C++ 关键字 const

9. Const

• declares a variable to have a constant value
• Constants are variables
  • Observe(遵守) scoping rules
  • Declared with "const" type modifier

9.1 Compile time constants

• 编译时刻，可以确定值
  • value must be initialized
  • unless you make an explicit extern declaration;

const int bufsize = 1024;

// extern 声明
extern const int bufsize;

9.2 Run-time constants

const int class_size = 12;
int finalGrade[class_size];  // ok

int x;
cin >> x;
const int size = x;
double classAverage[size];   // error

9.3 Pointers and const

char* const q = "abc";  // q is const
*q = 'c';   // OK
q++;        // ERROR


const char *p = "ABCD";  // (*p) is a const char
*p = 'b';  // ERROR


Person p1("Fred", 200);
const Person* p = &p1;
Person const* p = &p1;
Person *const p = &p1;

// const 位于星号前面，对象是 const
// const 位于星号后面，指针是 const

9.4 示例

• a.cpp

#include <iostream>
using namespace std;

int main()
{
    //char *s = "hello world";
    // "hello world": 位于代码段, 代码段的内容，不能写

    char s[] = "hello world";
    // "hello world": 位于栈中

    cout << s << endl;

    s[0] = 'B';

    cout << s << endl;

    return 0;
}

9.5 const 函数

• Repeat the const keyword in the definition as well as the declaration
• Function members that do not modify data should be declared const.
• const memeber functions are safe for const objects.

int Date::set_day(int d) {
    day = d;
}

int Date::get_day() const {
    day++;      // ERROR modifies data member
    
    set_day(12);    // ERROR calls non-const member

    return day;     // ok
}

9.6 类中的 const

• main.cpp

#include <iostream>
using namespace std;

class A {
    int i;
public:
    A() : i(0) {}
    void f() { cout << "f()" << endl; }
    void f() const { cout << "f() const" << endl; }
};

int main()
{
    const A a;
    a.f();

    return 0;
}

// 解释：
// void f()        实际为： void f(A* this)
// void f() const  实际为： void f(const A* this)

• 运行结果
  

参考链接：

• 翁凯 C++