Question

比方说我有一个

class Dictionary
{
vector<string> words;  
void addWord(string word)//adds to words
{
/...
}
bool contains(string word)//only reads from words
{
//...
}
}

Is there a way to make compiler check that contains isnt changing words vector. Ofc this is just an example with one class data member, I would like it to work with any number of data members. P.S. I know i have no public: and private:, I left it out intentionally to make code shorter and problem clearer.

Answer 1

如果希望编译器强制执行此操作，请声明成员函数＜code＞const＜/code＞：

bool contains(string word) const
{
    ...
}

const函数不允许修改其成员变量，只能调用其他const成员函数（其自身或其成员变量）。

此规则的例外情况是，如果成员变量被声明为＜code＞可变的[但是mutable不应用作通用的const解决方法；它只适用于对象的“可观察”状态应为const，但内部实现（如引用计数或延迟求值）仍需要更改的情况。]

还要注意，＜code＞const＜/code＞不会通过例如指针传播。

综上所述：

class Thingy
{
public:
    void apple() const;
    void banana();
};

class Blah
{
private:
    Thingy t;
    int *p;
    mutable int a;

public:
    Blah() { p = new int; *p = 5; }
    ~Blah() { delete p; }

    void bar() const {}
    void baz() {}

    void foo() const
    {
        p = new int;  // INVALID: p is const in this context
        *p = 10;      // VALID: *p isn t const

        baz();        // INVALID: baz() is not declared const
        bar();        // VALID: bar() is declared const

        t.banana();   // INVALID: Thingy::banana() is not declared const
        t.apple();    // VALID: Thingy::apple() is declared const

        a = 42;       // VALID: a is declared mutable
    }
};

Answer 2

将它们标记为＜code＞const＜/code＞：

bool contains(string word) const
//                        ^^^^^^

Another positive thing: You can only call other const member functions! :) Yet another good thing: You re allowed to call those functions on const objects of your class, example:

void foo(const Dictionary& dict){
  //  dict  is constant, can t be changed, can only call  const  member functions
  if(dict.contains("hi")){
    // ...
  }
  // this will make the compiler error out:
  dict.addWord("oops");
}

Answer 3

通常将方法声明为“const”可以实现以下目的：

bool contains(string word) const
// ...

编译器将告诉您是否在类成员上使用任何不是const的方法。还要注意，您可以通过引用传递字符串以避免复制（使＜code＞word＜/code＞参数＜code＞std:：string const&；＜/code〕）。

Answer 4

将函数声明为const：

void addWord(string word) const { /... }

If you try to change any members inside the body, the compiler will give an error. Also note that inside a method declared const, you can t call other methods that are not declared const.

Answer 5

使成员函数常量：

bool contains(string word) const
{
//...
}

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