C ++转换为派生类 [英] C++ cast to derived class
问题描述
如何投射到派生类?以下方法都会产生以下错误:
How can i cast to a derived class? The below approaches all give the following error:
无法从BaseType转换为DerivedType。没有构造函数可以接受
源类型,或者构造函数重载解析是不明确的。
Cannot convert from BaseType to DerivedType. No constructor could take the source type, or constructor overload resolution was ambiguous.
BaseType m_baseType;
DerivedType m_derivedType = m_baseType; // gives same error
DerivedType m_derivedType = (DerivedType)m_baseType; // gives same error
DerivedType * m_derivedType = (DerivedType*) & m_baseType; // gives same error
推荐答案
p>
Think like this:
class Animal { /* Some virtual members */ };
class Dog: public Animal {};
class Cat: public Animal {};
Dog dog;
Cat cat;
Animal& AnimalRef1 = dog; // Notice no cast required. (Dogs and cats are animals).
Animal& AnimalRef2 = cat;
Animal* AnimalPtr1 = &dog;
Animal* AnimlaPtr2 = &cat;
Cat& catRef1 = dynamic_cast<Cat&>(AnimalRef1); // Throws an exception AnimalRef1 is a dog
Cat* catPtr1 = dynamic_cast<Cat*>(AnimalPtr1); // Returns NULL AnimalPtr1 is a dog
Cat& catRef2 = dynamic_cast<Cat&>(AnimalRef2); // Works
Cat* catPtr2 = dynamic_cast<Cat*>(AnimalPtr2); // Works
// This on the other hand makes no sense
// An animal object is not a cat. Therefore it can not be treated like a Cat.
Animal a;
Cat& catRef1 = dynamic_cast<Cat&>(a); // Throws an exception Its not a CAT
Cat* catPtr1 = dynamic_cast<Cat*>(&a); // Returns NULL Its not a CAT.
现在回头看第一条语句:
Now looking back at your first statement:
Animal animal = cat; // This works. But it slices the cat part out and just
// assigns the animal part of the object.
Cat bigCat = animal; // Makes no sense.
// An animal is not a cat!!!!!
Dog bigDog = bigCat; // A cat is not a dog !!!!
您很少需要使用动态转换。
这是为什么我们有虚方法:
You should very rarely ever need to use dynamic cast.
This is why we have virtual methods:
void makeNoise(Animal& animal)
{
animal.DoNoiseMake();
}
Dog dog;
Cat cat;
Duck duck;
Chicken chicken;
makeNoise(dog);
makeNoise(cat);
makeNoise(duck);
makeNoise(chicken);
我可以想到的唯一原因是如果你将对象存储在一个基类容器中: p>
The only reason I can think of is if you stored your object in a base class container:
std::vector<Animal*> barnYard;
barnYard.push_back(&dog);
barnYard.push_back(&cat);
barnYard.push_back(&duck);
barnYard.push_back(&chicken);
Dog* dog = dynamic_cast<Dog*>(barnYard[1]); // Note: NULL as this was the cat.
但是如果你需要把特定的对象传回给Dogs,那么你的设计就有一个根本的问题。您应该通过虚拟方法访问属性。
But if you need to cast particular objects back to Dogs then there is a fundamental problem in your design. You should be accessing properties via the virtual methods.
barnYard[1]->DoNoiseMake();
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