How large is the stream buffer?

[Lambda]

As I know, when I use ifstream and ofstream to read and write file,
it will use a stream buffer internally.
How large is the stream buffer?

If I want to write a large file, need I define a stream buffer myself?
In what situation, user-defined stream buffer is useful?

 

[James Kanze]

As I know, when I use ifstream and ofstream to read and write file,
it will use a stream buffer internally.
How large is the stream buffer?

About so big.

It depends on the implementation, and will often be optimized
for the system you're running under.

If I want to write a large file, need I define a stream buffer
myself?
No.

In what situation, user-defined stream buffer is useful?

Any time the non-standard ones aren't sufficient. The most
frequent use is probably filtering streambuf's; it's hard to
imagine an application that doesn't use at least one (if only
for log dispatching). Non-filtering user written streambuf's
are used to interface with sockets, pipes, raw memory, or just
about anything else besides ordinary files and std::strings.