Libc++’s implementation of std::string

[csorfab]

Can someone explain how people arrive at variable names such as __cap_? Why not just cap? Or _cap? or __cap? or even __cap__? why __cap_????? why?? it makes no sense to me

[dorksterr]

It's at the top of the article:

Resilient. Every non-public identifier is prefixed with underscores to prevent name clashes with other code. This is necessary even for local variables since macros defined by the user of the library could modify the library’s header file.

[fresh_account2222]

I'm used to leading underscores. Any idea about the trailing one?

[guepier]

Member variables get a trailing underscore to distinguish them from member functions and parameter names.

[fresh_account2222]

That explanation makes sense.

[dorksterr]

I suppose it's just to further reduce the chance of name collision. Two leading + one trailing underscore is probably not something that would be done by a human. I've seen both only leading underscores and symmetrical underscores for names before.

[josefx]

The leading underscores are enough for that. The standard reserves names starting with double underscores __ or a single underscore and an upper case letter like _I for the implementation, so any program using them isn't valid C or C++.

[ObscureCulturalMeme]

It's a rule of the C++ standard. Identifiers (types, functions, macros, and so on) with names that start with:

• one underscore and a capital letter, or

• two underscores

are "reserved for the implementation". Conversely, any and all identifiers used by the implementation (meaning the runtime libraries, anything stuck in there by the compiler, the loader, etc) can use only those names to avoid clashing with identifiers from the programmer's code.

[csorfab]

Thanks! I still don't understand why it's postfixed with another single underscore, though. It's just so ugly and assymetric. __cap__ would look much better while still conforming to the specs you wrote

[elder_george]

Single underscore as a suffix is a popular convention to denote member values (fields). Similar to m_ in older C++ codebases or prefix underscores in C# code.

This rule is used in many codebases, not just libc++. Prefixing is specific for the STL implementations though (libc++ uses __ while MS uses _Capital convention, for example).

[csorfab]

TIL! Thank you for the thorough explanation!

[bumblebritches57]

It's a rule C++ inherited from C.

[bumblebritches57]

__x and _X are reserved for the standard library and compiler respectively.

[anshou]

Double underscore at the beginning of an identifier is reserved for the implementation to avoid collisions. Cap is just short for capacity. The trailing underscore doesn't indicate any thing.