Design direction for multiple format parsing

Question

I am writing an app to parse lines in a text file. The problem is that I need to be able to load different routines depending on a variable set at run-time. I can not change the format of the incoming file.

int intFormat = 1; //Loaded from INI file

void __fastcall TForm1::Button1Click(TObject *Sender) {
    myFileConverstion *myFC;
    switch(intFormat) {
        case 1:
            myFC = new FileConverstionCompanyA(); 
        case 2:
            myFC = new FileConverstionCompanyB();
    }
    myFileConverstion->Execute("fileName");
}

Within ->Execute(), I would be calling private (or protected) methods to do the parsing. There are some of the methods that could be used across all formats, too.

What would be the best OOP way to do this?

• Create a virtual object i.e: myFileConverstion? Then inherit from that for the CompanyA, B, C, etc.
• write myFileConverstion with all the common methods (private/protected) and a virtual Execute(). Then just change the Execute() internals for the various "companies"?

I'm looking for some guidance.

Haven't really tried anything yet, I'm in the planning stage.

Answer 1

There would also be the possibility to realize the whole thing with std:.function by inserting them into some kind of lookup map. Here is some example:

#include <functional>
#include <iostream>
#include <unordered_map>

class FileConverter
{
    using convert_engine_t = std::function< bool( const std::string& ) >;
    std::unordered_map< size_t, convert_engine_t > engines;
    convert_engine_t *activeEngine { nullptr };
public:
    template < class T >
    auto addEngine( size_t id, T && routine ) -> void
    {
        engines.emplace( id, std::forward< T >( routine ) );
    }

    auto setActiveEngine( size_t id ) -> bool
    {
        const auto iterFound = engines.find( id );

        if ( iterFound == engines.end() )
            return false;

        activeEngine = &iterFound->second;
        return true;
    }

    auto convert( const std::string fileName ) -> bool
    {
        if ( !activeEngine || !( *activeEngine ) )
            return false;

        return ( *activeEngine )( fileName );
    }
};

int main()
{
    struct MyConvertingEngineA
    {
        auto operator()( const auto& fn ) {
           std::cout << "Doing A" << std::endl; return true;
        }
    };

    auto myConvertingEngineB = []( const auto& fn ) {
        std::cout << "Doing B" << std::endl; return true;
    };

    FileConverter myFC;
    myFC.addEngine( 1, MyConvertingEngineA() );
    myFC.addEngine( 2, std::move( myConvertingEngineB ) );
    myFC.addEngine( 8, []( const auto& fn ){ std::cout << "Doing C" << std::endl; return true; } );

    myFC.setActiveEngine( 1 );
    myFC.convert( "MyFileA1" );
    myFC.convert( "MyFileA2" );

    myFC.setActiveEngine( 2 );
    myFC.convert( "MyFileB" );

    myFC.setActiveEngine( 8 );
    myFC.convert( "MyFileC" );

    myFC.setActiveEngine( 7 ); // Will fail, old engine will remain active
    return 0;
}

Output:
Doing A
Doing A
Doing B
Doing C

A few explanations about the code:
The addEngine function uses templates with a forwarding reference and perfect forwarding to provide the widest possible interface. Engines are copied or moved here, depending on the type of reference passing. The engines are passed in form of "callable objects". Different types like functors or lambdas can be added. Even (as in the example) generic lambdas can be passed (auto parameter) as long as the function signature of of std::function is fulfilled. This saves another redundant specification of the string type. If you have large and complex engines, use a separate class instead of a lambda to keep the local scope small

Shared parser-functionality could also be passed to the respective engines via smart pointer if you don't want to have any inheritance.

    class EngineA
    {
        std::shared_ptr< ParserCore > parserCore;
    public:
        EngineA( std::shared_ptr< ParserCore > parserCoreParam ) : 
            parserCore( std::move( parserCoreParam ) ) 
        {}
        
        auto operator()( const auto& fn ) {
           std::cout << "Doing A" << std::endl; return true;
           // -> parserCore->helperFuncForABC() ....;
        }
    };