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C++ Language/ErrorHandling/ReturningAnErrorCode
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
When a low-level feature of the operating system needs to report an error, it usually does so by returning an "error code".
The Win32 platform has four built-in enumerations of error codes, which the C++ language has combined into a single std::error_code type.
Additional information about error codes (includes interactive examples)