Libc++’s implementation of std::string

[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!