将类写入文件是比较通用的任务，是写文件索引的基础。进一步可以将b+ tree等东西写入文件
/*
* Filename: classtfile.cpp
*
* Description: 将类写入文件，必要时候都回到内存中
#include<iostream>
#include<fstream>
using namespace std;
class person
{
public:
enum {SIZE=50};
int id;
int age;
char name[SIZE];
char address[SIZE];
};
class Database
{
private:
fstream fs;
string filename;
void openf()
{
try
{
fs.open(this->filename.c_str(),fstream::in|fstream::out|fstream::app);
}catch(...)
{ }
}
public:
Database(string fn):filename(fn)
{
}
~Database()
{
if(fs.is_open())
fs.close();
}
//通过id查找位置
size_t query(size_t id)
{
size_t index=0;
if(!fs.is_open())
openf();
fs.seekg(0,ios_base::beg);
while(!fs.eof())
{
fs.read(reinterpret_cast<char*>(&index),sizeof(int));
if(index==id)
{
fs.seekg(-sizeof(int),ios_base::cur);
return fs.tellg();
}
fs.seekg(sizeof(person)-sizeof(int),ios_base::cur);
}
fs.close();
return -1;
}
//返回位置n的person记录
person retrieve(size_t n)
{
person p;
if(!fs.is_open())
openf();
int resultindex=query(n);
if(resultindex!=-1)//找到了
{
fs.seekg(resultindex);
fs.read(reinterpret_cast<char*>(&p),sizeof(person));
fs.close();
return p;
}
fs.close();
throw("no result");
}
//修改位置n的person记录
int update(size_t n, person& p)
{
if(!fs.is_open())
fs.open(filename.c_str(),fstream::in|fstream::out);
int resultindex=query(n);
if(resultindex!=-1)//找到了
{
fs.seekp(resultindex);
fs.write(reinterpret_cast<char*>(&p),sizeof(person));
fs.close();
return 0;
}
fs.close();
return 1;
}
//添加一个人的记录
void add(person& p)
{
if(!fs.is_open())
openf();
fs.seekp(0,ios::end);
fs.write(reinterpret_cast<char*>(&p),sizeof(person));
fs.close();
}
};
int main()
{
Database db("db");
person p;
p.id=1;
p.age=10;
strcpy(p.name,"person1");
strcpy(p.address,"hunan");
person p2;
p2.id=2;
p2.age=68;
strcpy(p2.name,"person2");
strcpy(p2.address,"guangxi");
person p3;
p3.id=3;
p3.age=333;
strcpy(p3.name,"person3");
strcpy(p3.address,"shenzhen");
db.add(p);
db.add(p2);
db.add(p3);
for(int i=0;i<3;i++)
{
try
{
person p=db.retrieve(i+1);
cout<<"p.id="<<p.id<<endl;
cout<<"p.age="<<p.age<<endl;
cout<<"p.name="<<p.name<<endl;
cout<<"p.address="<<p.address<<endl;
}catch(...)
{
cout<<"can’t not find the person "<<i+1<<endl;
}
}
cout<<"change 2 happend...."<<endl;
person ppp;
ppp.id=1;
ppp.age=222;
strcpy(ppp.name,"new person 1");
strcpy(ppp.address,"中华人们公共和国");
db.update(1,ppp);//修改
for(int i=0;i<3;i++)
{
try
{
person p=db.retrieve(i+1);
cout<<"p.id="<<p.id<<endl;
cout<<"p.age="<<p.age<<endl;
cout<<"p.name="<<p.name<<endl;
cout<<"p.address="<<p.address<<endl;
}catch(...)
{
cout<<"can’t not find the person "<<i+1<<endl;
}
}
}
责任编辑：小草
