C++ using的多种用法

1、引入命名空间

using namespace std;
using std::cout;

2、引入基类成员

class Base{
public:
	void func(){cout << "Base::func()" << endl;}
};
class Derived : public Base{
public:
	using Base::func;
	void func(int x){
		cout << "Derived::func(x)" << x << endl;
	}
};
int main(){
	Derived d;
	d.func();	//Base::func()
	d.func(1);	//Derived::func(x)
	return 0;
}

3、取别名

//给类型取别名
using type = int;
type x = 10;

//给FormatItem类型的智能指针取名为ptr
using ptr = shared_ptr<FormatItem>;

template<class T>
class A{
public:
	//给模板类型取别名
	using value_type = T;
	//T func(){}
	value_type func(){}
private:
	//T x;
	value_type x;
}