Re: confused about .h and .cpp definition vs implementation

"Alf P. Steinbach" <>
Sat, 26 Jan 2008 21:03:48 +0100

Hello everyone,
maybe some usefull information first: I'm a student, I can program and
most of my experience is in java but I always had the feeling java is
"the easy way out" so now I'm interested in some c++. I'm longing to
make a speedy raytracer and that is why I need c++ and need to learn
alot about how it works internally to see what choices are the best
way for an optimal speed result.

I have some troubles understanding the concepts of a header and all
around it. It seems that a header has the purpose to define classes
and functions. But its also possible to actually implement the classes
and functions in there. But why would someone do that? Is it better?

I see three options of implementation at the moment:

1) defining a header with the definitions and implement them in a .cpp
2) defining a header with the definitions and the implementations
3) using a cpp file and implement it all in there.

So what are the pro's and contras here? Currently I'm using using
option 1 since I learned it that way in a basic OO-language course.

Also, it is possible to define a range of multiple classes in the same
header file, why would I want that over the 1 header - 1 class
structure that I'm using now(probably a java influence here).

Headers and implementation files are a way to emulate logical separately
compiled modules in a language that has a pre-processor but no direct
support for separately compiled modules.

Separately compiled modules are a way to physically split and package a
large system so as to reduce complexity and build times, and allow
concurrent development of different parts.

Method (1) achieves these goals for pure OO code, but with current
compilers (except Comeau) fails to provide information hiding for
template code, because current compilers don't support "export" so that
the template implementations must be provided in the header file.

Method (2) supports template code, and can avoid some extra work, at the
cost of possibly longer build times and not providing information
hiding. In particular, it does not support the common PIMPL idiom.

Method (3) means essentially that the program is so small that there's
no need for physical packaging.

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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