C++ const

const qualifiers 在 C 和 C++ 裡面都很常見， 但是觀念不清楚的話就不能好好利用， 以下做些觀念紀錄。

加上 const 關鍵字可以確保不該被改到的變數不被更動， 如果不小心寫錯更動到的話， 會在編譯時噴出錯誤。

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const in declaration

由右至左解釋變數宣告：

const X p       // p is an X that is const
X const p       // p is a const X

const X * p     // p points to an X that is const
X const * p     // p points to a const X

X * const p     // p is a const pointer to an X

X const * const p     // p is a const pointer to a const X
const X * const p     // p is a const pointer to an X that is const

const X& p      // p is a reference to an X that is const
X const& p      // p is a reference to a const X

const in function parameters

void f1(const std::string& s);     // Pass by reference-to-const
void f2(const std::string* sptr);  // Pass by pointer-to-const
void f3(std::string s);            // Pass by value

const member function

const member function 又稱為 inspectors，不會更動 object 裡的東西，也就是對於這個 object 沒有副作用：

class MyClass {
public:
    void inspect() const;   // This member promises NOT to change *this
    void mutate();          // This member function might change *this
};

如果想要從 const member function 裡用 reference 的方式回傳 this object 裡的 member 的話， 必須使用 reference-to-const 或是 by value：

class MyClass {
public:

    // inspect member function will return ``this`` object's member

    const std::string& inspect_pass_1() const;    // Pass, the caller can't change the object
          std::string& inspect_fail_1() const;    // Compile-time error, the caller can change the object
    const std::string  inspect_pass_2() const;    // Pass, the caller can't change the object
          std::string  inspect_pass_2() const;    // Pass, the caller can't change the object

    const std::string& inspect_pass_3();          // Pass
          std::string& inspect_pass_4();          // Pass
    const std::string  inspect_pass_5();          // Pass
          std::string  inspect_pass_5();          // Pass
};

如果有 member 在 const member function 是會更動的， 但是仍然要把它視為 const member function， 則須把該 member 宣告為 mutable。

class MyClass {
public:

    void inspect() const { ++count; }

    mutable unsigned int count = 0;

};

const overloading

const-overloading 指的是有兩個名稱相同的 member function，差別只在 const， 所以 const 和 non-const 的 object 會呼叫到不同函數， 形成 overloading。

class MyClass {
public:

    // Subscript operators often come in pairs

    const MyClass& operator[] (unsigned index) const;
          MyClass& operator[] (unsigned index);
};