This class symbols implements a symbol table. The symbol table holds a dictionary of symbols where each symbol is a sequence of CharTs (a char, wchar_t, int, enumeration etc.) . The template class, parameterized by the character type (CharT), can work efficiently with 8, 16, 32 and even 64 bit characters. Mutable data of type T is associated with each symbol.

Traditionally, symbol table management is maintained separately outside the BNF grammar through semantic actions. Contrary to standard practice, the Spirit symbol table class symbols is-a parser. An instance of which may be used anywhere in the EBNF grammar specification. It is an example of a dynamic parser. A dynamic parser is characterized by its ability to modify its behavior at run time. Initially, an empty symbols object matches nothing. At any time, symbols may be added, thus, dynamically altering its behavior.

Each entry in a symbol table has an associated mutable data slot. In this regard, one can view the symbol table as an associative container (or map) of key-value pairs where the keys are strings.

The symbols class expects two template parameters (actually there is a third, see detail box). The first parameter T specifies the data type associated with each symbol (defaults to int) and the second parameter CharT specifies the character type of the symbols (defaults to char).

        typename T = int,
        typename CharT = char,
        typename SetT = impl::tst<T, CharT>
    class symbols;
Ternary State Trees

The actual set implementation is supplied by the SetT template parameter (3rd template parameter of the symbols class) . By default, this uses the tst class which is an implementation of the Ternary Search Tree.

Ternary Search Trees are faster than hashing for many typical search problems especially when the search interface is iterator based. Searching for a string of length k in a ternary search tree with n strings will require at most O(log n+k) character comparisons. TSTs are many times faster than hash tables for unsuccessful searches since mismatches are discovered earlier after examining only a few characters. Hash tables always examine an entire key when searching.

For details see

Here are some sample declarations:

    symbols<> sym;
    symbols<short, wchar_t> sym2;

    struct my_info
        int     id;
        double  value;

    symbols<my_info> sym3;

After having declared our symbol tables, symbols may be added statically using the construct:

    sym = a, b, c, d ...;

where sym is a symbol table and a..d etc. are strings. Note that the comma operator is separating the items being added to the symbol table, through an assignment. Due to operator overloading this is possible and correct (though it may take a little getting used to) and is a concise way to initialize the symbol table with many symbols. Also, it is perfectly valid to make multiple assignments to a symbol table to iteratively add symbols (or groups of symbols) at different times.

Simple example:

    sym = "pineapple", "orange", "banana", "apple", "mango";

Note that it is invalid to add the same symbol multiple times to a symbol table, though you may modify the value associated with a symbol artibrarily many times.

Now, we may use sym in the grammar. Example:

    fruits = sym >> *(',' >> sym);

Alternatively, symbols may be added dynamically through the member functor add (see symbol_inserter below). The member functor add may be attached to a parser as a semantic action taking in a begin/end pair:


where p is a parser (and sym is a symbol table). On success, the matching portion of the input is added to the symbol table.

add may also be used to directly initialize data. Examples:

    sym.add("hello", 1)("crazy", 2)("world", 3);

Assuming of course that the data slot associated with sym is an integer.

The data associated with each symbol may be modified any time. The most obvious way of course is through semantic actions. A function or functor, as usual, may be attached to the symbol table. The symbol table expects a function or functor compatible with the signature:

Signature for functions:

    void func(T& data);

Signature for functors:

    struct ftor
        void operator()(T& data) const;

Where T is the data type of the symbol table (the T in its template parameter list). When the symbol table successfully matches something from the input, the data associated with the matching entry in the symbol table is reported to the semantic action.

Symbol table utilities

Sometimes, one may wish to deal with the symbol table directly. Provided are some symbol table utilities.


    template <typename T, typename CharT, typename SetT>
    T*  add(symbols<T, CharT, SetT>& table, CharT const* sym, T const& data = T());

adds a symbol sym (C string) to a symbol table table plus an optional data data associated with the symbol. Returns a pointer to the data associated with the symbol or NULL if add failed (e.g. when the symbol is already added before).


    template <typename T, typename CharT, typename SetT>
    T*  find(symbols<T, CharT, SetT> const& table, CharT const* sym);

finds a symbol sym (C string) from a symbol table table. Returns a pointer to the data associated with the symbol or NULL if not found


The symbols class holds an instance of this class named add. This can be called directly just like a member function, passing in a first/last iterator and optional data:

    sym.add(first, last, data);

Or, passing in a C string and optional data:

    sym.add(c_string, data);

where sym is a symbol table. The data argument is optional. The nice thing about this scheme is that it can be cascaded. We've seen this applied above. Here's a snippet from the roman numerals parser

    //  Parse roman numerals (1..9) using the symbol table.

    struct ones : symbols<unsigned>
                ("I"    , 1)
                ("II"   , 2)
                ("III"  , 3)
                ("IV"   , 4)
                ("V"    , 5)
                ("VI"   , 6)
                ("VII"  , 7)
                ("VIII" , 8)
                ("IX"   , 9)

    } ones_p;

Notice that a user defined struct ones is subclassed from symbols. Then at construction time, we added all the symbols using the add symbol_inserter.

The full source code can be viewed here. This is part of the Spirit distribution.

Again, add may also be used as a semantic action since it conforms to the action interface (see semantic actions):


where p is a parser of course.