C++中using的三种用法举例详解

using std::string;
string s = "123";

class Base{
protected:
    int bn1;
    int bn2;
};
 
class Derived: private Base{	//私有继承
public:
    using Base::bn1;	//在当前作用域中引入了父类的保护成员, 在当前作用域中可以访问
};
 
class DerivedAgain: public Derived{	//在Derived的子类中仍然可以访问bn1
};
 
int main(){
    Derived d;
    DerivedAgain da; 
    d.bn1 = 1;
    d.bn2 = 2;   //error, 'bn2' is a private member of 'Base'
    da.bn1 = 3;  //ok
    std::cout<<d.bn1<<std::endl;
    return 0;
}

using namespace std;	//我们常用的std命名空间展开

using alias = typename;//使用别名去替代原始类型(重命名)

using ULL = unsigned long long;		//typedef unsigned long long ULL;
using func = void(*)(int, int);		//typedef void(*func)(int, int);

typedef std::map<int, int> mapIntInt;
typedef std::map<int, bool> mapIntBool;
typedef std::map<int, std::string> mapIntString;
//...

template <class Val>
struct mapInt{
    typedef std::map<int, Val> type;
};

int main(){
    mapInt<int>::type imap;		//将int映射到int的
    mapInt<bool>::type bmap;	//将int映射到bool的
    mapInt<std::string>::type smap;	//将int映射到string的
    return 0;
}

template <class Val>
using mapInt = std::map<int, Val>;

int main(){
    mapInt<int> imap;
    mapInt<bool> bmap;
    mapInt<std::string> smap;
    return 0;
}