递增迭代器:++it 比 it++ 更高效吗?

Incrementing iterators: Is ++it more efficient than it++?(递增迭代器:++it 比 it++ 更高效吗?)

问题描述

可能重复:
i++和++之间是否存在性能差异我在 C++ 中?

我正在编写一个程序，其中使用迭代器循环 std::vector.有人告诉我，在 for 语句中执行 ++it 会导致代码更高效.换句话说，他们是在说:

I am writing a program where an iterator is used to loop through a std::vector. Somebody told me that doing ++it in the for statement leads to more efficient code. In other words, they are saying that:

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); ++it )

跑得比

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); it++ )

这是真的吗?如果是，效率提升背后的原因是什么?它++/++所做的只是将迭代器移动到向量中的下一项，不是吗?

Is this true? If it is, what is the reason behind the efficiency improvement? All it++/++it does is move the iterator to the next item in the vector, isn't it?

推荐答案

前置增量更快的原因是后置增量必须复制旧值才能返回.正如 GotW #2 所说，前增量比后增量更有效，因为对于后增量对象必须自增，然后返回一个包含其旧值的临时值.请注意，即使对于像 int 这样的内置函数也是如此."

The reason behind the preincrement being faster is that post-increment has to make a copy of the old value to return. As GotW #2 put it, "Preincrement is more efficient than postincrement, because for postincrement the object must increment itself and then return a temporary containing its old value. Note that this is true even for builtins like int."

GotW #55 提供了 postincrement 的规范形式，这表明它必须做预增量加上更多的工作:

GotW #55 provides the canonical form of postincrement, which shows that it has to do preincrement plus some more work:

T T::operator++(int)
{
  T old( *this ); // remember our original value
  ++*this;        // always implement postincrement
                  //  in terms of preincrement
  return old;     // return our original value
}

正如其他人所指出的，某些编译器可能会在某些情况下对此进行优化，但如果您不使用返回值，最好不要依赖此优化.此外，对于具有微不足道的复制构造函数的类型，性能差异可能非常小，尽管我认为在 C++ 中使用预增量是一个好习惯.

As others have noted, it's possible for some compiler to optimize this away in some cases, but if you're not using the return value it's a good idea not to rely on this optimization. Also, the performance difference is likely to be very small for types which have trivial copy constructors, though I think using preincrement is a good habit in C++.

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