macro substitution

[Johan]

Hi,

Can somebody help me with this problem : I have the following member
functions

#define FUNC(x) get##()

class A
{
the declarations etc, etc.
};

string A::getDescripti on()
{
return desc;
}

int A::getNumber()
{
return number;
}

Here come the problem :

string A::makeSQLStrin g(string field)
{
stringstream s;

s << "select * from customer where " << field << " = " << XXXX

return s.str();
}

If want XXX to expand to getDescription or getNumber functions depending on
the value of field. I tried it with a macro FUNC ( see above ) but that
gives me FUNC(field) in the sql string. The func has to expanded to
getNumber or getDescription function.

How to do this

John

[Victor Bazarov]

Re: macro substitution

Johan wrote:[color=blue]
> Can somebody help me with this problem : I have the following member
> functions
>
> #define FUNC(x) get##()[/color]

Didn't you mean

#define FUNC(x) get##x()

??
[color=blue]
>
> class A
> {
> the declarations etc, etc.
> };
>
> string A::getDescripti on()
> {
> return desc;
> }
>
> int A::getNumber()
> {
> return number;
> }
>
> Here come the problem :
>
> string A::makeSQLStrin g(string field)
> {
> stringstream s;
>
> s << "select * from customer where " << field << " = " << XXXX
>
> return s.str();
> }
>
> If want XXX to expand to getDescription or getNumber functions depending on
> the value of field. I tried it with a macro FUNC ( see above ) but that
> gives me FUNC(field) in the sql string. The func has to expanded to
> getNumber or getDescription function.
>
> How to do this[/color]

There is no way. You're asking to expand a macro [at preprocessing-time]
depending on the value of an object ('field') during run-time? That's
done using 'if' statement.

std::string A::makeSQLStrin g(const std::string& field)
{
std::ostringstr eam s;
s << "select * from customer where " << field << " = "
<< (field == "Number" ?
getNumber() : (field == "Descriptio n" ?
getDescription( ) : "NOTHING" ) );
return s.str();
}

Victor

[David Harmon]

Re: macro substitution

On Fri, 29 Oct 2004 17:48:04 +0200 in comp.lang.c++,
"Johan" <me@knoware.n l> wrote,[color=blue]
>string A::makeSQLStrin g(string field)
>{
> stringstream s;
>
> s << "select * from customer where " << field << " = " << XXXX[/color]

First off, is the rightmost << supposed to be a
operator<<(ostr eam&, std::string)
or
operator<<(ostr eam&, std::int)


I'd suggest something like:

s << "select * from customer where " << field << " = ";
if (field == "Descriptio n")
s << getDescription( );
else if (field == "Number")
s << getNumber();

You will certainly get little or no help from macros when you need to make
a decision based on the run-time content of a string argument. It's
called the "preprocess or" because it does its stuff at a very early stage
in the compilation process.