C++ template meta-programming, number of member variables?

Question

hello

Is it possible in C++ to determine number of variables/fields in the generic class? for example

// suppose I need metaclass number_members determines number of members

struct example { int i, j; };
assert(number_members<example>::value==2);

I looked through mpl but could not find implementation.

thanks.

Answer 1

No. C++ does not provide general introspection into structures.

You can try a C++0x std::tuple, which has some of the features of a general POD struct. Or, try to roll your own from the Boost MPL library. That would be a bit advanced if you're just getting started with C++.

Answer 2

No. Unfortunately, C++ does not have that kind of introspection builtin. However, with some additional preprocessing such as Qt's Meta Object Compiler (moc), you can achieve something similar... the QMetaObject class provides a propertyCount(); however, your class would need to inherit from QObject, use the Q_OBJECT macro, and register the properties for all that to work... so, in short, it's not automatic.

Answer 3

You can't do that directly. The obvious question then, is what you're trying to accomplish -- chances are that you can do what you need to, but the way to do it may be rather different.