C++ 为什么在方法或函数名之前和之后使用 const 关键字？
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Why using the const keyword before and after method or function name?
提问by Vick_Pk
I have the following code in my application. Why do we use the constkeyword with the return type and after the method name?
我的应用程序中有以下代码。为什么我们const在返回类型和方法名称之后使用关键字？
const T& data() const { return data_; }
回答by stardust
const T& data() const { return data_; }
^^^^^
means it will return a constreference to T(here data_)
意味着它将返回const对T（此处data_）的引用
Class c;
T& t = c.data() // Not allowed.
const T& tc = c.data() // OK.
const T& data() const { return data_; }
^^^^^
means the function will not modify any member variables of the class (unless the member is mutable).
意味着函数不会修改类的任何成员变量（除非成员是mutable）。
void Class::data() const {
this->data_ = ...; // is not allowed here since data() is const (unless 'data_' is mutable)
this->anything = ... // Not allowed unless the thing is 'mutable'
}
回答by Collin Dauphinee
The const(and volatile) qualifier binds to the left. This means that any time you see const, it is being applied to the token to the left of it. There is one exception, however; if there's nothing to the left of the const, it binds to the right, instead. It's important to remember these rules.
的const（和volatile）限定符结合到左侧。这意味着任何时候你看到const，它都会被应用到它左边的标记上。然而，有一个例外；如果 左边没有任何东西const，它会绑定到右边。记住这些规则很重要。
In your example, the first consthas nothing to the left of it, so it's binding to the right, which is T. This means that the return type is a reference to a const T.
在您的示例中，第一个const左侧没有任何内容，因此它绑定到右侧，即T. 这意味着返回类型是对 a 的引用const T。
The second const does have something to the left of it; the function data(). This means that the constwill bind to the function, making it a constfunction.
第二个 const 的左边确实有一些东西；功能data()。这意味着const将绑定到函数，使其成为一个const函数。
In the end, we have a const function returning a reference to a const T.
最后，我们有一个const 函数返回对 const T 的引用。
回答by Ed Heal
The first constmeans the function is returning a const Treference.
第一个const意味着该函数正在返回一个const T引用。
The second one says that the method is not changing the state of the object. I.e. the method does not change any member variables.
第二个说该方法不会改变对象的状态。即该方法不会更改任何成员变量。
回答by shivakumar
const T& data() const { return data_; }
const after member function indicates that data is a constant member function and in this member function no data members are modified.
const after member function 表示 data 是一个常量成员函数，并且在这个成员函数中没有修改数据成员。
const return type indicates returning a constant ref to T
const 返回类型表示向 T 返回一个常量 ref
