This is step 5 of the calculator project in Exploration 68 of Exploring C++: The Programmer's Introduction to C++.

This version of the calculator adds library files. The save command saves the global symbol table to a file, and the load command loads symbols from a file into the global symbol table.

This calculator has a simple syntax:

start ::= statement-list
statement-list ::= empty | statement-list statement
statement ::= definition | quit | expression | "load" string | "save" string
definition ::= "def" identifier optional-parameters "=" expression
optional-parameters ::= empty | "(" optional-identifier-list ")"
optional-identifier-list ::= empty | identifier-list
identifier-list ::= identifier | identifier-list "," identifier
expression ::= multiplicative-expression "+" multiplicative-expression |
               multiplicative-expression "-" multiplicative-expression
multiplicative-expression ::= unary-expression "*" unary-expression |
                              unary-expression "/" unary-expression
unary-expression ::= "-" primary | "+" primary | primary
primary ::= number | function-call | "(" expression ")"
function-call ::= identifier optional_arguments
optional_arguments ::= empty | "(" optional-expression-list ")"
optional-expression-list ::= empty | expression-list
expression-list ::= expression | expression-list "," expression

Function arguments are evaluated lazily. There are no variables, only functions, but function definitions can look just like variable assignments:

def pi = 3.141592653589792

so there really isn't any difference between a variable and a function.

The calculator reads expressions from the standard input, evaluates them, and prints the results to the standard output. It's that simple.

The calculator parses expression into a parse tree. The parser uses node objects. The node class uses the pimpl idiom with a tree of node_impl-derived classes.