在for循环中计算向量大小是昂贵的，每次迭代？ [英] Is it expensive to compute vector size in for loops, each iteration?

问题描述

c ++编译器是否处理像，建筑是矢量这样的情况：

  for（int i = 0; i< ; buildings.size（）; i ++）{} 
  

在循环中或不是，然后
基于不评估它每次迭代？或者也许我应该自己做，
不是那么漂亮，但：

  int n = buildings.size 
 for（int i = 0; i   

解决方案

buildings.size（）可能会被编译器直接访问向量< ; T> 类。所以你不应该分开对 size 的调用。这种微优化是你不想担心的东西（除非你在一些真正紧密的循环被识别为瓶颈通过分析）。

Does the c++ compiler take care of cases like, buildings is vector:

for (int i = 0; i < buildings.size(); i++) {}

that is, does it notice if buildings is modified in the loop or not, and then based on that not evaluate it each iteration? Or maybe I should do this myself, not that pretty but:

int n = buildings.size();
for (int i = 0; i < n; i++) {}

解决方案

buildings.size() will likely be inlined by the compiler to directly access the private size field on the vector<T> class. So you shouldn't separate the call to size. This kind of micro-optimization is something you don't want to worry about anyway (unless you're in some really tight loop identified as a bottleneck by profiling).

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