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Understanding C++14 in embedded systems (Part 1)

Posted: 20 Feb 2015     Print Version  Bookmark and Share

Keywords:C99  microcontroller  compiler  constructor  destructors 

In 1998, I wrote an article for Embedded Systems Programming titled C++ in Embedded Systems – Myth and Reality. The article was intended to inform C programmers concerned about adopting C++ in embedded systems programming.

A lot has changed since 1998. Many of the myths have been dispelled, and C++ is used a lot more in embedded systems. There are many factors that may contribute to this, including more powerful processors, more challenging applications, and more familiarity with object-oriented languages.

C99 (an informal name for ISO/IEC 9899:1999) adopted some C++ features including const qualification and inline functions. C++ has also changed. C++11 and C++14 have added some cool features (how did I manage without the auto type specifier?) and some challenges, like deciding when to use constexpr functions.

But C++ has not displaced C, as I thought it would in 1998. C is alive and well in the Linux kernel, and there is a body of opinion implacably opposed to C++ in that environment.

The suspicion lingers that C++ is somehow unsuitable for use in small embedded systems. For 8- and 16-bit processors lacking a C++ compiler, that may be a concern, but there are now 32-bit microcontrollers available for under a dollar supported by mature C++ compilers. As this article series will make clear, with the continued improvements in the language most C++ features have no impact on code size or on speed. Others have a small impact that is generally worth paying for. To use C++ effectively in embedded systems, you need to be aware of what is going on at the machine code level, just as in C. Armed with that knowledge, the embedded systems programmer can produce code that is smaller, faster and safer than is possible without C++.

My history with C++
When I started a new microcontroller project a few years ago, I had to choose a tool-chain for the project. The MCU used (NXP LPC2458) was a 72MHz ARM7 with 512KB FLASH and 64KB RAM. Some toolchain vendors were surprised to be asked about the memory footprint of C++ libraries. When one vendor was pressed on the issue of a bloated library component, they said not many people are using C++ in such resource-constrained devices and it's hard to justify the cost of improving the library. Bear in mind that this "resource-constrained device" was somewhat more powerful than the DOS platform that ran commercial software written in C++ in the 90s.

So in 2015, it seems that there's still a need to de-mystify C++ for software engineers who are expert in embedded systems and in C, but wary of C++. If you're not familiar with C++, if you find that not many people are using it for applications like yours and if it's considered unsuitable for the Linux kernel, this wariness is understandable.

This is a revised version of the 1998 article addressing this issue. Less attention is given to features present in C99, since C programmers are likely to be familiar with them. The reader is assumed to be familiar with C99, which is used in the C code examples. The reader is also assumed to understand the C++ language features discussed, but doesn't need to be a C++ expert. A reader that is unfamiliar with some language features can still get value from this article by skipping over those features. The intended use of C++ language features and why they might be preferable to alternatives is also beyond the scope of this article.

This article aims to provide a detailed understanding of what C++ code does at the machine code level, so that readers can evaluate for themselves the speed and size of C++ code as naturally as they do for C code.

To examine the nuts and bolts of C++ code generation, we will discuss the major features of the language and how they are implemented in practice. Implementations will be illustrated by showing pieces of C++ code followed by the equivalent (or near equivalent) C code.

We will then discuss some pitfalls specific to embedded systems and how to avoid them.

We will not discuss the uses and subtleties of the C++ language or object-oriented design, as these topics have been well covered elsewhere. See http://en.cppreference.com/w/ for explanations of specific C++ language features.


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