IDE 往往封装了 compile 和 link

std::

调用C++标准库时，要写上std::
仅仅使用非标准库文件iostream.h，则不用写

#include<iostream>
using namespace std;
int main()
{
    cout<<"hello world";
    cout<<endl;
    return 0;
}

#include <iostream>
#include<fstream>
#include<string>
using namespace std;
int main()
{
   ofstream ofile("test.txt",ios::app);
   if(!ofile.fail())
   {
       cout<<"start write"<<endl;
       ofile<<"Mary";
       ofile<<"girl";
       ofile<<"20";
    }
    else
        cout<<"can not open";
    return 0;
}

#include <fstream>
#include <vector>
static std::vector<char> readFile(const std::string& filename) {
    std::ifstream file(filename, std::ios::ate | std::ios::binary);
    if (!file.is_open()) {
        throw std::runtime_error("failed to open file!");
    }
}