声明C ++变量const帮助还是损害性能？ [英] Does declaring C++ variables const help or hurt performance?

问题描述

我了解 const - 资格类型的行为。我很好奇，如果有任何性能获得或损失从过热或缺乏热情的合格变量 const 。我特别想到在一个孤立的代码块中声明和使用的变量。例如：

I understand the behavior of const-qualified data types. I am curious, though, if there is any performance gain or loss from over- or under-zealousness of qualifying variables as const. I am thinking particularly of variables declared and used exclusively within an isolated code block. For example, something like:

const qreal padding = CalculatePadding();
const QSizeF page_size = CalculatePagePreviewSize(padding);
const QRectF content_rect = CalculatePagePreviewContentRect(page_size);
const QList<QRectF> pages = renderer.BuildPrintPages(printer_, map_scene_);
const QFont page_number_font = CalculatePageNumberFont();
const QFontMetrics metrics(page_number_font);

假设我只需要 const 在所有这些（和更多。）是否有任何性能获益，宣布他们 const ？

Suppose I only need const-qualified methods on all of these (and more.) Is there any performance gain in declaring them all const? Or, conversely, does this actually hurt performance?

我对运行时的性能感到好奇（我猜这没有什么区别，因为 const 只是编译时检查 - 有人可以确认吗？）和编译时性能。我没有足够的经验与c ++有这种感觉，我想知道如果我应该错误的过度或低于应用程序 const 当所有其他事情（维护性等）相等。

I am curious for both run-time performance (I am guessing this makes no difference as the const is exclusively a compile-time check--can someone confirm?) and compile-time performance. I do not have enough experience with c++ to have a feel for this, and am wondering if I should err on the side of over- or under-applying const when all other things (maintainability, etc.) are equal.

推荐答案

const 一个编译时事情，然而，声明一些 const 有时允许某些优化。如果有问题的代码不是性能瓶颈，我不担心它，只是使用 const 意图：产生更清晰的代码，防止自己做蠢事。

const is mainly a compile-time thing, however, declaring something as const sometimes allows for certain optimizations. If the code in question isn't a performance bottleneck, I wouldn't worry about it and just use const as intended: to produce clearer code and prevent yourself from doing stupid things.

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