#ifndef MYHEAD_H
#define MYHEAD_H
……//头文件中的内容
#endif

#ifndef MYHEAD_H
#define MYHEAD_H

#include "myhead.h"

  ......

#endif

 int main()
 {
  #ifdef DEBUG
  cout<<"Beginning execution of main()\n";
  #endif
  string word;
  vector<string> text;
  while(cin>>word)
  {
 #ifdef DEBUG
 cout<<"word read:"<<word<<"\n";
 #endif
 text.push_back(word); 
  }
 //..... }

#ifndef MYHEAD_H
#define MYHEAD_H
#ifdef __cplusplus
extern "C" {
#endif
int DMpostprocessing();
#ifdef __cplusplus
}
#endif
#endif

if(element_count==0)
 cerr<<"Error:"<<__FILE__
   <<":line"<<__LINE__
   <<"element_count must be non-zero.\n";

 assert(filename!=0);

 #include <fstream>
//为了打开一个输出文件，先声明一个ofstream类型的对象：
 ofstream outfile("name-of-file");
//为了测试是否已经成功打开了一个文件，如下判断：
 //如文件不能打开值为false
 if(!outfile)
   cerr<<"Sorry! We were unable to open the file!\n";

//为了打开一个输入文件，先声明一个ifstream类型的对象：
  ifstream infile("name of file");
  if(!infile)
   cerr<<"Sorry! We were unable to open the file!\n";

一个简单程序：
  #include <iostream>
  #include <fstream>
  #include <string>

  int main()
  {
   ofstream outfile("out_file");
 ifstream infile("in_file");
 if(!infile){
   cerr<<"error:unable to open input file!\n";
   return -1;
 }
 if(!outfile) {
   cerr<<"error:unable to open output file!\n";
   return -2;
 }
 string word;
 while (infile>>word)
  outfile<<word<<' ';
 return 0;
  }

//shape.h
#include "math.h"//在计算三角形面积时要用到正弦函数
const double pi=3.14159265358;//常量定义
struct circle//类型声明
{
  double r;
};
struct square
{
  double a;
};
struct rectangle
{
  double a,b;
};
struct triangle
{
  double a,b,c,alpha,beta,gamma;
};
double perimeter_of_circle(double r)//函数定义
{
  return 2*pi*r;
}
double area_of_circle(double r)
{
  return pi*r*r;
}
double perimeter_of_square(double a)
{
  return 4*a;
}
double area_of_square(double a)
{
  return a*a;
}
double perimeter_of_rectangle(double a,double b)
{
  return 2*(a+b);
}
double area_of_rectangle(double a,double b)
{
  return a*b;
}
double perimeter_of_triangle(double a,double b,double c)
{
  return a+b+c;
}
double area_of_triangle(double a,double b,double gamma)
{
  return sin(gamma/180*pi)*a*b/2;
}
//main.cpp
#include "iostream.h"
#include "shape.h"//包含我们编写好的shape.h
int main()
{
  circle c={2};
  square s={1};
  rectangle r={2,3};
  triangle t={3,4,5,36.86989,53.13011,90};
  cout <<"Perimeter of circle " <<perimeter_of_circle(c.r) <<endl;
  cout <<"Area of square " <<area_of_square(s.a) <<endl;
  cout <<"Perimeter of rectangle " <<perimeter_of_rectangle(r.a,r.b) <<endl;
  cout <<"Area of triangle " <<area_of_triangle(t.b,t.c,t.alpha) <<endl;
  return 0;
}

Perimeter of circle 12.5664
Area of square 1
Perimeter of rectangle 10
Area of triangle 6