内容简介:void
在C++中类型转换包含 static_cast、const_cast、dynamic_cast、reinterpret_cast 四种类型转换。使用原始类型转换,所有转换都是一种写法,可读性不高,有可能有潜在的风险
1、static_cast
该类型转换函数用于不同类型的属性之间的转换
#include <iostream> using namespace std; void* func(){ int i = 0; return &i; } void main(){ //自动类型转换 //int i = 0; //double d = i; //或 //double d = 9.5; //int i = d; // int i = 8; // double d = 9.5; // i = static_cast<int>(d); // cout << "i = " << i << endl; int i = 9; int *i_p = &i; }
void 转 char
void* func(int type){ switch(type){ case 1: int i = 11; return &i; case 2: int a = 'X'; return &a; default: return NULL; } } void main(){ //void* 转 char* //char* c_p = (char*)func(2); char* c_p = static_cast<char*>(func(2)); //C++ func //C }
2、const_cast
该类型转换函数用于去常量,一般使用在常量属性转为非常量属性时使用
void func(char c[]){ //可以修改 c[0] = 'H'; c[1] = 'a'; } void func2(const char c[]){ //c[0] = 'H'; //不能修改 //c[1] = 'a';//不能修改 //通过指针间接赋值 //其他人并不知道,这次转型是为了去常量 //char* c_p = (char*)c; //c_p[1] = 'a'; //cout << c << endl; //C++ 去常量,可读性高 char* c_p = const_cast<char*>(c); c_p[1] = 'y'; cout << c << endl; } void main(){ char c[] = "hello"; func2(c); getchar(); }
3、dynamic_cast
该类型转换用于父类对象转换为子类时使用,处理父类转为子类时的不确定性(类型不匹配时转型失败,返回NULL)
class Person{ public: virtual void print(){ cout << "人" <<endl; } }; class Man : public Person{ public: void print(){ cout << "男人" <<endl; } void smoking(){ cout << "吸烟" <<endl; } }; class Woman : public Person{ public: void print(){ cout << "女人" <<endl; } void makeup(){ cout << "化妆" <<endl; } }; void func(Person* obj){ obj->print(); //调用子类的特有函数,需要转为实际类型 //当传入Woman对象时,转型为Man,转型失败,但是编译器并为察觉 //Man* man = (Man*)obj; //man->print(); //转型失败,返回NULL //Man* man = dynamic_cast<Man*>(obj); //man->print(); Man* man = dynamic_cast<Man*>(obj); if(man!=NULL){ man->smoking(); } Woman* woman = dynamic_cast<Woman*>(obj); if(woman!=NULL){ woman->makeup(); } } void main(){ Woman w1; //父类类型的指针指向子类对象 Person* p = &w1; func(p); getchar(); }
4、reinterpret_cast
该类型转换用于函数指针之间的转型,在使用函数指针转型时使用
void func1(){ } char* func2(){ return "abc"; } typedef void(*f_p)(); void main(){ f_p f_array[6]; f_array[0] = func1; //C 方式 f_array[1] = (f_p)func2; f_array[1] = reinterpret_cast<f_p>func2; getchar(); }
二、IO 流
在 C++ 中,通过 头文件中的 ifstream 读取文件,ofstream 写入文件,完成 IO 流操作
1、文本文件操作
对于文本文件,可进行使用 ifstream 中的 get(ch) 读取一个字符;使用 fout << 写入一个字符串
#include <fstream> void main(){ char fname[] = "D://dest.text"; //输出流 ofstream fout(fname); //创建失败 if(fout.bad()){ return -1; } fout << "Jack" << endl; fout << "Rose" << endl; fout.close(); //读取 ifstream fin(fname); if(fin.bad()){ return -2; } char ch; while(fin.get(ch)){ cout << ch; } fin.close(); }
2、二进制文件操作
对于二进制文件,可进行使用 ifstream中的read(buff,1024) 读取二进制文件,通过 ofstream中的write(buff,1024) 写入到对应文件中
#include <fstream> void main(){ char src[] = "D://xueshan.jpg"; char src_copy[] = "D://xueshan_copy.jpg"; //读取 ifstream fin(src,ios::binary); //输出流 ofstream fout(src_copy,,ios::binary); //创建失败 if(fin.bad()||fout.bad()){ return -1; } while(!fin.eof()){ char buff[1024] = {0}; fin.read(buff,1024); //写入 fout.write(buff, 1024); } //关闭 fin.close(); fout.close(); }
3、对象持久化
class Person{ private: char* name; int age; public: Person(){} Person(char* name,int age){ this->name = name; this->age = age; } char* getName(){ return this->name; } int getAge(){ return this->age; } void print(){ cout << this->name << "," << this->age << endl; } }; void main(){ Person p1(const_cast<char*>("Jack"),25); Person p2(const_cast<char*>("Rose"),22); //输出流 ofstream fout("D://c_obj.data",ios::binary); //指针能够读取到正确的数据,读取内存区的长度 fout.write((char*)(&p1), sizeof(Person)); fout.write((char*)(&p2), sizeof(Person)); fout.close(); //输入流 ifstream fin("D://c_obj.data",ios::binary); Person tmp; fin.read((char*)(&tmp), sizeof(Person)); tmp.print(); fin.read((char*)(&tmp), sizeof(Person)); tmp.print(); }
三、STL(standard template library)标准模板库
在 C++ 中包含很多标准模板库,可以让代码更加简洁
1、string 模板库
使用 string 模板库,在边写代码时更改简洁易用,但是在JNI中 string 需要转为 char* ,,在转为jstring返回
1.1 string 初始化
#include <string> void main(){ string s1 = "Hello "; string s2(" every day"); string s3 = s1 + s2; cout << s3 << endl; //string 转 c 字符串,在JNI中要转为C的str 然后再转为jstring返回 const char* c_str = s1.c_str(); cout << c_str << endl; }
1.2 string 遍历
#include <string> //string遍历 void main() { string s1 = "craig david"; // ^ //1 数组方式 for (int i = 0; i < s1.length(); i++) { cout << s1[i] << endl; } //2 迭代器指针 for (string::iterator it = s1.begin(); it != s1.end(); it++) { cout << *it << " "; } cout << endl; //3 at函数(charAt) // 可能会抛出异常 try { for (int i = 0; i < s1.length() + 3; i++) { cout << s1.at(i) << " "; } } catch (...) { cout << "异常" << endl; } system("pause"); }
1.3 string 字符串查找替换
#include <string> #include <iostream> using namespace std; //字符串查找替换 void main() { string s1 = "apple google apple iphone"; //从0开始查找"google"的位置 int idx = s1.find("google", 0); cout << idx << endl; //统计apple出现的次数 int idx_app = s1.find("apple",0); //npos大于任何有效下标的值 int num = 0; while (idx_app != string::npos) { num++; cout << "找到的索引:" << idx_app << endl; idx_app+=5; idx_app = s1.find("apple", idx_app); } cout << num << endl; system("pause"); }
1.4 string 删除(截取)和插入
#include <string> #include <iostream> using namespace std; //删除(截取)和插入 void main() { string s1 = "apple google apple iphone"; //删除a,找到a所在的指针 string::iterator it = find(s1.begin(),s1.end(),'g'); //只能删除一个字符 s1.erase(it); //开头末尾插入字符串 s1.insert(0, "macos"); s1.insert(s1.length(), " facebook"); cout << s1 << endl; system("pause"); }
1.5 string 大小写转换
#include <string> #include <iostream> #include <algorithm> //算法 using namespace std; //java StringBuffer才可变 //String 不可变 //大小写转换 void main() { string s1 = "JASON"; //原始字符串的起始地址,原始字符串的结束地址, 目标字符串的起始地址, 函数名称 transform(s1.begin(), s1.end()-1,s1.begin(), tolower); cout << s1 << endl; transform(s1.begin(), s1.end() - 1, s1.begin(), toupper); cout << s1 << endl; system("pause"); }
2、容器 vector 模板库
使用 vector 模板库,不需要使用动态内存分配,就可以使用动态数组
2.1 vector 初始化与遍历
#include <vector> void printVector(vector<int> &v) { //通过数组的方式遍历 for (int i = 0; i < v.size(); i++) { cout << v[i] << endl; } } void main(){ //动态数组 vector<int> v; v.push_back(12); v.push_back(118); v.push_back(1); v.push_back(52); v.push_back(20); //遍历 for(int i = 0; i<v.size(); i++){ cout << v[i] << endl; } //部分复制 vector<int> v3(v1.begin(),v1.begin()+2); for(int i = 0; i<v3.size(); i++){ cout << v3[i] << endl; } }
2.2 vector 添加和删除
//添加 删除 void main() { //添加到结尾 vector<int> v1; v1.push_back(20); v1.push_back(40); v1.push_back(15); v1.push_back(7); //访问头部 v1.front() = 11; //访问尾部 v1.back() = 90; //删除结尾的元素 //v1.pop_back(); while (v1.size() > 0) { cout << "末尾的元素:" << v1.back() << endl; v1.pop_back(); } printVector(v1); system("pause"); }
2.3 vector 数组方式
//数组的方式 void main() { vector<int> v1; v1.push_back(20); v1.push_back(40); v1.push_back(15); v1.push_back(7); v1[2] = v1[2] +10; //容器等价于动态数组 vector<int> v2(10); for (int i = 0; i < v2.size(); i++) { v2[i] = i + 1; } printVector(v2); system("pause"); }
2.4 vector 迭代器遍历
//迭代器遍历 //迭代器的种类(正向,反向迭代器) void main() { vector<int> v1; v1.push_back(20); v1.push_back(40); v1.push_back(15); v1.push_back(7); //正向 for (vector<int>::iterator it = v1.begin(); it < v1.end(); it++) { cout << *it << endl; } cout << "-----------------" << endl; //反向迭代 for (vector<int>::reverse_iterator it = v1.rbegin(); it < v1.rend(); it++) { cout << *it << endl; } system("pause"); }
2.5 vector 插入和删除
void main() { vector<int> v1(10); for (int i = 0; i < v1.size(); i++) { v1[i] = i + 1; } //删除指定位置 vector<int>::iterator it = v1.begin(); it += 3; v1.erase(it); //distance(v1.begin(), it); //删除区间 v1.erase(v1.begin(), v1.begin() + 3); for (vector<int>::iterator it = v1.begin(); it < v1.end(); it++) { if (*it == 5) { printf("%x\n", it); vector<int>::iterator tmp = v1.erase(it); //注意以后开发中编译器版本问题 printf("%x,%x\n",it,tmp); } } //插入 v1.insert(v1.begin() + 2, 100); v1.insert(v1.end() - 1, 200); printVector(v1); system("pause"); }
3、队列 deque 模板库
3.1 双向队列
//双向队列 #include <deque> void printDeque(deque<int>& q) { for (int i = 0; i < q.size(); i++) { cout << q[i] << endl; } } void main() { deque<int> d1; //添加到尾部 d1.push_back(2); d1.push_back(10); //添加到头部 d1.push_front(-90); d1.push_front(-30); //printDeque(d1); //cout << d1.front() << endl; //cout << d1.back() << endl; //两个方向弹出 //d1.pop_back(); //d1.pop_front(); printDeque(d1); //查找第一个-90元素索引位置,无需遍历 deque<int>::iterator it = find(d1.begin(), d1.end(), -90); if (it != d1.end()) { int idx = distance(d1.begin(), it); cout << "索引位置为:" << idx << endl; } system("pause"); }
3.2 没有迭代器的队列
#include <iostream> #include <queue> using namespace std; void main() { queue<int> q; q.push(78); q.push(18); q.push(20); q.push(33); //q.front(); //q.back(); while (!q.empty()) { int tmp = q.front(); cout << tmp << endl; q.pop(); } system("pause"); }
3.3 优先级队列
#include <iostream> #include <queue> using namespace std; void main() { //默认 最大值优先级 priority_queue<int> pq1; pq1.push(12); pq1.push(3); pq1.push(40); pq1.push(15); while (!pq1.empty()) { int tmp = pq1.top(); cout << tmp << endl; pq1.pop(); } cout << "----------" << endl; //最小值优先级队列 priority_queue<int, vector<int>, greater<int>> pq2; pq2.push(12); pq2.push(3); pq2.push(40); pq2.push(15); while (!pq2.empty()) { int tmp = pq2.top(); cout << tmp << endl; pq2.pop(); } system("pause"); }
4、stack 栈 模板
4.1 stack 栈的初始化
#include <iostream> #include <stack> using namespace std; void main() { stack<int> s; for (int i = 0; i < 10; i++) { s.push(i + 1); } while (!s.empty()) { int tmp = s.top(); cout << tmp << endl; s.pop(); } system("pause"); }
5、list 模板
5.1 list 初始化
#include <iostream> #include <list> using namespace std; void printList(list<int>& lst) { //迭代器 //没有重载“<”运算符 for (list<int>::iterator it = lst.begin(); it != lst.end(); it++) { cout << *it << endl; } } //基本操作 /* void main() { list<int> lst; for (int i = 0; i < 10; i++) { //尾部插入元素 lst.push_back(i); } //头部插入元素 lst.push_front(80); lst.push_front(90); list<int>::iterator it = lst.begin(); it++; cout << *it << endl; //it = it + 3; 注意:不支持随机访问 printList(lst); system("pause"); }
5.2 list 插入
#include <iostream> #include <list> using namespace std; void main() { list<int> lst; for (int i = 0; i < 10; i++) { //尾部插入元素 lst.push_back(i); } list<int>::iterator it = lst.begin(); it++; lst.insert(it, 100); printList(lst); system("pause"); }
5.3 list 初始化
#include <iostream> #include <list> using namespace std; void main() { list<int> lst; for (int i = 0; i < 10; i++) { //尾部插入元素 lst.push_back(i); } list<int>::iterator it = lst.begin(); //删除 it++; //删除第二个元素 //lst.erase(it); //删除区间(已经被删除了元素不能再删除) list<int>::iterator it_begin = lst.begin(); list<int>::iterator it_end = lst.begin(); it_end++; it_end++; it_end++; lst.erase(it_begin, it_end); //直接根据内容删除元素 lst.remove(5); printList(lst); system("pause"); }
6、set 模板
6.1 set 初始化
#include <iostream> #include <set> using namespace std; //set 元素唯一 默认从小到大 void printSet(set<int> &s) { for (set<int>::iterator it = s.begin(); it != s.end(); it++) { cout << *it << endl; } } void main() { set<int> s; //添加元素 for (int i = 0; i < 10; i++) { s.insert(i+1); } s.insert(20); s.insert(15); s.insert(15); //删除 set<int>::iterator it = s.begin(); it++; s.erase(it); printSet(s); system("pause"); }
6.2 set 元素按照从大到小排列
#include <iostream> #include <set> #include <functional> using namespace std; void main() { //同 Java 中:Map<String,List<String>> set<int,greater<int>> s; s.insert(10); s.insert(5); s.insert(20); s.insert(99); for (set<int,greater<int>>::iterator it = s.begin(); it != s.end(); it++) { cout << *it << endl; } system("pause"); }
6.3 set 自定义 排序 规则
#include <iostream> #include <set> #include <functional> using namespace std; //元素类型为Teacher对象,按照年龄排序 class Teacher { public: Teacher(char* name, int age) { this->name = name; this->age = age; } void print() { cout << name << "," << age << endl; } public: char* name; int age; }; //自定义排序规则 //仿函数 struct MyAgeSorter { bool operator()(const Teacher &left, const Teacher &right) { return left.age < right.age; } }; void main() { set<Teacher, MyAgeSorter> s; s.insert(Teacher(const_cast<char*>("jack"),18)); s.insert(Teacher(const_cast<char*>("rose"), 20)); s.insert(Teacher(const_cast<char*>("jason"), 22)); s.insert(Teacher(const_cast<char*>("alan"), 5)); //s.insert(Teacher("jimy", 5)); //不会插入 for (set<Teacher>::iterator it = s.begin(); it != s.end(); it++) { cout << (*it).name << "," << (*it).age << endl; } system("pause"); }
6.3 set 查找
#include <iostream> #include <set> using namespace std; void main() { set<int> s; //添加元素 for (int i = 0; i < 10; i++) { s.insert(i + 1); } //printSet(s); //等于4的元素指针 set<int>::iterator s_4 = s.lower_bound(4); //cout << *s_4 << endl; //大于4的元素指针 set<int>::iterator s_5 = s.upper_bound(4); //cout << *s_5 << endl; //一次性获取等于4的元素指针,和大于4的元素指针\ //BasicNameValuePair pair<set<int>::iterator, set<int>::iterator> p = s.equal_range(4); cout << *p.first << endl; cout << *p.second << endl; system("pause"); }
6.4 multiset 允许重复的元素
#include <iostream> #include <set> using namespace std; void main() { multiset<int> s; s.insert(2); s.insert(8); s.insert(2); s.insert(8); for (multiset<int>::iterator it = s.begin(); it != s.end(); it++) { cout << *it << endl; } system("pause"); }
7、map 模板
7.1 map 初始化
#include <iostream> #include <map> #include <string> using namespace std; void main() { //key -> value //1. map<int, string> map1; map1.insert(pair<int, string>(1, "jack")); map1.insert(pair<int, string>(2, "rose")); //2 map1.insert(make_pair(3, "jason")); //3 map1.insert(map<int, string>::value_type(4, "alan")); //4 map1[5] = "jimmy"; //map["NO1"] = 90; //前三种方式,如果key已经存在,重复添加会报错 //第四种方式,如果key已经存在,重复添加会覆盖 //遍历输出 for (map<int, string>::iterator it = map1.begin(); it != map1.end(); it++) { cout << it->first << "," << it->second << endl; } system("pause"); }
7.2 map 删除元素的方式
#include <iostream> #include <map> #include <string> using namespace std; void printMap(map<int, string> &map1) { for (map<int, string>::iterator it = map1.begin(); it != map1.end(); it++) { cout << it->first << "," << it->second << endl; } } void main() { map<int, string> map1; map1.insert(pair<int, string>(1, "jack")); map1.insert(pair<int, string>(2, "rose")); map1.insert(pair<int, string>(3, "jason")); map<int, string>::iterator it = map1.begin(); it++; map1.erase(it); printMap(map1); system("pause"); }
7.3 map 添加元素的结果
#include <iostream> #include <map> #include <string> using namespace std; void main() { map<int, string> map1; map1.insert(pair<int, string>(1, "jack")); map1.insert(pair<int, string>(2, "rose")); map1.insert(pair<int, string>(3, "jason")); //获取添加的结果(first元素指针,second 是否成功) pair<map<int, string>::iterator, bool> res = map1.insert(pair<int, string>(3, "alan")); if (res.second) { cout << "添加成功" << endl; } else { cout << "添加失败" << endl; } printMap(map1); system("pause"); }
7.3 map 查找
#include <iostream> #include <map> #include <string> using namespace std; void main() { map<int, string> map1; map1.insert(pair<int, string>(1, "jack")); map1.insert(pair<int, string>(2, "rose")); map1.insert(pair<int, string>(3, "jason")); printMap(map1); cout << "---------" << endl; //获取key等于大于5的元素的值 pair<map<int, string>::iterator, map<int, string>::iterator> p = map1.equal_range(2); if (p.first != map1.end()){ //等于2的元素key value cout << p.first->first << p.first->second << endl; //大于2的元素key value cout << p.second->first << p.second->second << endl; } system("pause"); }
7.3 multimap
#include <iostream> #include <map> #include <string> using namespace std; //一个key对应多个value //一个部门多个员工 //multimap class Employee { public: Employee(char* name,int age) { this->name = name; this->age = age; } public: char* name; int age; }; void main() { multimap<string, Employee> map1; //开发部 map1.insert(make_pair("开发", Employee(const_cast<char*>("搁浅"), 20))); map1.insert(make_pair("开发", Employee(const_cast<char*>("彪哥"), 20))); //财务 map1.insert(make_pair("财务", Employee(const_cast<char*>("小颖"), 16))); map1.insert(make_pair("财务", Employee(const_cast<char*>("rose"), 20))); //销售 map1.insert(make_pair("销售", Employee(const_cast<char*>("阿呆"), 30))); map1.insert(make_pair("销售", Employee(const_cast<char*>("呵呵"), 30))); //遍历输出 for (multimap<string, Employee>::iterator it = map1.begin(); it != map1.end(); it++) { cout << it->first << "," << it->second.name << "," << it->second.age << endl; } cout << "----------------" << endl; //只获取“财务”部的员工 //获取“财务部”员工的个数,key对应的value的个数 int num = map1.count("财务"); multimap<string, Employee>::iterator it = map1.find("财务"); int c = 0; //控制循环的次数 while (it != map1.end() && c < num) { cout << it->first << "," << it->second.name << "," << it->second.age << endl; it++; c++; } system("pause"); }
8、深拷贝与浅拷贝
#include <iostream> #include <map> #include <string> #include <vector> using namespace std; class Employee { public: //构造函数 Employee(char* name, int age) { this->name = new char[strlen(name) + 1]; strcpy(this->name, name); this->age = age; } //析构函数 ~Employee() { if (this->name != NULL) { delete[] this->name; this->name = NULL; this->age = 0; } } //拷贝构造函数 //Employee e = Employee(const Employee &obj) { this->name = new char[strlen(obj.name) + 1]; strcpy(this->name, obj.name); this->age = obj.age; } //重载= //e1 = e2; Employee& operator=(const Employee &obj) { //释放旧的内存 if (this->name != NULL) { delete[] this->name; this->name = NULL; this->age = 0; } //重新分配 this->name = new char[strlen(obj.name) + 1]; strcpy(this->name, obj.name); this->age = obj.age; return *this; } public: char* name; int age; }; void func() { vector<Employee> v1; Employee e1(const_cast<char*>("jack"), 20); v1.push_back(e1); } void main() { //vector<Employee> v1; //Employee e1("jack",20); //将e1拷贝到vector中 //v1.push_back(e1); func(); system("pause"); }
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