Dynamic variables and function calls in XML files

Question

I'm using libxml2 to parse xml documents for my application. I want the ability to use variables and even function calls in the XML. For example, something like:

< Element attr1="$variable1" attr2="=rand(1, 2, 3, 4)" />

The variables may be defined in the XML and reused, or defined/hardcoded somewhere in the code loading the XML. The functions may be defined in the code (i.e. calling C functions from the XML).

My application already loads XML documents and I can load and modify the XML in memory, but I would like to know: what is the best way to evaluate the variables and function calls?

Are there any XML libraries or interpreter libraries that I can use?

Answer 1

You will have to implement a simple parser (or complex, depending on what you need), which can understand variables and functions.

First step is to extract the expressions from attr1="$variable1" - in the simple case, that's just a matter of looking at the first character inside the string to see if it's a '$' or '='.

If all variables are integers, you can easily store variables in a std::map<std::string, int>. If the value is allowed to be multiple types, then you can either store the value as a string (and convert as needed), or have a struct/class that includes information about what the actual content type is.

Evaluating expressions like =rand(1,2,3,4) would involve some sort of "string to function" construct. A very simple way is an if/else chain. Here we assume that you have extracted the function name as a standalone string called funcname

if (funcname == "rand") 
{
    ... do stuff here... 
} 
else if (funcname == "sin")
{
    .... 
}

A more complex, but often more effective solution is to have a structure that contains a string and a function pointer. Search for the string, and call the function.

Another alternative is to use std::map<std::string, functionType> where functionType is a typedef of a standardized function.

Bot of the latter two solutions pretty much relies on having a standard function type. You can standardise the function by for example having a std::vector<Arg> args as the arguments - the functions can check the number of arguments with if (args.size() != 4) print_error("wrong number of arguments");.

I hope this helps you along the way...