++迭代器和迭代器++之间的性能差异？ [英] Performance difference between ++iterator and iterator++?

问题描述

我的同事宣称，对于对象类型，preincrement比post increment更有效

My workmate claims that for object types preincrement is more efficient than post increment

例如

std::vector<std::string> vec;

... insert a whole bunch of strings into vec ...

// iterate over and do stuff with vec.  Is this more efficient than the next 
// loop?
std::vector<std::string>::iterator it;
for (it = vec.begin(); it != vec.end(); ++it){

}

// iterate over and do stuff with vec.  Is this less efficient than the previous loop?
std::vector<std::string>::iterator it;
for (it = vec.begin(); it != vec.end(); it++){

}

推荐答案

Postincrement必须返回迭代器在递增之前的值;所以，以前的值需要被复制到某个地方，然后改变它与适当的增量，所以它可以返回。额外的工作可能有点或很多，但它肯定不能小于零，相比预增量，它可以简单地执行增量，然后返回刚刚更改的值 - 无复制//保存//等等。

Postincrement must return the value the iterator had BEFORE it was incrementing; so, that previous value needs to be copied somewhere before altering it with the increment proper, so it's available to return. The extra work may be a little or a lot, but it certainly can't be less than zero, compared to a preincrement, which can simply perform the incrementing and then return the just-altered value -- no copying // saving // etc necessary.

所以，除非你特别必须有postincrement（因为你在某种程度上使用value before increment），你应该总是使用preincrement。

So, unless you specifically MUST have postincrement (because you're using the "value before increment" in some way), you should always use preincrement instead.

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