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"Forwarding" iterators without losing const?


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#1 TheComet   Members   -  Reputation: 1519

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Posted 21 October 2013 - 02:56 AM

The title is a little misleading, but I'm not sure what else to call it.

 

Consider the following.

class Bar {};

class Foo
{
public:
    typedef std::vector<Bar*>::iterator iterator;
    iterator begin(){ return m_List.begin(); }
    iterator end(){ return m_List.end(); }
private:
    std::vector<Bar*> m_List;
};

void doStuff( const Foo& foo )
{
    for( Foo::iterator it = foo.begin(); it != foo.end(); ++it ) // ERROR: passing 'const foo' as 'this' argument of 'Foo::iterator Foo::begin()' discards qualifiers
        // do something here with "it"
        ;
}

Changing the function doStuff do the following fixes it, but I'd rather not lose const-ness:

void doStuff( Foo& foo )

What do?


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#2 Brother Bob   Moderators   -  Reputation: 8090

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Posted 21 October 2013 - 03:06 AM

You're calling non-const member functions on a const object. You need two variants of the begin and end function: one set of non-const member function that returns iterator, and one set of const member functions that return const_iterator.

 

edit: You need both if you actually need mutable iterators. The const variant is sufficient if you only need non-mutable iterators.


Edited by Brother Bob, 21 October 2013 - 03:07 AM.


#3 TheComet   Members   -  Reputation: 1519

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Posted 21 October 2013 - 03:16 AM

Thanks! This fixed it:

class Bar {};

class Foo
{
public:
    typedef std::vector<Bar*>::iterator iterator;
    typedef std::vector<Bar*>::const_iterator const_iterator;
    iterator begin(){ return m_List.begin(); }
    iterator end(){ return m_List.end(); }
    const const_iterator begin() const { return m_List.begin(); }
    const const_iterator end() const { return m_List.end(); }
private:
    std::vector<Bar*> m_List;
};

void doStuff( const Foo& foo )
{
    for( Foo::const_iterator it = foo.begin(); it != foo.end(); ++it )
        // do something here with "it"
        ;
}

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#4 NightCreature83   Crossbones+   -  Reputation: 2763

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Posted 21 October 2013 - 06:19 AM

Just as a side question but why are you not handling the iteration inside of Foo? You are kind of exposing class internals in this structure you present here and that is generally speaking bad design.


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#5 TheComet   Members   -  Reputation: 1519

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Posted 21 October 2013 - 07:26 AM


Just as a side question but why are you not handling the iteration inside of Foo? You are kind of exposing class internals in this structure you present here and that is generally speaking bad design.

Below is all of the relevant code (compressed to the minimum) to do with my question, along with explanations. If there is a more elegant way of handling this, please do let me know (note: may contain typos, as it is not a direct copy of the source code).

 

The project is a Sokoban back-end. If you want to review the entire project, you can do so here: https://github.com/TheComet93/chocobun/tree/master/chocobun-core/core

Be warned though, for I am not that experienced yet, and it may make you face-palm repeatedly.

// container for levels, and basically acts as the interface to the front-end
class Collection
{
public:

    typedef std::vector<Level*>::iterator level_iterator
    typedef std::vector<Level*>::const_iterator const_level_iterator;

    level_iterator level_begin(){ return m_Levels.begin(); }
    level_iterator level_end(){ return m_Levels.end(); }
    const const_level_iterator level_begin() const { return m_Levels.begin(); }
    const const_level_iterator level_end() const{ return m_Levels.end(); }

    // loads all levels from a file and stores them in this collection
    void parse( const std::string& fileName ){
        m_FileName = fileName;
        CollectionParser cp;
        cp.parse( m_FileName, *this );
    }

    // saves all levels in this collection to a file
    void save( const std::string& fileName ){
        CollectionParser cp;
        cp.save( m_CollectionName, *this );
    }
private:

    std::vector<Level*> m_Levels;
    std::string m_FileName;
};

// handles everything to do with level data (tiles), player movement, and so forth
class Level
{
public:
    // stuff do do with the levels
};

// base class for all collection parsers
class CollectionParserBase
{
public:
    virtual void save( std::ofstream& file, const Collection& collection ){}
    virtual void parse( std::ifstream& file, Collection& collection ){}
};

// SOK implementation of a parser
class CollectionParserSOK : public CollectionParserBase
{
public:
    void parse( std::ifstream& file, Collection& collection )
    {
        // parses the file, creates levels using the object "collection" and writes level data to those levels
    }

    void save( std::ofstream& file, const Collection& collection )
    {
        for( Collection::const_level_iterator it = collection.level_begin(); it != collection.level_end(); ++it ) // <---- this is where I need it
        {
            // stream everything to the file (e.g. file << data);
        }
    }
}

// other parser implementations here...

// this is used to save and load collections
class CollectionParser
{
public:
    void parse( const std::string& fileName, Collection& collection ){
        std::ifstream file( fileName.c_str() );
        // check if file is open, etc...

        // determine the file format of fileName here, and create instance of corresponding derived parser
        std::auto_ptr<CollectionParserBase> parser = new // whatever parser was chosen

        parser->parse( file, collection );
    }

    void save( const std::string& fileName, const Collection& collection ){
        // open file, determine format, etc. etc. (see parse method above, it's basically the same)
        parser->save( file, collection );
    }
};

The parser requires access to all Level objects contained within the Collection object, and I found it to be easiest to pass Collection to the parser. In an older version, I passed the level array (std::vector<Level*>) to the parser, but the problem was the parser needed access to various other parts of the Collection object (such as the collection name).


Edited by TheComet, 21 October 2013 - 07:29 AM.

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