Apache C++ Standard Library User's Guide

## 31.3 The Open Modes

The only open modes that have an effect on string streams are `std::ios_base::in`, `std::ios_base::out`, `std::ios_base::ate`, and `std::ios_base::app`. They have more or less the same meaning that they have with file streams (see Section 30.3). The `std::ios_base::binary` open mode is irrelevant, because there is no conversion to and from the dependent file format of the operating system. The `std::ios_base::trunc` open mode is simply ignored.