C++中调用std::stringstream的flush()方法会发生什么？

Great question—let's break this down clearly, since in-memory streams like std::stringstream behave differently than file-based streams when it comes to flushing.

What does flush() actually do? #

Unlike streams that interact with external devices (like std::ofstream writing to a disk file), std::stringstream keeps all its data in an in-memory buffer. Normally, calling flush() on an output stream forces any buffered data to be synced with the underlying device. But for std::stringstream, there's no external device to sync with.

Under the hood, std::ostream::flush() calls the pubsync() method of the stream's underlying buffer (std::stringbuf). The C++ standard explicitly defines std::stringbuf::sync() to return 0 without performing any actual synchronization or data movement. In plain terms: calling flush() on a std::stringstream is a no-op—it does absolutely nothing.

Is this behavior standard or implementation-dependent? #

This is fully specified by the C++ standard. All compliant compilers (GCC, Clang, MSVC, etc.) will follow this rule. You don't have to worry about different implementations treating std::stringstream::flush() differently; it's guaranteed to be an empty operation across all standard-compliant environments.

Are any operations mandatory when calling flush()? #

No, there are no mandatory operations required. Since the standard defines it as a no-op, calling flush() on a std::stringstream won't trigger any data writes, buffer resets, or other actions. You could call it repeatedly and your program's behavior won't change at all.

The only scenario where you might see flush() called on a std::stringstream is in generic code that's designed to work with any std::ostream (including file streams, console streams, and string streams). For those cases, the call is harmless even if it does nothing for the string stream.

内容的提问来源于stack exchange，提问作者Antonio Perez