c++ - 为什么有人将对象定义为指针？

Question

我目前正在学习C++，我无法理解为什么没有明显的理由将一个类的对象定义为指针(即Car *c = new Car;而不是Car c;)？
举个例子:

#include <iostream>

class Car {
   public:
   void accelerate() { std::cout << "Accelerating!.\n"; }
};

int main() {
   Car *c = new Car;

   c->accelerate();

   return 0;
}

当这样做更直观时，为什么要这样做呢？

#include <iostream>

class Car {
   public:
   void accelerate() { std::cout << "Accelerating!.\n"; }
};

int main() {
   Car c;

   c.accelerate();

   return 0;
}

如果需要指向该对象的指针，则可以始终声明一个指针，不是吗？

Answer 1

why one would define an object of a class as a pointer

一个不应该。通常应尽可能避免使用指针。但是，当您进行多态时，它们是必需的。在这种情况下，请使用智能指针 shared_ptr， unique_ptr代替原始指针。
以下是使用指针的错误示例，因为现在您还有另一个问题，即“释放分配的内存”。

int main() {
   Car *c = new Car;
   c->accelerate();
   return 0;
}

没错，第二个示例要好得多，它应该是默认的处理方式。
每当出现此类问题时，最好看看 C++ Core Guidelines怎么说:

R.11: Avoid calling new and delete explicitly

Reason

The pointer returned by new should belong to a resource handle (that can call delete). If the pointer returned by new is assigned to a plain/naked pointer, the object can be leaked.

ES.60: Avoid new and delete outside resource management functions

Reason

Direct resource management in application code is error-prone and tedious.

R.3: A raw pointer (a T*) is non-owning

Reason

There is nothing (in the C++ standard or in most code) to say otherwise and most raw pointers are non-owning. We want owning pointers identified so that we can reliably and efficiently delete the objects pointed to by owning pointers.(Owning pointers are pointers which take ownership of a pointer and are responsible for freeing it.)

Example

void f()
{
   int* p1 = new int{7};           // bad: raw owning pointer
   auto p2 = make_unique<int>(7);  // OK: the int is owned by a unique pointer
   // ...
}

因此，答案是仅在绝对需要时才使用指针，否则请坚持引用和值。
什么时候使用指针？

在做多态时(使用智能指针)

当由于堆栈大小有限而需要一个巨大的数组(> 1MB)时。 (2 - 8 MB(usually) on linux，1 MB on windows)。如果可以的话，在这种情况下，最好使用std::vector。

当您使用“C”库或处理旧版C++代码时，有时可能需要使用指针。