C++成员函数中const的使用详解

void A::show(const int *b) {
    cout << "show const";
    //  error:  read-only variable is not assignable
    // *b = 2;
    cout << b << endl;
}

void A::show(const int b)
// void A::show(int b) // error class member cannot be redeclared
void display(int *num); // overload
void display(const int *num); // overload
void fun(A &a); // overload
void fun(const A &a); // overload

#include <iOStream>
using namespace std;
class A {
private:
    int a;
public:
    A(int a) {
        this->a = a;
    }
    void show(int b);
    // error redeclared
    // void show(const int b);
    void display(int *num); // ok
    void display(const int *num); // ok
    void fun(A &a);
    void fun(const A &a);
    void happy(int * h);
    void hour(const int * h);
};
void A::show(int b) {
    cout << "show: " << b << endl;
}
void A::display(int *num) {
    cout << "display:" << *num << endl;
}
void A::display(const int *num) {
    cout << "const display:" << *num << endl;
}
void A::fun(A &obj) {
    cout << "fun: " << obj.a << endl;
}
void A::fun(const A &obj) {
    cout << "const fun: " << obj.a << endl;
}
void A::happy(int *h) {
    cout << "happy:" << *h << endl;
}
void A::hour(const int *h) {
    cout << "const hour:" << *h << endl;
}
int main() {
    A a(1);
    const A a2(11);
    int b1 = 2;
    const int b2 = 3;
    // test overload
    a.show(b1);
    a.show(b2);
    a.display(&b1);
    a.display(&b2);
    a.fun(a);
    a.fun(a2);
    // test const
    a.happy(&b1);
    // a.happy(&b2); // error cannot initialize a parameter of type 'int *' with an rvalue of type 'const int *'
    a.hour(&b1);
    a.hour(&b2);
    return 0;
}
// ouptut
show: 2
show: 3
display:2
const display:3
fun: 1
const fun: 11
happy:2
const hour:2
const hour:3

#include <iostream>
using namespace std;
class Alice {
private:
    int a;
public:
    Alice(int a): a(a) {}
    int get_a() {return a;}
    const int* get_const_ptr() {return &a;}
    int* get_ptr() {return &a;}
};
int main() {
    Alice alice(1);
    int a1 = alice.get_a(); // ok
    cout << a1 << endl;
    const int a2 = alice.get_a(); // ok
    cout << a2 << endl;
    // error cannot initialize a variable of type 'int *' with an rvalue of type 'const int *'
    // int* b1 = alice.get_const_ptr();
    const int* b2 = alice.get_const_ptr(); // ok
    cout << *b2 << endl; // ok
    // *b2 = 3; // error read-only variable is not assignable
    *(alice.get_ptr()) = 3;
    cout << alice.get_a() << endl; // 3
    return 0;
}

class Alice {
private:
    int a;
public:
    Alice(int a): a(a) {}
    void show();
};
void Alice::show() {
    cout << "hello Alice" << endl;
}
int main() {
    const Alice a(1);
    //  error: 'this' argument to member function 'show' has type 'const Alice', but function is not marked const
    // a.show();
    return 0;
}

#include <iostream>
using namespace std;
class R {
public:
    R(int r1, int r2) {
        a = r1;
        b = r2;
    }
    void print();
    void print() const;
private:
    int a;
    int b;
};
void R::print() {
    cout << "nORMal print" << endl;
    cout << a << ", " << b << endl;
}
void R::print() const {
    cout << "const print" << endl;
    cout << a << ", " << b << endl;
}
int main() {
    R a(5, 3);
    a.print();
    const R b(6 ,6);
    b.print();
    return 0;
}
// output
normal print
5, 3
const print
6, 6