//my class
class a{
....
public:
a(){...}
a(a&){...}
~a(){...}

....
bool operator<{...}
bool operator=={...}

};

int main(){
a a1,a2... an;
a1 ={...}

a2={...}

an={...}

std::vector<a> v1;
v1.push_back(a1);
{...}
v1.push_back(an);  

std::sort(v1.begin(),v1,end());  //error!!!

{...}

}

#include <iostream>
#include <vector>
#include <algorithm>

class A
{
    int _a;
public:
    A( int a ) : _a( a ) {}

    int dump() const { return _a; }
    bool operator<( const A & rhs ) const { return _a < rhs._a; }
};

int main()
{
    A a1( 10 );
    A a2( 30 );
    A a3( 20 );

    std::vector<A> v;
    v.push_back(a1);
    v.push_back(a2);
    v.push_back(a3);

    std::sort( v.begin(), v.end() );

    for( std::vector<A>::const_iterator i = v.begin(); i != v.end(); ++i )
    {
        std::cout << (*i).dump() << std::endl;
    }
}

//head.file

#ifndef BIGN_H_
#define BIGN_H_

#include<string.h>
#include<iostream>
#include<string>
#include<sstream>

std::string int2str(int);

struct Nrep{
    char si;
    std::string name;
    int sz;
    
    Nrep(int n){    ...      }
    
    ~Nrep(){}
private:
    Nrep(const Nrep&);
    Nrep& operator=(const Nrep&);
};

class bign
{		
	Nrep* num;
	int* ref;
public:
	bign():num(new Nrep(0)),ref(new int(1)){}
	bign(int i):num(new Nrep(i)),ref(new int(1)){}
	bign(const bign& b):num(b.num),ref(b.ref){++*b.ref;}
	virtual ~bign();
	
	int size() const{ return num->sz;}
	char operator[](int i) const {if((i<=num->sz)&&(i>0)) return num->name[i-1];else return 0;}
	char getSi() const{ return num->si;}
	
	bign& operator=(const bign&);
	bign& operator+=(const bign&);	
	bign& operator-=(const bign&);
	bign& operator*=(const bign&);
	bool operator==(const bign&) const;
	bool operator>(const bign&)const;
	bool operator<(const bign&) const;
	friend std::ostream& operator<<(std::ostream& oo,const bign& b){if (b.getSi()=='-')return oo<<b.getSi()<<b.num->name;else return oo<<b.num->name;}
	
};

bool big_than(bign& ,bign&);
bool less(bign& ,bign&);
bign& operator* (bign&, const bign&);

#endif /*BIGN_H_*/

//bign.cpp

......

//main cpp

#include<iostream>
#include<fstream>
#include<string>
#include<vector>
#include<algorithm>
#include"bign.h"

int main(int argc,char* argv[]){

	std::string from;
	
	if(argc!=2)
		from="a1.in";
	else
		from=argv[1];
	
	std::ifstream ii(from.c_str());	
	if(ii==0){
		if(argc <=1)
			std::cout<<"Usage: "<<argv[0]<<" [filename] \n\tPlease try again!\n";
		else
			std::cout<<"Can't open the file: "<<from<<std::endl;
		return -1;
	}
	
	std::string to;
	if(argc>=2)
		to=argv[2];
	else
		to=from;
	
	int n=to.find_last_of('.');
	if (n<1)
		to =to + ".out";
	else
		to.replace(n,from.length(),".out");	
	std::ofstream oo(to.c_str());

	ii>>n;
	for(int loop=0;loop<n;loop++){
		int re1=0;
		int num=0;
		std::vector<bign> v1,v2;
		ii>>num;
		for(int i=0;i<num;i++){
			ii>>re1;
			v1.push_back(re1);
		}
		for(int i=0;i<num;i++){
			ii>>re1;
			v2.push_back(re1);
		}
		
		std::sort(v1.begin(),v1.end());
		std::sort(v2.begin(),v2.end());
		
		bign result;
		for(int i=0;i<num;i++)
			result += v1[i] * v2[num-i-1];
		
		std::cout<<"Case #"<<loop+1<<": "<<result<<std::endl;
		oo<<"Case #"<<loop+1<<": "<<result<<std::endl;
	}
	
	return 0;
	
}