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Inline Variant Switch? Macro/Function?

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I've recently begun my first dealings with a variant type. My only gripe with the pattern is that I would need to create a switch anywhere I use it to select the right data member from the union. I was going to make a macro as it would evade running code every time I go to use the data, but then quickly realized that was doomed to failure due to being a preprocessor directive in the first place. Although I could just implement all the data selection code within the macro every time I need to use it, or use auto assignment with the macro. auto p = myvariant.MacroGetData(); However then the issue is how to get the macro to compile inline at all. So then perhaps I could pass in a variable to the macro to assign the correct data to, but then how do I know what variable type to create.

 

I am having some difficulty thinking of a solution, if one even exists.

 

Has anybody encountered a variant type which simplifies the data selection down to a function-like call to be made inline?

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Usually I try to write code that already knows the type of data it's dealing with. So, at compile-time the type is known and just be directly casted. If the type is not known at compile-time then a run-time solution must be used, which means some kind of small performance hit (perhaps negligibly small). Usually this would be called run-time polymorphism. Good example of compile-time polymorphism is function overloading, or C++ templates (and template specialization), or any other code-generation tool.

 

Some options to implement run-time polymorphism come to mind:

  • function pointers for polymorphism
  • switch statements
  • jump table (enums/function pointers/virtual dispatch)

So I'd recommend trying to make sure a compile-time option will not work for you. If not then look into using some kind of run-time method. Good candidates for run-time polymorphism are pieces of code that do a common algorithmic task, but can operate on different kinds of data:

  • Serializing data
  • Settor/Gettor for member variables
  • Conversions from one type to another
  • etc.

So then we could write something like:

var a = 10;
var b = 12.0f;

printf( "%s/n", a.ToString( ) ); // "10"
printf( "%s/n", b.ToString( ) ); // "10.000000f"

Where ToString can be a polymorphic procedure. Inside of ToString we can use switches, or a jump table, or any other means of implementing polymorphism at run-time.

Edited by Randy Gaul

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