刨根问底系列之C++ const 挖掘
#include <iostream>using namespace std;const int MAX_FRIEND = 100;class Player{ public: enum { boy = 1, girl = 0 }; Player() : user_id_(0), user_sex_(boy){} ~Player(){} Player(const Player&); const Player& operator=(const Player&); void SetUserId(int user_id); int GetUserId() const; void SetGroupId(int group_id) const; int GetGroupId() const; void SetUserAge(int user_age) const; int GetUserAge() const; void SetUserName(const char* user_name); void PrintPlayerInfo() const; private: int user_id_; mutable int group_id_; int user_age_; string user_name_; const char user_sex_; static const int friend_count_ = MAX_FRIEND;};Player::Player(const Player& player):user_sex_(boy) {}const Player& Player::operator=(const Player& other) {}void Player::SetUserId(int user_id) { user_id_ = user_id;}int Player::GetUserId() const { return user_id_;}void Player::SetUserAge(int user_age) const { const_cast<Player*>(this)->user_age_ = user_age;}int Player::GetUserAge() const { return user_age_;}void Player::SetGroupId(int group_id) const { group_id_ = group_id;}int Player::GetGroupId() const { return group_id_;}void Player::SetUserName(const char* user_name) { user_name_ = user_name;}void Player::PrintPlayerInfo() const { cout<<"user id:\t"<<user_id_<<endl; cout<<"user name:\t"<<user_name_<<endl; cout<<"user sex:\t"<<user_sex_<<endl; cout<<"user age:\t"<<user_age_<<endl; cout<<"user group:\t"<<group_id_<<endl;}int main() { const Player player1; //player1.SetUserId(100001); player1.GetUserId(); Player player2; player2.SetUserId(100001); player2.SetGroupId(100); player2.SetUserAge(21); player2.SetUserName("Jack"); player2.PrintPlayerInfo(); return 0;}
通过例子2.3可以看出const objects只能读取内存的值，不能改写内存的值。对于const member function，member data group_id_ 是按逻辑申明的可改变变量。而在SetUserAge(int user_age)函数里通过强转this指针，来改变member data。
文章里所示程序在gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)环境里编译通过，全文完。
参考：
c++ 编程思想卷一
effective c++ 第三版
