Questions about operator[]

[toptony]

Hi,guys;

This is an exemple of chapter 11 section 8, Subscripting,The C++ Program Language Special Edition,

#include <iostream>
#include <iterator>
#include <vector>
#include <string.h>
#include <algorithm>
using namespace std;

class Assoc{
		struct Pair{
		string name;
		double value;
		Pair(string n="", double v=0):name(n),value(v) {}
		};
		
		vector<Pair> vec;

		Assoc(const Assoc&);
		Assoc& operator=(const Assoc&);
	public:
		Assoc(){};
		const double& operator[] (const string&);
		double& operator[] (string&);
		void print_all() const;
	};

double& Assoc::operator[] (string& s){

	for(vector<Pair>::const_iterator it = vec.begin();it != vec.end();++it)
		if(s==it->name) return it->value;
	vec.push_back(Pair(s,0));
	return vec.back().value;
}

void print_all(){

	for(vector<Pair>::const_iterator p= vec.begin(); p != vec.end();++p)
		std::cout<<p->name<<" : "<<p->value;
}

int main()
{

	string buf;
	Assoc vec;
	while(cin>>buf)vec[buf]++;
	vec.print_all();

	
	return 0;
}

When I run this program, it have some problems:

One is :

The complier find errors, one of them is

error C2784:"" bool std::operator ==(const std::vector<_Ty,_Alloc> &,const std::vector<_Ty,_Alloc> &)" can't get the "const std::vector<_Ty,_Ax> &" from the template argument of "std::string""

Did I miss any head files in this program or another errors?

The next is:
struct Pair is defined in class Assoc. Does it waste same one memory space in every instance?

question 3:
why defined two funcion of operator[], one is const, is it necessary?

question 4:
This instruction

while(cin>>buf)vec[buf]++;

The "vec[buf]" returned the reference of value of vec[buf] , and the operator++ is not defined in class Assoc.
Can this instruction correct?
if it runs, then this instruction modified the members of class Assoc, is it? and is it correct?

Thanks help!

[dwk]

Hi guy,

I can't reproduce your compiler error using g++ (GCC) 3.4.5 (mingw special).

Anyway, your code will not work, because
1) line 34 should read
void Assoc::print_all() const {
2) in the non-const operator[] you have to use a non-const iterator. The g++ gives a more reasonable error message:
error: invalid initialization of reference of type 'double&' from expression of type 'const double'

Does it work now?

[toptony]

Thanks dwk

#include <iostream>
#include <string.h>
#include <vector>
#include <algorithm>

using namespace std;

class Assoc{
		struct Pair{
		string name;
		double value;
		Pair(string n="", double v=0):name(n),value(v) {}
		};
		
		vector<Pair> vec;

		Assoc(const Assoc&);
		Assoc& operator=(const Assoc&);
	public:
		Assoc(){};
		const double& operator[] (const string&);
		double& operator[] (string&);
		void print_all() const;
	};

double& Assoc::operator[] (string& s){

	for(vector<Pair>::iterator it = vec.begin();it != vec.end();++it)
		if( s == it->name) return it->value;
	vec.push_back(Pair(s,0));
	return vec.back().value;
}

void Assoc::print_all() const {

	for(vector<Pair>::const_iterator p= vec.begin(); p != vec.end();++p)
		std::cout<<p->name<<" : "<<p->value;
}


int main(int argc, char *argv[])
{
  
	string buf;
	Assoc vec;
	while(cin>>buf)vec[buf]++;
	vec.print_all();

  system("PAUSE");	
  return 0;
}

It works when I use the dev-c++(v4.9.8) complimer of bloodshed software.

but it still doesn't work under the VS2003.

same error:
error C2784: "bool std::operator ==(const std::vector<_Ty,_Alloc> &,const std::vector<_Ty,_Alloc> &) "could not get "const std::vector<_Ty,_Ax> &" from "std::string";

I thought there is a ambiguity of operator== since I missed some head files.

Is anyone can help me about Virual Studio C++?

and the questions that I asked last time is not be answered.

any one can help me too.

question 1:
How the complier deal with the define of struct Pair, is it waste memorey space of every instance?

question 2:
Why defined two function of operator[], and one is const?

const double& operator[] (const string&);
double& operator[] (string&);

question 3:
Why the instruction

vec[buf]++;

can modify the value of Assoc's member?

[dwk]

Hi!
Finally got the source of your compiler error:
Don't include string.h but only string:

#include <string> 

For MS-Copiler this makes a difference... (tested on MSVC++ 7.1.3088)
And te iterator in line 28 must not be const.

Reganding your other questions:

ad 1): The definitoin of Struct as an inner class to Assoc doesn't waste any space at runtime, because it is like a type definition. the struct Pair {...} doesn't generate executable code. btw, I don't see the reason fpr definig Pair on your own - std::pair<std::string, double> does basically the same.

ad 2) You have a const and a non-const version of the the [] operator. This is important if you use the [] operator inside a method which is defined const itself. In order to work your declaration shall be
const double& operator[] (const string&) const ; and you have to implement it, of course. The compiler will then pick the const operator if the this -pointer ist const and the volatile version otherwise.

ad 3) Yes, definitely. this is obviously the non-const [] operator...

Try it out by placing this

	std::string key("johndoe");
	std::cout << (*this)[key] << std::endl;

in the printAll method while not having implemented the const-version of the [] operator - the compiler will complain!

[toptony]

:)

haaa.

Thanks dwk!

I found the problem at the same time!

I included the sting.h file, but it must be the sring file!

haaaa.....

where are you, dwk? thank you for your help.again!