const-correctness的解释是什么？ [英] What is the definition of const-correctness?

问题描述

我认为常量正确性的概念定义得很好，但是当我与其他人讨论时，我们对它的含义似乎有不同的看法。有人说这是关于在尽可能多的位置使用 const注释的程序。其他人则是在且仅当使用const注释的地方没有违反constness的情况（即，编译器会为您检查的属性）时，才将程序定义为const正确。

I thought the concept of "const-correctness" was pretty well defined, but when I talked to other people about it, it seemed we had different ideas of what it means. Some people say it's about a program having the "const" annotations in as many places as possible. Others define a program as const-correct if and only if there are no violations of constness where the const annotation is used (ie. it's a property that the compiler checks for you).

所以我想知道以下哪个函数是const正确的：

So I'd like to know, which of these functions are const correct:

struct Person {
    string getName1() const { return _name; }
    string getName2() { return _name; }
    string getName3() const { _name = "John"; return _name; }
    string getName4() { _name = "John"; return _name; }

    string _name;
};

我在互联网上搜索过定义，但找不到真正的答案，我也怀疑可能会发生 citogenesis 的情况。那么有人可以为定义提供任何可靠的引用吗？

I've searched on the Internet for definitions but couldn't really find definite answer, and I also have suspicions that there might be a case of citogenesis at play. So could anyone provide any solid citation for a definition?

推荐答案

在我看来，常量正确性的意思是：

In my view "const correctness" means:

所有不打算修改的内容都应标记为 const 。

这意味着编译器可以在您输入错误时告诉您。 （在某些情况下，它可能还会对优化产生影响，但这更多是次要的收益，取决于许多其他因素。）

This means that the compiler can tell you when you make a mistake. (It also possibly has implications for optimisations in certain conditions, but that's more of a secondary benefit and depends on a lot of other factors)

很有用：

1. 成员函数可以标记为 const ，如下所示：你的例子。这意味着您从该函数内部访问成员变量就像变量本身是 const 。 （这是您显示的示例，尽管 getName3（）不起作用）

1. Member functions can be marked const, as in your examples. This means that your accesses to member variables from within that function will be as though the variables themselves were const. (This is the example you've shown, although getName3() won't work)

免费变量，局部变量和成员变量本身也可以标记为 const -初始化后就不能更改。示例-局部变量：

"Free" variables, local and member variables themselves may also be marked const - once initialised they may not be changed. Example - local variable:

int f(int i) {
   const int square = i*i;
   // do something here that uses, but doesn't change square
   return square;
}

或自由变量：

extern const double PI; // set somewhere else but not changed.

或成员变量：

class foo {
   const int state; // can't be changed once constructed
   foo(int s) : state(i) {}
};

函数自变量也可以标记为 const ，该定义仍可以匹配非const声明：

Function arguments can also be marked const, the definition can match a non-const declaration still:

void f(int i);
void f(const int i) {
    // the fact that i is const is an implementation detail of f here
}

作为旁注，在某些情况下， const 引用的正确性是必需的：

As a side note const correctness for references is required in some cases:

void g(const std::string& s);
void f(std::string& s);

这两个可以在更多地方使用：

Of these two one can be used in more places:

g("hi"); // Fine
f("hi"); // Not fine - you can't bind a temporary to a reference

当然，如果您 meant 更改 s 然后通过临时变量还是没有任何意义的。

Of course if you meant to change s then passing a temporary in makes little sense anyway.

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