C++: Using inheritance when not all of the derived classes are known up front?

Question

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Is there a way to instantiate objects from a string holding their class name?

I'm working on a problem in some C++ code that has a base class InputFile, and a number of derived classes: TxtInputFile, ASCInputFile, etc, where each derived class is a specific input type.

What I'd like to be able to do is to take a variable off the command line, and then generate the correct derived class object to deal with the indicated file type (e.g. user indicated TXT off the command line, so, I generate a TXTInputFile object and return under the InputFile label for use in the rest of the program).

I could do this with a string of IF / 'ELSE` statements, comparing the user-input with a bunch of predetermined file codes, but I'd like to be able to add support for new file types in the future without editing a string of if statements and adding new file codes, etc.

Is there any way to get access to some compiler generated table of all derived classes to a base class at runtime?

Or perhaps some sort of polymorphic constructor that is dynamically bound based on what the passed parameter is equal to?

(e.g. InputFile(string)... TXTInputFile(string temp = "TXT"), ASCInputFile(string temp = ASC")... I realize that's the format for default values to parameters, just trying to suggest where I was going with that train of thought.)

Thanks in advance.

Answer 1

You can make a mapping (perhaps with std::map) from strings to factory functions.

Answer 2

Don't roll your own parser if the parsing is at all complex. There are lots of options, ranging from the old standby lex/yacc (or flex/bison) to Boost::Spirit to ANTLR. Lex/yacc is a LALR parser and it generates code. The generated code is a bit hard to follow and debug. Boost::Spirit is much more in line with modern programming concepts, as is ANTLR. The basic idea is that you specify the grammar and how that grammar is to be processed.