Building an AST

The ASTContainer class is intended to be used as the superclass for most AST nodes. Any ASTPtr and ASTList fields of subclasses of this class will automatically be created from the AST stack. Each AST class that is constructed is pushed onto the stack in the order that it is constructed and then popped off by its parents.

AST nodes do not, by default, keep around the InputRange of the text that they matched. This is to save space for cases where it is not required. If you intend to do helpful error reporting after semantic analysis, then it is strongly recommended that you do keep such a reference.

After you have defined your grammar and AST, all that remains is to bind the two together. To do this, create a subclass of ASTParserDelegate, with one BindAST field for each AST node, initialised with the corresponding grammar rule. The following is the parser for the Calculator example:

	class CalculatorParser : public ASTParserDelegate
		BindAST<AST::Number> num = CalculatorGrammar::get().num;
		BindAST<AST::AddExpression> add = CalculatorGrammar::get().add_op;
		BindAST<AST::SubtractExpression> sub = CalculatorGrammar::get().sub_op;
		BindAST<AST::MultiplyExpression> mul = CalculatorGrammar::get().mul_op;
		BindAST<AST::DivideExpression> div = CalculatorGrammar::get().div_op;
		const CalculatorGrammar &g = CalculatorGrammar::get();

Invoking the parse() method on this class will cause an AST::Number class to be created for every terminal matching the num rule in the grammar, and so on. Note that this parser is reentrant. It is safe to use it from multiple threads to parse different strings. It is therefore safe to also make the parser a singleton.