Question

I try to build a very simple calculator supporting x,y,+,-,*,/(,). The idea is simple:

1. Build syntax tree from input string (which may be: “x+3y”)
2. For given x,y calculate the result by parsing the syntax tree, and inserting ints for x and y.

With boost 1.46, you can easily do both steps at once, using parse_phrase(). However, I have to execute step 2 a million of times, while the parsing does not change. So I’d like to separate this (like it was usual in the classic spirit versions). How can I do this? (Please note: boost 1.46 has no utree data structure yet)

Answer 1

I got a solution meanwhile. It is pretty simple, look at the boost docs linked here.

The main() function contains this code:

expression_ast ast;
ast_print printer;
bool r = phrase_parse(iter, end, calc, space, ast);

if (r && iter == end)
{
    // [...]
    printer(ast);
    // [...]
}

There are actually multiple phrase_parse() functions in boost. There are variations of the phrase_parse without and “variation[s] of the phrase_parse with an additional argument: the parser’s attribute.”, as stated in the boost docs. In the above code, ast is the parser’s attribute; printer then evaluates the ast.

If we wanted to evaluate the printer with different values from outside, we would pass them to printer before each call of printer(ast).

Note for 1.46: There’s no boost::utree in yet. However, the AST in this example is a replacement with full functionaliy.