When not to comment

Without usable and accurate documentation of how to use an API, developers can find themselves deterred from reusing relevant code. In C++, one place developers can find documentation is in a header file. When information is missing, they may look at the corresponding implementation code. To understand what's missing from C++ API documentation and the factors influencing whether it will be fixed, we conducted a mixed-methods study involving two experience sampling surveys with hundreds of developers at the moment they visited implementation code, interviews with 18 of those developers, and interviews with 8 API maintainers. In many cases, updating documentation may provide only limited value for developers, while requiring effort maintainers don't want to invest. We identify a set of questions maintainers and tool developers should consider when improving API-level documentation.