
};
没有详细信息您的实际问题是什么,很难找到更好的解决方案。
使用:
template<typename Base, typename Derived>
bool is_dominant_descendant(Derived * x) {
return std::abs(
std::distance(
static_cast<char*>(static_cast<void*>(x)),
static_cast<char*>(static_cast<void*>(dynamic_cast<Base*>(x)))
)
) <= sizeof(Derived);
};
class Creature
{
public:
virtual void print()
{
std::cout << "I'm a creature" << std::endl;
}
};
class Walker : public virtual Creature
{
public:
void print()
{
if (is_dominant_descendant<Creature>(this))
Creature::print();
std::cout << "I can walk" << std::endl;
}
};
class Swimmer : public virtual Creature
{
public:
void print()
{
if (is_dominant_descendant<Creature>(this))
Creature::print();
std::cout << "I can swim" << std::endl;
}
};
class Flier : public virtual Creature
{
public:
void print()
{
if (is_dominant_descendant<Creature>(this))
Creature::print();
std::cout << "I can fly" << std::endl;
}
};
class Duck : public Flier, public Swimmer, public Walker
{
public:
void print()
{
Walker::print();
Swimmer::print();
Flier::print();
std::cout << "I'm a duck" << std::endl;
}
};
使用Visual Studio 2015,输出为:
I'm a creature
I can walk
I can swim
I can fly
I'm a duck
但is_dominant_descendant
没有便携式定义。我希望这是一个标准的概念。
您要求在函数级别继承自动调用继承函数并添加更多代码。您也希望它像类继承一样以虚拟方式完成。伪语法:
class Swimmer : public virtual Creature
{
public:
// Virtually inherit from Creature::print and extend it by another line of code
void print() : virtual Creature::print()
{
std::cout << "I can swim" << std::endl;
}
};
class Flier : public virtual Creature
{
public:
// Virtually inherit from Creature::print and extend it by another line of code
void print() : virtual Creature::print()
{
std::cout << "I can fly" << std::endl;
}
};
class Duck : public Flier, public Swimmer
{
public:
// Inherit from both prints. As they were created using "virtual function inheritance",
// this will "mix" them just like in virtual class inheritance
void print() : Flier::print(), Swimmer::print()
{
std::cout << "I'm a duck" << std::endl;
}
};
所以你的问题的答案
有没有一些内置的方法来做到这一点?
没有。 C ++中不存在这样的东西。另外,我不知道有任何其他语言有这样的东西。但这是一个有趣的想法......