.. index::
   single: Parse Tree API

Main methods to handle ANTLR ParseTree nodes
============================================

ANTLR generates a visitor method for each rule in the grammar.
The visitor method looks like:

::
   
   std::any visitRULE(AslParser::RULEContext *ctx)
    
where ``RULE`` is the name of the rule.

The parameter of each visitor method is a pointer ``ctx`` to an object of class ``RULEContext``, which is derived from ANTLR class ``ParseRuleContext``.

The pointer ``ctx`` allows access to information in the node currently being visited, and to its children.

For instance, if our grammar contains the rule:

::
   
   expr : expr (MUL|DIV) expr

then our visitor will have a method:

::
   
   std::any visitExpr(AslParser::ExprContext *ctx)


Inside this method we can, for instance:

* Check whether the node has a ``MUL`` child:
    
   ::

      if (ctx->MUL()) ...


* Get the string that matched the ``MUL`` regex:
    
   ::
   
      ctx->MUL()->getText()

   (not very useful for ``MUL``, but very useful e.g. for ``ID`` or ``INTVAL``)
    
* Check whether the rule has some ``expr`` child:

   ::
   
      if (ctx->expr())...
    
* Get how many ``expr`` children the rule has:
    
   ::
   
      ctx->expr().size()

  Note: Only when more than one ``expr`` are expected in the rule/subrule, ``ctx->expr()`` is a ``std::vector<ParseRuleContext>``. Otherwise, ``ctx->expr()`` is *the* only ``expr`` child.
    
* Get the i-th ``expr`` child (counting from 0):
    
   ::
   
      ctx->expr(i)
    
* Get the total number of children of any kind:
    
   ::
   
      ctx->children.size()

* Get the i-th child of any kind:
    
   ::
   
      ctx->children[i]

* Both ``ctx->expr()`` and ``ctx->children`` can be iterated if needed:
    
   ::
   
      for (int i=0; i<ctx->expr().size(); ++i) visit(ctx->expr(i));
      for (auto ex : ctx->expr()) visit(ex);

      for (int i=0; i<ctx->children.size(); ++i) visit(ctx->children[i]);
      for (auto ch : ctx->children) visit(ch);

  The last two iterations are equivalent to a simple call to ``visitChildren(ctx)``.