C++基本语法

#include <iostream>
#include <cstdio> //C++风格
int main(void)
{
    //输出语句
    std::cout << "hello world!" << std::endl;
    printf("hello world!\n");
    return 0;
}

#include <iostream>
namespace ns1{
    void func(void){
        std::cout << "ns1的func" << std::endl;
    }
}
namespace ns2{
    void func(void){
        std::cout << "ns2的func" << std::endl;
    }
}
int main(void)
{
    //func();//名字空间成员不能直接访问
    //通过"空间::"访问
    ns1::func();
    ns2::func();
    return 0;
}

#include <iostream>
//标准名字空间指令,以后省略"std::"
using namespace std;
namespace ns1{
    void func(void){
        cout << "ns1的func" << endl;
    }
}
namespace ns2{
    void func(void){
        cout << "ns2的func" << endl;
    }
}
int main(void)
{
    using namespace ns1;//名字空间指令
    func();//ns1的func
    using namespace ns2;//名字空间指令
    //func();//歧义错误
    ns2::func();
    return 0;
}

#include <iostream>
//标准名字空间指令,以后省略"std::"
using namespace std;
namespace ns1{
    void func(void){
        cout << "ns1的func" << endl;
    }
}
namespace ns2{
    void func(void){
        cout << "ns2的func" << endl;
    }
}
int main(void)
{
    using ns1::func;//名字空间声明
    func();
    //using ns2::func;
    //func();//歧义错误
    using namespace ns2;//名字空间指令
    func();//ns1的func,局部优先
    ns2::func();//ns2的func
    return 0;
}

#include <iostream>
using namespace std;
namespace ns1{
    int num = 100;
}
namespace ns2{
    int num = 200;
}
//int num = 300;//全局作用域
namespace { //无名名字空间
    int num = 300;
}
int main(void)
{
    cout << num << endl;//300
    using ns1::num;//名字空间声明
    cout << num << endl;//100
    cout << ns2::num << endl;//200
    cout << ::num << endl;//300
    return 0;
}

#include <iostream>
using namespace std;
struct Teacher{
    //成员变量
    char name[20];
    int age;
    double salary;
    //成员函数
    void who(void){
        cout << "我叫" << name << ",今年" <<
            age << "岁,工资是" << salary <<
            endl;
    }
};
int main(void)
{
    /*struct*/
    Teacher wangjl = {"王建立",48,800.5};
    wangjl.who();
    return 0;
}

#include <iostream>
#include <cstdio>
using namespace std;
int main(void)
{
    union{//匿名联合
        unsigned int ip1;
        unsigned char ip2[4];
    };
    ip1 = 0x12345678;
    for(int i=0;i<4;i++){
        printf("%#x ",ip2[i]);    
    }
    printf("\n");
    return 0;
}

#include <iostream>
using namespace std;
int main(void)
{
    enum Color{RED,YELLOW,BLUE};
    cout << RED << ',' << YELLOW << ','
        << BLUE << endl;//0 1 2
    /*enum*/ Color c;
    //c = 2;//C:ok  C++:error
    c = BLUE;//C:ok  C++:ok
    cout << c << endl;//2
    return 0;
}

#include <iostream>
#include <cstring>
using namespace std;
int main(void)
{
    //定义
    string s = "hello";
    cout << "s=" << s << endl;
    //拷贝
    string s2;
    s2 = s;
    cout << "s2=" << s2 << endl;
    //连接
    string s3 = " world";
    s3 = s + s3;
    cout << "s3=" << s3 << endl;
    s += " world";//s = s + " world"
    cout << "s=" << s << endl;
    //比较
    if(s == s3){
        cout << "比较成立" << endl;
    }
    else{
        cout << "比较不成立" << endl;
    }
   
    //获取字符串中某个字符
    s[0] = 'H';
    s[6] = 'W';
    cout << s << endl;//Hello World
   
    //获取字符串长度
    cout << s.size() << endl;//11
    cout << s.length() << endl;//11
    //转换为C风格的字符串
    cout << strlen(s.c_str()) << endl;//11
    return 0;
}

#include <iostream>
using namespace std;
int main(void)
{
    string str;
    cout << "请输入一个字符串:" << endl;
    //cin >> str;//碰到空白符结束
    getline(cin,str);//碰到"\n"结束
    int count = 0;
    for(int i=0;i<str.size();i++){
        if(str[i]=='A' || str[i]=='a'){
            ++count;
        }
    }
    cout << "包含A/a的个数是:" << count << endl;
    return 0;
}

#include <iostream>
using namespace std;
int main(void)
{
    string str;
    cin >> str;
    int len = str.size();
    //"abcdef"==>"fedcba"
    for(int i=0; i<len/2; i++){
        str[i] = str[i] ^ str[len-1-i];
        str[len-1-i] = str[i] ^ str[len-1-i];
        str[i] = str[i] ^ str[len-1-i];
    }
    cout << str << endl;
    return 0;
    //char c1 = 3;
    //char c2 = 5;
    //c1 = c1 ^ c2;
    //c2 = c1 ^ c2;
    //c1 = c1 ^ c2;
}