Hi,
I'm working with Borland C++ Builder 6.2.
My project uses the std::string class. However, Borland in
its infinite wisdom has its own string class, AnsiString.
To make my life easier, I want to create a function,
preferably a conversion operator, to convert from a
std::string to an AnsiString. Please note that I don't
want to change either std::string or AnsiString.
The Background
--------------
In using Borland's Database Engine (BDE), there is
a variant type "TVariant" which it uses for its
field types. The TVariant is a union:
class TVarRec
{
public:
union
{
Integer VInteger;
Boolean VBoolean;
Char VChar;
PExtended VExtended;
PShortString VString;
Pointer VPointer;
PChar VPChar;
TObject* VObject;
TClass VClass;
WideChar VWideChar;
PWideChar VPWideChar;
Pointer VAnsiString;
PCurrency VCurrency;
PVariant VVariant;
};
union
{
Byte VType;
};
};
I'm writing my own generic wrapper for fields,
and I have a function to fill in a TVarRec
variable:
class Field
{
public:
virtual void fill_variant(TV arRec * pv) const = 0;
};
class Integer_Field
: public Field
{
int value;
public:
virtual void fill_variant(TV arRec * p_variant) const
{*p_variant = value;}
};
So far, so good (I hope). Now I'm trying to make
a templated field class. The issue is that TVarRec
has no assignment for std::string, but for AnsiString.
Which means that I need a conversion for the
fill_variant method:
class String_Field
: public Field
{
std::string value;
public:
virtual void fill_variant(TV arRec * p_variant) const
{*p_variant = AnsiString(valu e.c_str());}
// The AnsiString type has constructor for C style strings.
};
When I convert String_Field to use a template, the content
of the fill_variant method differs from the Integer field.
template <typename Field_Value>
class Typed_Field
: public Field
{
Field_Value value;
public:
virtual void fill_variant(TV arRec * p_variant) const
{*p_variant = value;}
};
The thorn in this design is that Typed_Field<std ::string>
causes an error because there is no std::string type
defined in the TVarRec. The PShortString type will
convert from an AnsiString type though.
I have other cases where I am having to convert from
a std::string type to an AnsiString type (such as all
the windowing APIs). This is where a global conversion
function would come in handy. The function would be
really handy if it could be implicit so I don't have
to write something like:
edit_box.Text = AnsiStringFromS TLString(s);
I'd rather write:
string hello("hello");
edit_box.Text = hello; // Implied conversion
Any ideas?
BTW, this is not off-topic, as I am searching for
a conversion operator between types; regardless
of whether the types are STL, C++ runtime or
external libraries.
--
Thomas Matthews
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