How to access operator from base class?

[Ulf Rimkus]

Hallo!

Now I've tried out for quite a long time how to access the []-operator from
a base class. But I cannot find out how to do this. Well, it's an example
from "Think in C++".
So, I guess I need some help from any real C++ programmer.

Greetings, Ulf

// 14.9 Inherit a class StringVector from vector<void*> and redefine the
push_back( ) and operator[] member
// functions to accept and produce string*. What happens if you try to
push_back( ) a void*?

//StringVector.cp p

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class StringVector: public vector<void*> {
public:
void push_back(strin g* str){cout << "My_push_
back called\n";vecto r<void*>::push_ back(str);}
// following code does not work!!
string* operator[](int i) {
string* t= (string*)(vecto r<void*>&)this[i]; //wrong! What can I do?
return (t);
}
};

int
main(){
StringVector s;
string *str1 = new string("Hallo") ;
string *str2 = new string("Welt");
s.push_back(str 1);
s.push_back(str 2);
cout << *(string*)s[0] << endl;
cout << *(string*)s[1] << endl;
return 0;
}

[Ron Natalie]

"Ulf Rimkus" <ul********@arc or.de> wrote in message news:3f******** **************@ newsread2.arcor-online.net...

Hallo!

string* operator[](int i) {
string* t= (string*)(vecto r<void*>&)this[i]; //wrong! What can I do?

You're applying the vector& cast to the return of this[i] (which is recursive). You want to apply it
to "this". You need more parens.
(string*) ((vector<void*> &)this)[i];

[John Dibling]

On Mon, 14 Jul 2003 17:09:59 +0200, Ulf Rimkus <ul********@arc or.de>
wrote:

Hallo!

Now I've tried out for quite a long time how to access the []-operator from
a base class. But I cannot find out how to do this. Well, it's an example
from "Think in C++".
So, I guess I need some help from any real C++ programmer.

Greetings, Ulf

// 14.9 Inherit a class StringVector from vector<void*> and redefine the
push_back( ) and operator[] member
// functions to accept and produce string*. What happens if you try to
push_back( ) a void*?

//StringVector.cp p

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class StringVector: public vector<void*> {
public:
void push_back(strin g* str){cout << "My_push_
back called\n";vecto r<void*>::push_ back(str);}
// following code does not work!!
string* operator[](int i) {
string* t= (string*)(vecto r<void*>&)this[i]; //wrong! What can I do?
return (t);
}
};

int
main(){
StringVector s;
string *str1 = new string("Hallo") ;
string *str2 = new string("Welt");
s.push_back(str 1);
s.push_back(str 2);
cout << *(string*)s[0] << endl;
cout << *(string*)s[1] << endl;
return 0;
}

In general:

(*this)[]

or, if you like:

this->operator[]

or simply :

operator[]

Will do.

</dib>

John Dibling
email: dib@substitute_ my_full_last_na me_here.com
Witty banter omitted for your protection

[John Harrison]

"Ulf Rimkus" <ul********@arc or.de> wrote in message
news:3f******** **************@ newsread2.arcor-online.net...

Hallo!

Now I've tried out for quite a long time how to access the []-operator from a base class. But I cannot find out how to do this. Well, it's an example
from "Think in C++".
So, I guess I need some help from any real C++ programmer.

Greetings, Ulf

// 14.9 Inherit a class StringVector from vector<void*> and redefine the
push_back( ) and operator[] member
// functions to accept and produce string*. What happens if you try to
push_back( ) a void*?

//StringVector.cp p

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class StringVector: public vector<void*> {
public:
void push_back(strin g* str){cout << "My_push_
back called\n";vecto r<void*>::push_ back(str);}
// following code does not work!!
string* operator[](int i) {
string* t= (string*)(vecto r<void*>&)this[i]; //wrong! What

can I do?

string* t = (string*)(*(vec tor<void*>*)thi s)[i];

I have my doubts about the whole concept (deriving from std::vector is not
recommended) but at least that should compile.

john

[Glen Low]

> Now I've tried out for quite a long time how to access the []-operator from

a base class. But I cannot find out how to do this. Well, it's an example
from "Think in C++".
So, I guess I need some help from any real C++ programmer.
class StringVector: public vector<void*> {
public:
void push_back(strin g* str){cout << "My_push_
back called\n";vecto r<void*>::push_ back(str);}

The danger of publicly inheriting from vector<void*> is that
operator[] is not virtual in the vector class. So if you pass a
StringVector to a function that expects a vector<void*>, the compiler
will happily accept it, but if that function then calls operator[], it
will call the original vector<void*> operator not the one you've just
created.

The easiest (and slightly safer) way to do what you want to do is to
privately inherit from vector<void*>, then selectly bring back the
member functions that you want and redefine the ones you want to
redefine e.g.

class StringVector: private vector<void*> {
public:
using vector<void*>:: size;
// etc.
void push_back (string* str); // redefine as the others have
suggested
};

[Ulf Rimkus]

John Harrison wrote:

string* t = (string*)(*(vec tor<void*>*)thi s)[i];

I have my doubts about the whole concept (deriving from std::vector is not
recommended) but at least that should compile.

john

Hallo John!

Thank you! This one compiled and did what I intented it should do. Now I
try to really understand this line of code.

Ulf