模块化（modularity）能够实现接口（interface）定义（definition，或声明，declaration）、实现（implementaion）、使用（或调用，calling）的分离，并且能够分散到同一项目的不同文件中，通过预处理指令include建立联系，且最后能通过链接器实现链接。同时，使函数库或类库能得以实现。

1 接口放到头文件（或放到单文件的前面）

Typically, we place the declarations that specify the interface to a module in a file with a name indicating its intended use.

//vector.h
class Vector {
public:
	Vector(int s);
	int size();
	double& operator[](int i);
	~Vector() { delete[] elem; } // destructor: release resources, inline
private:
	double* elem; // pointer to the elements
	int sz; // the number of elements
};
double read_and_sum(int s);

2 接口实现

To help the compiler ensure consistency, the .cpp file providing the implementation of Vector will also include the .h file providing its interface:

//vector.cpp
#include <iostream>
#include "vector.h"
using namespace std;
Vector::Vector(int s) // constr uct a Vector
{
	sz = s;
	elem = new double[s];
	for (int i=0; i!=s; ++i) 
		elem[i]=i; // initialize elements
}

double& Vector::operator[](int i) // element access: subscripting
{ 
	return elem[i]; 
} 
int Vector::size() 
{ 
	return sz; 
}
double read_and_sum(int s)
{
	Vector v(s); // make a vector of s elements
	//for (int i=0; i!=v.size(); ++i)
		//cin>>v[i]; //read into elements
	double sum = 0;
	for (int i=0; i!=v.size(); ++i)
		sum+=v[i]; //take the sum of the elements
	return sum;
}

3 接口使用（访问或调用）

This declaration would be placed in a file Vector.h, and users will include that file, called a header file, to access that interface.

//user.cpp
#include <iostream>
#include "vector.h"
using namespace std;
void main()
{
 cout<< read_and_sum(100)<<endl; //4950
 system("pause");
}

－End－