any idear?

H

Hill

I want a class Table which saves one list of Pair<int,std::string>.
And I want that it has following behaviour:

Table t;//now t is empty
try{
int i = t["me"];// I want this call to an empty table will throw a
exception [case 1]
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
t["me"] = 0;//and i hope this call can work well [case 2]
}


Could some one tell me how to define this class Table?
How to define operator for case 1 and case 2?

class Table
{
public:
class Bad_op{};
struct Pair
{
int data;
std::string str;
};
Table(const Table& table);
Table();
~Table();
Table& operator= (const Table& table);
//int& operator[](const std::string& str);???????
//nst int& operator[](const std::string& str)const;???????
private:
std::vector<Pair> m_Vec;
};
 
K

Kai-Uwe Bux

Hill said:
I want a class Table which saves one list of Pair<int,std::string>.
And I want that it has following behaviour:

Table t;//now t is empty
try{
int i = t["me"];// I want this call to an empty table will throw a
exception [case 1]
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
t["me"] = 0;//and i hope this call can work well [case 2]
}
Click to expand...

First, a clarification about the specs: what about

int i = t["here"];

where t is not empty but no entry for "here" is to be found?
Could some one tell me how to define this class Table?
How to define operator for case 1 and case 2?

class Table
{
public:
class Bad_op{};
struct Pair
{
int data;
std::string str;
};
Table(const Table& table);
Table();
~Table();
Table& operator= (const Table& table);
//int& operator[](const std::string& str);???????
//nst int& operator[](const std::string& str)const;???????
private:
std::vector<Pair> m_Vec;
};
Click to expand...

Why a vector of pairs? It seems that all you need is a thin wrapper around
std::map< std::string, int >.


Best

Kai-Uwe Bux
 
H

Hill

Hill said:
I want a class Table which saves one list of Pair<int,std::string>.
And I want that it has following behaviour:
Click to expand...
Table t;//now t is empty
try{
int i = t["me"];// I want this call to an empty table will throw a
exception [case 1]
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
t["me"] = 0;//and i hope this call can work well [case 2]
}
Click to expand...

First, a clarification about the specs: what about

int i = t["here"];

where t is not empty but no entry for "here" is to be found?






Could some one tell me how to define this class Table?
How to define operator for case 1 and case 2?
Click to expand...
class Table
{
public:
class Bad_op{};
struct Pair
{
int data;
std::string str;
};
Table(const Table& table);
Table();
~Table();
Table& operator= (const Table& table);
//int& operator[](const std::string& str);???????
//nst int& operator[](const std::string& str)const;???????
private:
std::vector<Pair> m_Vec;
};
Click to expand...

Why a vector of pairs? It seems that all you need is a thin wrapper around
std::map< std::string, int >.

Best

Kai-Uwe Bux- Òþ²Ø±»ÒýÓÃÎÄ×Ö -

- ÏÔʾÒýÓõÄÎÄ×Ö -
Click to expand...

Yes , When i say "empty" , i mean the table has a emplty list.
It's just a exercise. Not realistic class.
 
K

Kai-Uwe Bux

Hill said:
Hill said:
I want a class Table which saves one list of Pair<int,std::string>.
And I want that it has following behaviour:
Click to expand...
Table t;//now t is empty
try{
int i = t["me"];// I want this call to an empty table will throw a
exception [case 1]
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
t["me"] = 0;//and i hope this call can work well [case 2]
}
Click to expand...

First, a clarification about the specs: what about

int i = t["here"];

where t is not empty but no entry for "here" is to be found?






Could some one tell me how to define this class Table?
How to define operator for case 1 and case 2?
Click to expand...
class Table
{
public:
class Bad_op{};
struct Pair
{
int data;
std::string str;
};
Table(const Table& table);
Table();
~Table();
Table& operator= (const Table& table);
//int& operator[](const std::string& str);???????
//nst int& operator[](const std::string& str)const;???????
private:
std::vector<Pair> m_Vec;
};
Click to expand...

Why a vector of pairs? It seems that all you need is a thin wrapper
around std::map< std::string, int >.

Best

Kai-Uwe Bux- ??????? -

- ??????? -
Click to expand...

Yes , When i say "empty" , i mean the table has a emplty list.
It's just a exercise. Not realistic class.
Click to expand...

In that case, you could try something like this:


#include <map>
#include <stdexcept>
#include <string>
#include <iostream>

template < typename Key, typename Mapped >
class table : public std::map< Key, Mapped > {

struct proxy {

table * the_table;
Key the_key;

proxy ( table * ptr, Key const & key )
: the_table ( ptr )
, the_key ( key )
{}

proxy & operator= ( Mapped const & rhs ) {
static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) = rhs;
return ( *this );
}

operator Mapped & ( void ) const {
if ( the_table->empty() ) {
throw ( std::exception() );
}
return ( static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) );
}

};

struct const_proxy {

table * the_table;
Key the_key;

const_proxy ( table * ptr, Key const & key )
: the_table ( ptr )
, the_key ( key )
{}

operator Mapped const & ( void ) const {
if ( the_table->empty() ) {
throw ( std::exception() );
}
return ( static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) );
}

};

public:

table ( void ) :
std::map< Key, Mapped > ()
{}

template < typename A >
table ( A a ) :
std::map< Key, Mapped > ( a )
{}

template < typename A, typename B >
table ( A a, B b ) :
std::map< Key, Mapped > ( a, b )
{}

proxy operator[] ( Key const & key ) {
return ( proxy( this, key ) );
}

const_proxy operator[] ( Key const & key ) const {
return ( proxy( this, key ) );
}

};

int main ( void ) {
table< std::string, int > t;
{
try{
int i = t[ "me" ];
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
}
}
{
t[ "me" ] = 0;
}
{
try{
int i = t[ "me" ];
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
}
}
}


Best

Kai-Uwe Bux
 
H

Hill

Hill said:
Hill wrote:
I want a class Table which saves one list of Pair<int,std::string>.
And I want that it has following behaviour:
Table t;//now t is empty
try{
int i = t["me"];// I want this call to an empty table will throw a
exception [case 1]
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
t["me"] = 0;//and i hope this call can work well [case 2]
}
First, a clarification about the specs: what about
int i = t["here"];
where t is not empty but no entry for "here" is to be found?
Could some one tell me how to define this class Table?
How to define operator for case 1 and case 2?
class Table
{
public:
class Bad_op{};
struct Pair
{
int data;
std::string str;
};
Table(const Table& table);
Table();
~Table();
Table& operator= (const Table& table);
//int& operator[](const std::string& str);???????
//nst int& operator[](const std::string& str)const;???????
private:
std::vector<Pair> m_Vec;
};
Why a vector of pairs? It seems that all you need is a thin wrapper
around std::map< std::string, int >.
Best
Kai-Uwe Bux- ??????? -
- ??????? -
Click to expand...
Click to expand...
Yes , When i say "empty" , i mean the table has a emplty list.
It's just a exercise. Not realistic class.
Click to expand...

In that case, you could try something like this:

#include <map>
#include <stdexcept>
#include <string>
#include <iostream>

template < typename Key, typename Mapped >
class table : public std::map< Key, Mapped > {

struct proxy {

table * the_table;
Key the_key;

proxy ( table * ptr, Key const & key )
: the_table ( ptr )
, the_key ( key )
{}

proxy & operator= ( Mapped const & rhs ) {
static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) = rhs;
return ( *this );
}

operator Mapped & ( void ) const {
if ( the_table->empty() ) {
throw ( std::exception() );
}
return ( static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) );
}

};

struct const_proxy {

table * the_table;
Key the_key;

const_proxy ( table * ptr, Key const & key )
: the_table ( ptr )
, the_key ( key )
{}

operator Mapped const & ( void ) const {
if ( the_table->empty() ) {
throw ( std::exception() );
}
return ( static_cast< std::map< Key, Mapped > * >
( the_table )->operator[]( the_key ) );
}

};

public:

table ( void ) :
std::map< Key, Mapped > ()
{}

template < typename A >
table ( A a ) :
std::map< Key, Mapped > ( a )
{}

template < typename A, typename B >
table ( A a, B b ) :
std::map< Key, Mapped > ( a, b )
{}

proxy operator[] ( Key const & key ) {
return ( proxy( this, key ) );
}

const_proxy operator[] ( Key const & key ) const {
return ( proxy( this, key ) );
}

};

int main ( void ) {
table< std::string, int > t;
{
try{
int i = t[ "me" ];
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
}
}
{
t[ "me" ] = 0;
}
{
try{
int i = t[ "me" ];
}
catch(...){
std::cout << "It's a empty table,bad operator" << std::endl;
}
}

}

Best

Kai-Uwe Bux- Òþ²Ø±»ÒýÓÃÎÄ×Ö -

- ÏÔʾÒýÓõÄÎÄ×Ö -
Click to expand...

By overloading operator= and operator Mapped& , We distinguish the
lvalue and rvalue.
It's a good idear. Thanks.
 

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