Why would anybody use C over C++?

I like minimalism & simplicity.

Fears of performance or bloat are not good reason to forgo C++. Every language has its potential pitfalls and trade offs - good programmers learn about these and where necessary develop coping strategies, poor programmers will fall foul and blame the language.

Interpreted Python is in many ways considered to be a "slow" language, but for non-trivial tasks a skilled Python programmer can easily produce code that executes faster than that of an inexperienced C developer.

In my industry, video games, we write high performance code in C++ by avoiding things such as RTTI, exceptions, or virtual-functions in inner loops. These can be extremely useful but have performance or bloat problems that it's desirable to avoid. If we were to go a step further and switch entirely to C we would gain little and lose the most useful constructs of C++.

The biggest practical reason for preferring C is that support is more widespread than C++. There are many platforms, particularly embedded ones, that do not even have C++ compilers.

There is also the matter of compatibility for vendors. While C has a stable and well-defined ABI (Application Binary Interface) C++ does not. The ABI in C++ is more complicated due to such things as vtables and constructurs/destructors so is implemented differently with every vendor, and even versions of a vendors toolchain.

In real-terms this means you cannot take a library generated by one compiler and link it with code or a library from another which creates a nightmare for distributed projects or middleware providers of binary libraries.

• Because they already know C
• Because they're building an embedded app for a platform that only has a C compiler
• Because they're maintaining legacy software written in C
• You're writing something on the level of an operating system, a relational database engine, or a retail 3D video game engine.

Joel's answer is good for reasons you might have to use C, though there are a few others:

• You must meet industry guidelines, which are easier to prove and test for in C
• You have tools to work with C, but not C++ (think not just about the compiler, but all the support tools, coverage, analysis, etc)
• Your target developers are C gurus
• You're writing drivers, kernels, or other low-level code
• You know the C++ compiler isn't good at optimizing the kind of code you need to write
• Your app not only doesn't lend itself to be object-oriented but would be harder to write in that form

In some cases, though, you might want to use C rather than C++:

• You want the performance of assembler without the trouble of coding in assembler (C++ is, in theory, capable of 'perfect' performance, but the compilers aren't as good at seeing optimizations a good C programmer will see)

• The software you're writing is trivial, or nearly so - whip out the tiny C compiler, write a few lines of code, compile and you're all set - no need to open a huge editor with helpers, no need to write practically empty and useless classes, deal with namespaces, etc. You can do nearly the same thing with a C++ compiler and simply use the C subset, but the C++ compiler is slower, even for tiny programs.

• You need extreme performance or small code size and know the C++ compiler will actually make it harder to accomplish due to the size and performance of the libraries.

You contend that you could just use the C subset and compile with a C++ compiler, but you'll find that if you do that you'll get slightly different results depending on the compiler.

Regardless, if you're doing that, you're using C. Is your question really "Why don't C programmers use C++ compilers?" If it is, then you either don't understand the language differences, or you don't understand the compiler theory.