On Sun, May 29, 2011 at 11:12:09PM -0400, Gene Smith wrote:
> >
> >I really wonder about the logic behind the use of C++ in small embedded
> >devices.
> >
>
> I agree. But sometimes it is decreed on high that you will use c++
> and so and so's c++ library code. :(
At RTSI, we've been using C++ in embedded systems of all sizes, from 32k ROM
on up, for about 15 years now. We've found major advantages over C in code
quality and reuseability. Many things you _should_ be doing in C to make
your code correct, robust, and maintainable are directly supported in C++:
- Encapsulation: limiting the scope over which names can be seen, and
enforcing access restrictions to data and code (like C file-static data
and functions, but more powerful)
- Inheritance: supporting re-use of common data structures and algorithms
without copy-paste coding (like nested structures in C)
- Polymorphism (virtual functions): allowing common code to vary its
operation based on the situation (like C callbacks, but built into the
language)
- Generics (templates): supporting algorithms that can work on many
different data types (replaces C macro functions, but type-safe).
It is true, of course, that not every big-system practice is appropriate for
embedded systems. For example, the usual approach of layering iostreams
over stdio is over-complex. Instead, at RTSI we simply derive our serial
port drivers directly from class streambuf, so we then can use istreams and
ostreams to perform parsing and formatting:
class UartBuf : public streambuf
{
// ...
// override the required streambuf virtual functions:
virtual int overflow(int);
virtual int underflow();
// ...
};
void main()
{
UartBuf uartBuf(/* port address, baud rate, etc */);
ostream os(uartBuf);
os << "Hello, world!" << endl;
}
This technique also works well for things like TCP connections.
C++ is definitely a more sophisticated language than C; it took me some
considerable time to become truly proficient in it. Written properly, a C++
program generates no more code (and in some cases less code) than the
corresponding C program, but the C++ program can be shorter, clearer, more
likely correct, and more robust under maintenance.
On our website are a couple of articles showing how we use C++ for embedded
control: http://www.realtime.bc.ca/articles
Regards,
David Querbach
Real-Time Systems Inc.
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