Omitting parameter with default value in a subclass function override - c++

Question

I am writing an interface for several I/O classes.
There is a function that looks for information in different kinds of files (sometimes html, sdb, txt, ...):
bool Search(std::string file, std::string field)
However, one of these requires an additional parameter to complement the SQL query. In this case the sdb needs to specify in what table the field is located.
I am trying something like the following (it does not compile, I am aware):
class fileIO{
public:
virtual ~FileIO(){};
virtual bool Search(std::string file, std::string field,
std::string additional = 0 /* for sdb only */) = 0;
}
class readHTML : fileIO{
public:
bool Search(std::string file, std::string field); //does NOT override virtual method
Is there anything that can give me the behavior I am looking for?
Is such strategy according to C++ standards?
What else could I add to replace such enforcement on the interface?
I am sorry if the title is misleading, I am looking for an alternative with that behavior. I could not find it so far.

Answer 1

You don't need it, I'd say.
At the caller site, there is only two possibilities: you know your specific fileIO instance is a sdbIO or you don't. If you do, you can call an overloaded version of Search defined in sdbIO which takes this additional info. If you don't, you don't and sdbIO::Search should be defined in terms of its overloaded version.
struct fileIO
{
virtual bool Search(std::string file, std::string field) = 0;
}
struct sdbIO : fileIO
{
bool Search(std::string file, std::string field, std::string additional);
bool Search(std::string file, std::string field) override
{
Search(file, field, "");
}
};
At the caller site:
void f(fileIO& io)
{
// I know this is a sdb:
dynamic_cast<sdbIO&>(io).Search("/file", "text", "WHERE answer=42");
// I don't
io.Search("/file", "text");
}
notes: do you really need a copy of those strings?

Answer 2

You can hide the virtual function in the non-public interface and make the public interface (with the default argument) non-virtual.
struct Interface
{
...
// public interface calls the abstract members.
bool Search(string const&a, string const&b, string const&c = "")
{
if(c.empty() && need_third_string())
throw runtime_error("search requires an additional string argument");
return search(a,b,c);
}
protected:
virtual bool need_third_string() const = 0;
virtual bool search(string const&, string const&, string const&) const=0;
};
with obvious derivations:
struct A : Interface
{
protected:
bool need_third_string() const override
{ return false; }
bool search(string const&a, string const&b, string const&) const override
{ /* search ignoring third argument */ }
};
struct B : Interface
{
protected:
bool need_third_string() const override
{ return true; }
bool search(string const&a, string const&b, string const&c) const override
{ /* search ignoring using all 3 arguments */ }
};

Answer 3

I don't see any problem with above two way to handle things. Still, I have just one more.
#include<bits/stdc++.h>
#include <stdexcept>
using namespace std;
typedef struct
{
std::string arg1;
std::string arg2;
std::string arg3;
} Param;
class FileIO{
public:
virtual ~FileIO(){};
virtual void Search(Param param) = 0;
};
class ReadHTML : public FileIO{
public:
void Search(Param param)
{
if(param.arg3.length() > 0) // Some logic to handle things here.
search3(param.arg1, param.arg2, param.arg3);
else
throw std::runtime_error("Bad call with param");
}
private:
void search3(std::string arg1, std::string arg2, std::string arg3)
{
std::cout << " I am called with Html::Search3" << std::endl;
}
};
class ReadTxt : public FileIO{
public:
void Search(Param param)
{
if(param.arg1.length() && param.arg2.length()) // Some logic to handle things here.
search2(param.arg1, param.arg2);
else
throw std::runtime_error("Bad call with param");
}
private:
void search2(std::string arg1, std::string arg2)
{
std::cout << " I am called with Txt::Search2" << std::endl;
}
};
// Driver program to test above function
int main()
{
FileIO *io = new ReadHTML();
Param paramHtml = {"a", "b", "c"};
io->Search(paramHtml); // Put some try .. catch
Param paramTxt = {"a", "b"};
io = new ReadTxt(); // Put some try...catch
io->Search(paramTxt);
return 0;
}