問題描述

在 Stack Overflow 上閱讀有關 C++ 迭代器和性能的各種問題**，我開始懷疑 for(auto& elem : container) 是否被編譯器擴展"為最佳版本?(有點像 auto，編譯器會立即推斷出正確的類型，因此永遠不會變慢，有時會更快).

Reading various questions here on Stack Overflow about C++ iterators and performance**, I started wondering if for(auto& elem : container) gets "expanded" by the compiler into the best possible version? (Kind of like auto, which the compiler infers into the right type right away and is therefore never slower and sometimes faster).

** 比如你寫的有沒有關系

** For example, does it matter if you write

for(iterator it = container.begin(), eit = container.end(); it != eit; ++it)

或

for(iterator it = container.begin(); it != container.end(); ++it)

對于非失效容器?

推薦答案

標準是你的朋友，見[stmt.ranged]/1

The Standard is your friend, see [stmt.ranged]/1

對于表單的基于范圍的語句

For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

讓 range-init 等價于括號括起來的表達式

let range-init be equivalent to the expression surrounded by parentheses

( expression )

以及基于范圍的 for 形式的語句

and for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

讓 range-init 等同于花括號初始化列表.在每種情況下，基于范圍的 for 語句等效于

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
             __end = end-expr;
        __begin != __end;
        ++__begin )
  {
    for-range-declaration = *__begin;
    statement
  }
}

是的，該標準保證實現最佳形式.

So yes, the Standard guarantees that the best possible form is achieved.

對于許多容器，例如 vector，在此迭代期間修改(插入/擦除)它們是未定義的行為.

And for a number of containers, such as vector, it is undefined behavior to modify (insert/erase) them during this iteration.

這篇關于基于范圍的 for 循環對性能有益嗎?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！