关于std::errc、std::io_errc与std::future_errc错误类型分类差异的设计疑问及error_condition可移植错误代码含义咨询

1. Is the difference in enum specializations intentional? #

Absolutely, this is a deliberate design choice tied to the core distinction between std::error_code and std::error_condition in the C++ standard:

• std::error_code represents concrete, platform or library-specific error values. These are tied to a specific "category" (like system errors, IO library errors, or future library errors) and map directly to underlying implementation details.
• std::error_condition represents abstract, semantic error conditions that are portable across platforms. They describe what went wrong in a way that's not tied to a specific system or library's error numbering.

Here's how this applies to your examples:

• std::errc is specialized for is_error_condition_enum because it's meant to represent abstract error semantics (like errc::permission_denied, errc::file_not_found) that translate to different system-specific error codes across platforms. It's a common vocabulary for talking about errors regardless of the OS.
• std::io_errc and std::future_errc are specialized for is_error_code_enum because they're library-specific error codes tied directly to the C++ standard library's IO and future components. These errors aren't meant to map to system-level errors—they're internal to the library's logic, so they don't need the abstract, cross-platform translation that error_condition provides.

2. What does "portable error code" mean for std::error_condition, if std::errc values differ across platforms? #

Great catch—this is a common point of confusion, and the key here is to separate numeric values from semantic meaning. When we say std::error_condition is "portable," we're not talking about the underlying integer values being the same across platforms. We're talking about the semantics being consistent.

For example:

• On Linux, std::errc::permission_denied maps to the POSIX EPERM value (usually 1).
• On Windows, it might map to a different system error code (like ERROR_ACCESS_DENIED, which is 5).

But when you write code like this:

if (some_error_condition == std::errc::permission_denied) {
    // Handle "permission was denied"
}

This check works identically across platforms because you're checking the semantic condition, not the raw numeric value. The std::error_condition abstraction handles mapping the platform-specific error_code to the correct abstract condition for you.

In short, "portable" here means you can reason about errors using a common, OS-agnostic vocabulary instead of having to memorize or conditionalize on platform-specific error numbers.

内容的提问来源于stack exchange，提问作者Kingsley Chen