C++命名空间

cir.h

namespace cir{
    double PI = 3.141592653;
    //获取圆周长
    double getLengthOfCir(double radius){
        return 2*PI*radius;
    }
    //获取圆面积
    double getAreaOfCir(double radius){
        return PI*radius*radius;
    }
}

main.cpp 

#include <iostream>
#include<cir.h>
#include<stdio.h>
using namespace std;
using namespace cir;
int main()
{
    cout << "Hello World!" << endl;
    double myRadius=5;

    printf("length::%lf,area::%lf\n",cir::getLengthOfCir(myRadius),
           cir::getAreaOfCir(myRadius));

    cout<<"length::"<<cir::getLengthOfCir(myRadius)<<endl;
    cout<<"area::"<<cir::getAreaOfCir(myRadius)<<endl;

    cout<<"length::"<<getLengthOfCir(myRadius)<<endl;
    cout<<"area::"<<getAreaOfCir(myRadius)<<endl;
    return 0;
}

运行结果