标记所有不修改const的变量是否有不利之处？ [英] Are there any downsides to marking all variables you don't modify const?

问题描述

经过大量的搜索之后，我发现了很多关于将函数及其参数标记为 const 的信息，但是没有有关将变量标记为 const的指南。

After much googling I've found a lot about marking functions and their parameters as const, but no guide on marking variables as const.

这是一个非常简单的示例：

Here's a really simple example:

#include <string>
#include <iostream>

void example(const std::string& x) {
  size_t length = x.length();
  for (size_t i = 0; i < length; ++i) {
    std::cout << x.at(i) << std::endl;
  }
}

int main() {
  example("hello");
}

为什么不赚钱

size_t length = x.length();

const like

const like

const size_t length = x.length();

按惯例？

我知道这样一个小而简单的示例确实并没有显示出任何大的好处，但是似乎在较大的代码库中很有用，在该代码库中，您可能会偶然地突变了不应该突变的变量。

I know such a small, simple example really doesn't show any huge benefit to this, but it seems like it'd be helpful in a larger codebase where you might accidentally mutate a variable you shouldn't have mutated.

尽管有这样的好处，但我并没有真正看到它使用太多（在我见过的C ++代码库中），也几乎没有提到使函数及其参数 const 。

Despite that benefit, I don't really see it used that much (in the C++ codebases I've seen) or mentioned nearly as much as making functions and their parameters const.

除了必须输入5个额外字符以外，这样做还有其他缺点吗？我没有在这个主题上找到太多东西，如果有这么多const的问题，我也不想to脚。

Is there some downside to doing this other than having to type 5 extra characters? I haven't found much on the topic, and I don't want to shoot myself in the foot if it's a problem having so many consts.

推荐答案

标记没有修改的变量没有任何缺点 const 。

There are no downsides to marking variables you don't modify const.

有但是有一些好处：编译器将帮助您诊断，当您无意中修改了您不应该/不想要的变量时，编译器可能（尽管由于该语言的 const_cast 和 mutable 这种情况很少）会生成更好的代码。

There are some up-sides though: the compiler will help you diagnose when you unintentionally modify a variable you shouldn't/didn't mean to and the compiler may (although due to the language having const_cast and mutable this is rare) generate better code.

因此，我建议;在可能的地方使用 const 。没有缺点，编译器可以帮助您发现错误。没有理由（除了一些额外的输入）。

So, I'd advise; use const where you can. There are no downsides and your compiler can potentially help you spot bugs. No reason not to (except for a bit of extra typing).

请注意，这也扩展到成员函数。尽可能使它们为 const -它使它们可以在更多上下文中使用，并帮助用户推理代码（调用此函数不会修改对象是宝贵的信息）。

Note that this extends to member functions as well. Make them const when you can - it lets them be used in more contexts and helps users reason about the code ("calling this function won't modify the object" is valuable information).

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