Re: Shared code for multiple consrtuctors

From:

NULL@NULL.NULL ("Tom1s")

Newsgroups:

comp.std.c++

Date:

Sat, 13 May 2006 14:16:16 GMT

Message-ID:

<xel9g.9118$j7.305238@news.indigo.ie>

David R Tribble posted:

Java has the feature of allowing a constructor to invoke another
constructor within the same class having a different signature.
This is useful for initilializing class members without duplicating a
lot of code, and comes in handy when default function parameters
are not convenient to use.

Two ways.

1) The not-so-good way:

class Monkey {

    int k;

    char* r;

    void CommonConstructor()
    {
        /* Do the common-denominator stuff in here */
    }

public:

    Monkey( char* arg_r ) : r(arg_r)
    {
        /* Do some particular stuff */

        CommonConstructor();
    }

    Monkey( int arg_k ) : k(arg_k)
    {
        /* Do some particular stuff */

        CommonConstructor();
    }

};

This runs out of steam however if you have const member objects, or
references as members (assuming you don't want to duplicate code).

2) The better way

/* Start off with a bare-bones base class */

class BB_Monkey {
protected:

    int k;

    char* const r;

    int& j;

public:

    BB_Monkey( int const arg_k, char* const arg_r
        int& arg_j)
        : k(arg_k), r(arg_r), j(arg_j) {}
};

class Monkey : public BB_Monkey {
public:

    /* Now put all the fancy constructors
       in here, and have them call the
       common-denominator base-class constructor */

    Monkey( int ) : BB_Monkey(...

};

What you want can be done... I suppose the only question is:

Should it be made more convenient by adding certain functionality to the
language?

-Tom=E1s

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