问题描述
问题在于自我分配.例如，将向量复制到自身中:
The question is about self-assignment. For example copying a vector into itself:
std::vector<std::string> vec(5, "hello");
vec = vec;
上面的代码应该对自己执行5个字符串赋值操作，还是什么都不做?我的意思是以下检查是否有效:
Should the code above perform 5 assignment operations of strings into themselves, or just do nothing? I mean whether the following checking is valid:
std::vector operator=(const std::vector &rhs)
{
if (this == &rhs)
{ return *this; }
...
}
我正在开发自己的std::variant类的实现(只是为了好玩)，并且对我是否应该在自赋值运算符的开头添加自赋值检查感兴趣，还是应该将包含的元素复制到自身中而感兴趣?
I'm working on my own implementation of std::variant class (just for fun) and interested if I should add a self-assignment check to the beginning of the assignment operator, or should I just copy the contained element into itself?
我知道通常这并不重要.您不应该创建一个利用复制本身的事实的类.但是我很想知道标准是否对此有所说明.
I understand that generally this doesn't matter. You should not make a class that utilizes the fact of copying into itself. But I'm interested if the standard says anything about this.
推荐答案
由标准指定的容器分配的前提条件(引用最新草案):
The pre/post conditions of assignment of a container specified by the standard (quoting latest draft):
r = a
确保:r == a.
这允许但不强制执行自我分配检查.
This allows but does not mandate self assignment check.
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