C++ tree output error

Question

for my data structures class, we are making a data structure that we can use to easily store and organize data. I am having an issue with the output function of my tree. The error message that I get is:

AccountDB.cpp: In member function ‘void AccountDB::output(std::ostream&) const’:
AccountDB.cpp:23:21: error: passing ‘const AccountDB’ as ‘this’ argument of ‘void    
AccountDB::output(std::ostream&, const AccountDB::Elem*)’ discards qualifiers [-fpermissive]

I've been looking around and my output code looks pretty similar to what other people have done. I have no idea, and I don't really understand what the error is trying to tell.

Thanks for your help.

Header:

#ifndef ACCOUNTDB_H
#define ACCOUNTDB_H


#include <iostream>

using namespace std;


#include "AccountRecord.h"

class AccountDB {

public:
    AccountDB();
    ~AccountDB();
    void insert( const AccountRecord &v );
    AccountRecord * get( const AccountRecord &v );
    void output( ostream &s ) const;

private:
    struct Elem {
        AccountRecord info;
        Elem *left;
        Elem *right;
    };

Elem *root;

void insert( const AccountRecord &v, Elem *&e );
AccountRecord * get( const AccountRecord &v, Elem *&e );
void output( ostream &s, const Elem *e );

};

ostream &operator << ( ostream &s, const AccountDB &v );

#endif

Source

#include "AccountDB.h"

//default constructor
AccountDB::AccountDB() {
    root = 0;
}

//destructor
AccountDB::~AccountDB() {

}

//public
void AccountDB::insert( const AccountRecord &v ) {
    return insert( v, root );
}

AccountRecord * AccountDB::get( const AccountRecord &v ) {
    return get( v, root );
}

void AccountDB::output( ostream &s ) const {
    output( s, root );
}

//private
void AccountDB::insert( const AccountRecord &v, Elem *&e ) {
    if( e == NULL ) {
        e = new Elem();
        e->info = v;
    }

    else if( v < e->info )
        insert( v, e->left );
    else if( v > e->info )
        insert( v, e->right );
}

AccountRecord * AccountDB::get( const AccountRecord &v, Elem *&e ){
    if( e->info == v )
        return &(e->info);
    else if( v < e->info && e->left != NULL )
        get( v, e->left );
    else if( v > e->info && e->right != NULL )
        get( v, e-> right );
    else
        return NULL;
}

void AccountDB::output( ostream &s, const Elem *e ) {

    if( e != NULL ) {
        output( s, e->left );
        s << e->info << endl;
        output( s, e->right );
    }
}

ostream &operator << ( ostream &s, const AccountDB &v ) {
    v.output( s );
    return s;
}

Sergey Kalinichenko · Accepted Answer · 2012-04-02 21:43:51Z

5

Your output function is not declared const, so when you call

output( s, root );

the compiler tells you that you are calling a non-const function from inside a const function.

There are several ways to deal with this - one is to make output const; the other is to make output static (if you can).

answered Apr 2, 2012 at 21:43

Sergey Kalinichenko

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6 Comments

Sergey Kalinichenko Over a year ago

@LuchianGrigore Yeah, I know, I'm a slow typist :(

Luchian Grigore Over a year ago

+1 for the suggestion to make it static. He actually can and should.

josephthomas Over a year ago

Heh, you guess still beat me when I was just about to post the answer.

dajee Over a year ago

Thanks dasblinkenlight, what is the advantage of using static instead?

Sergey Kalinichenko Over a year ago

@David Generally, it is a good idea to use static functions to indicate their logical independence of the instance. This conveys your intent to human readers of your programs. Less importantly, you can save some CPU cycles by not passing the hidden "this" pointer to your functions.

|

Luchian Grigore · Accepted Answer · 2012-04-02 21:43:48Z

2

The error is that

void AccountDB::output( ostream &s, const Elem *e )

is not declared as const, but you call it from a const method.

Change the declaration (and definition) to:

void output( ostream &s, const Elem *e ) const;

You can do this since you're not modifying any members inside the function.

answered Apr 2, 2012 at 21:43

Luchian Grigore

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