C++: Dynamically access class properties

Question

Hi, I'm wondering if it is possible to access a property of a class dynamically without knowing the name of the property accessed at run-time.

To give you a better understanding of what I'm aiming at, this php code should demonstrate what I want to do:

<?php
$object = new object();
$property = get_name_out_of_textfile();
echo $object->$property;
?>

I know that is it not possible to do the exact same thing but let's say I got the name of an attribute in a cstring of std::string and want to access the attribute matching the name. Is there a way to do that?

The background is that I want to dynamically load and save data from a class with lots of attributes. I can save the in any format I want but I need a way to load them back in again without specifying the exact attribute every time.

Thanks in advance for any help, Robin.

Answer 1

If serialization is your main concern, take a look at existing solutions like Boost.Serialization which opts for a streaming approach:

// save data to archive:
// create and open a character archive for output
std::ofstream ofs("filename");
boost::archive::text_oarchive oa(ofs);
// write class instance to archive
oa << myInstance;

// ...

// restore data from archive:
// create and open an archive for input
std::ifstream ifs("filename");
boost::archive::text_iarchive ia(ifs);
// read class state from archive
ia >> myInstance;

Answer 2

Basically, you need to create an extra function, ala:

std::string get(const std::string& field, const std::string& value_representation)
{
    std::ostringstream oss;
    if (field == "a")
        oss << a;
    else if (field == "b")
        oss << b;
    else
        throw Not_Happy("whada ya want");
    return oss.str();
}

There are lots of existing frameworks to do this, such as the boost serialization library. Most involve declaring your class using some kind of markup, or worse - delocalised secondary metadata.

If you really need something less invasive, then tools such as Gcc-XML and OpenC++ allow you to automate the generation of these functions (the former can easily be combined with XML libraries, a little python or ruby, for an easy but low performance win).

Just to be absolutely clear, C++ does not provide any automated, Standard-compliant way to do this. No popular compilers provide extensions for this. In desparation, you may be able to get somewhere parsing your own debug information, but that's definitely not recommended.

Answer 3

Yes it's possible... Python, php, etc... are implemented on top of C or C++, and they offer it.

But it's also extremely awkward.

What you are asking for, the ability to inspect a type at runtime, is called introspection. It is difficult to implement, so given the way you phrased your question... don't think about it yet, you need more experience.

If you want introspection, think about another language, really.