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::getDescription()
{
return desc;
}

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

Here come the problem :

string A::makeSQLString(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]

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

#define FUNC(x) get##()

Didn't you mean

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

??

class A
{
the declarations etc, etc.
};

string A::getDescription()
{
return desc;
}

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

Here come the problem :

string A::makeSQLString(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

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::makeSQLString(const std::string& field)
{
std:stringstream s;
s << "select * from customer where " << field << " = "
<< (field == "Number" ?
getNumber() : (field == "Description" ?
getDescription() : "NOTHING" ) );
return s.str();
}

Victor

 

[David Harmon]

On Fri, 29 Oct 2004 17:48:04 +0200 in comp.lang.c++,

string A::makeSQLString(string field)
{
stringstream s;

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

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


I'd suggest something like:

s << "select * from customer where " << field << " = ";
if (field == "Description")
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 "preprocessor" because it does its stuff at a very early stage
in the compilation process.