默认assigment operator =在c ++中是一个浅拷贝？ [英] Default assigment operator= in c++ is a shallow copy?

问题描述

只是一个简单的快速问题，我无法找到一个坚实的答案其他地方。

Just a simple quick question which I couldn't find a solid answer to anywhere else. Is the default operator= just a shallow copy of all the class' members on the right hand side?

Class foo {
public:
  int a, b, c;
};

foo f1, f2;
...
f1 = f2;

将等同于：

f1.a = f2.a;
f1.b = f2.b;
f1.c = f2.c;

这似乎是真的，当我测试它，但我需要确保我不缺少一些具体情况。

This seems to be true when I test it but I need to be sure I'm not missing some specific case.

推荐答案

我会说，默认 operator = 。它复制每个成员。

I'd say, default operator= is a copy. It copies each member.

浅拷贝和深拷贝之间的区别不会出现，除非被拷贝的成员是某种类型的间接，如指针。至于默认的 operator = ，由被复制的成员是什么复制的意思，它可以是深或浅。

The distinction between a shallow copy and a deep copy doesn't arise unless the members being copied are some kind of indirection such as a pointer. As far as the default operator= is concerned, it's up to the member being copied what "copy" means, it could be deep or shallow.

具体来说，虽然，复制一个原始指针只是复制指针值，它不做任何事情与referand。因此，包含指针成员的对象默认是浅复制的 operator = 。

Specifically, though, copying a raw pointer just copies the pointer value, it doesn't do anything with the referand. So objects containing pointer members are shallow-copied by default operator=.

对复制执行克隆操作，所以如果你使用那些无处不在的原始指针，那么默认的 operator = 将执行深拷贝。

There are various efforts at writing smart pointers that perform clone operations on copying, so if you use those everywhere in place of raw pointers then the default operator= will perform a deep copy.

如果你的对象有任何标准容器作为成员，那么（例如）Java程序员可能会说 operator = 浅拷贝。在Java中， Vector 成员只是一个引用，所以shallow copy意味着 Vector ：源和目标是指相同的底层向量对象。在C ++中，一个向量成员将被复制，连同其内容，因为成员是一个实际对象而不是引用（和 vector :: operator = 保证内容被复制）

If your object has any standard containers as members, then it may be confusing to (for example) a Java programmer to say that operator= is a "shallow copy". In Java a Vector member is really just a reference, so "shallow copy" means that Vector members aren't cloned: source and destination refer to the same underlying vector object. In C++ a vector member will be copied, along with its contents, since the member is an actual object not a reference (and vector::operator= guarantees the contents are copied with it).

如果你的数据成员是一个指针的向量，没有深度副本或。你有一个半深的副本，其中源和目标对象有独立的向量，但每个静态对应的向量元素指向同一个未克隆的对象。

If your data member is a vector of pointers, then you don't have either a deep copy or a shallow copy. You have a semi-deep copy, where the source and destination objects have separate vectors, but the corresponding vector elements from each still point to the same, uncloned object.

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