Re: C++ implementation question

"Jim Langston" <>
Tue, 20 Mar 2007 03:12:32 -0700
"Dilip" <> wrote in message

I was wondering if you guys can help me out with a design issue I am
running into.

I have a class that needs to maintain a mapping between a table name
and its corresponding list of columns.

The column class is defined thusly:

struct Column
enum Type
BOOL = 1,
INT = 2,
// etc. and so on
Type type(); // returns the type of the column

There is also an API that gives me the value of a column given a
column pointer. Its overloaded for various types like:

APIPtr->get(Column* c, <overloads_for_various_types>& colvalue);

APIPtr->get(Column* c, int& value);
APIPtr->get(Column* c, char*& value);
// etc

I went down this direction:

class TableHandler
std::map<std::string, std::vector<Column*> > Table2ColumnMap;
Table2ColumnMap table2Col_; // this map is set at construction time
Column* c1 = APIPtr->getColDetailsFromSomewhere("COLNAME1");
Column* c2 = APIPtr->getColDetailsFromSomewhere("COLNAME2");
Column* c3 = APIPtr->getColDetailsFromSomewhere("COLNAME3");
vector<Column*> colvec;
tale2Col_.insert(std::make_pair("TABLE1", colvec));

I was wondering if there is a way to write a method in TableHandler
that can give iteratively extract the values of all the columns given
a table name (using that APIPtr->get() thingy).

So I came all the way here:

void TableHandler::ExtractColValues(std::string const& tablename)
// 1. drill into map; 2. locate associated vector of columns
// 3. IT is at this point I am a little confused


As I noted in (3), each Column* already maintains an enum that is
indicative of its type. I want to write a clean mechanism to extract
the value of that column using APIPtr->get() and somehow using its
type. I just am not able to translate it into code.

Can someone point me in the right direction? Just a hint would be
appreciated -- I can expand it from there.

I just did it this way. I'm not saying if it's good or bad, it's just the
way I handled it.

class CField

    CField( const std::string& Name, const std::string Type, const bool /*
Null */, const bool /* PriKey */, const std::string& Default, const
std::string& /* Extra */);

    void operator=( const std::string& Value ) { Value_ = Value; Set_ =
true; }
    void operator=( const int& Value ) { Value_ = jml::StrmConvert( Value );
Set_ = true; }
    void operator=( const unsigned int& Value ) { Value_ =
jml::StrmConvert( Value ); Set_ = true; }
    void operator=( const bool& Value ) { Value_ = Value ? "true" : "false";
Set_ = true;}
    void operator=( const short int& Value ) { Value_ = jml::StrmConvert(
Value ); Set_ = true; }
    void operator=( const unsigned short int& Value ) { Value_ =
jml::StrmConvert( Value ); Set_ = true; }
    void operator=( const float& Value ) { Value_ = jml::StrmConvert(
Value ); Set_ = true; }
    void operator=( const double& Value ) { Value_ = jml::StrmConvert(
Value ); Set_ = true; }

    std::string Value() const { return Value_; }
    std::string Name() const { return Name_; }
    void Reset() { Set_ = false; Value_ = ""; }
    bool Set() const { return Set_; }
    void Set( bool Value ) { Set_ = Value; }
    ESQL_FieldTypes FieldType() { return FieldType_; }

    std::string EscapeString( MYSQL* Sock ) const;


    std::string Value_;
    bool Set_;

    std::string Name_;
    std::string Default_;

    ESQL_FieldTypes FieldType_;
    unsigned short int FieldLength_;


I was mainly worrying about setting the value. If I wanted to think about
getting the value I would of added oeprator int, operator float, etc...

operator int() { if ( Set_ && FieldType_ == SQL_FieldType_Int || FieldType_
== SQL_FieldType_UnsignedInt /* ... */ ) { return jml::StrmConvert<int>(
Value_ ) else return 0 /* or throw */; }
operator std::string() ( if ( Set_ ) return Value_ else return ""; // or
throw */ }
operator bool { if ( Set_ } { if ( Value == "TRUE" } return true; else
return false; } else return false /* or throw */; }


I really can't tell you why I dont' have those operator int's etc.. in there
now, or how I'm getting the values since it wound up being a lot of
dependant classes that is pretty much a nightmare to maintain right now.
I'm going to have to go through and refactor it all. I made it, and started
using it but don't like it right now. I think it could be better.

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