Re: [Boost-users] callbacks to function template (solved)

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-05-30 11:20:57

Just because of curiosity: Did you take a look at boost::spirit library?

http://spirit.sourceforge.net/distrib/spirit_1_8_3/libs/spirit/doc/preface.html

Best Regards,
Ovanes

-----Ursprüngliche Nachricht-----
Von: Ares Lagae [mailto:ares.lagae_at_[hidden]] 
Gesendet: Mittwoch, 30. Mai 2007 15:24
An: boost-users_at_[hidden]
Betreff: [Boost-users] callbacks to function template (solved)

With the help of the boost lists I finally solved the problem of callbacks to a function template. I will describe what exactly the problem was, and how I have solved it, in the hope that it will be usefull for somebody. I am also open for suggestions and comments.

Problem description
-------------------

I am building a set of parsers. The parser classes pass data to the application using callbacks. Because there are potentially a lot of callbacks, and some of the callbacks are optional, I wrap each callback into a std::tr1::function object, and then pass it to the parser.

The problem I came across can be simplified and abstracted to parsing a format like this one.

bool 1
char 2
short 3
int 4
long 5
float 6.0
double 7.0

Each line contains a type and is followed by a value of that type. This data should be parsed and passed to the application. The types are fixed (bool, char, short, int, long, float, double). (In the actual real problem, there are more types, and "declarations" involving several types.)

Solution 1
----------

The first solution (see listing_1.cpp) registers a callback for each data type. Because each data type is handled in a similar way, there is a lot of code repetition.

Solution 2
----------

The second solution (see listing_2.cpp and variable_list.hpp) replaces the callback function by a function template. Instead of having a member variable for the callback for each data type, the member variables are grouped into a "variable_list". This is a class template that, for each type in a type list, keeps a variable.

The code repetition in the parser class is eliminated, but the user still has to call "set_callback" to register a callback for each of the data types.

Solution 3
----------

The third solution (see listing_3.cpp) eliminates the similar code at the user side. Instead of calling "set_callback" to register a callback for each of the data types, a functor is constructed, that for each type constructs a std::tr1::function object that wraps the callback. That function object is passed to the parser, and the parser sets all the callbacks. This again results in similar code in the parser "set_callbacks".

Solution 4
----------

The fourth solution (see listing_4.cpp) eliminates the similar code in the parser "set_callbacks" by executing a functor for each type in the type list. This code is approximately equivalent to callbacks to a function template.

Best regards,

--
Ares Lagae
Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/