我需要一些帮助来理解Nicolai Josuttis的这本书

Question

I'd like to have some explanation regarding this paragraph on page 518 of Nicolai Josuttis book "The C++ Standard Library" (first edition):

These flags are maintained by the class basic_ios and are thus present in all objects of type basic_istream or basic_ostream . However, the stream buffers don't have state flags. One stream buffer can be shared by multiple stream objects, so the flags only represent the state of the stream as found in the last operation. Even this is only the case if goodbit was set prior to this operation. Otherwise the flags may have been set by some earlier operation.

I don't understand what does he mean by "the stream buffer don't have state flags" and right below this paragraph there's a table with the title "Member functions for stream states".

Answer 1

Streams consist of two objects:

1. The actual stream object ( std::istream or std::ostream , derived from std::ios ).
2. The stream buffer, ie, a class derived from std::streambuf .

The state flags are present in std::ios but not in std::streambuf .

Answer 2

There are "stream buffer objects", and a "stream objects". One stream buffer can be shared between multiple stream objects. Each stream object has its own set of flags - so one stream may be "reached end of file", where another is not - or the flags for output in decimal or hex may be completely different for two output streams using the same buffer.

[Of course, if you are using the same buffer for multiple streams, you will have to take care that you don't mess things up - and it's not a common thing to share the buffer over multiple streams, but it can be done!]

Answer 3

The iostate flags store things about output formatting: whether you want numbers printed in decimal or hex, capitals or lowercase, etc. Stream objects control formatting, so the flags are inside the stream object.

In iostreams, buffering is separate from formatting. Linked to the iostream object is a stream buffer object, which controls sending and/or receiving characters from an underlying source. The buffer object has no such flags; its only state variables deal with preparing (encoding) the characters and optionally storing (buffering) them to reduce the number of times the operating system is asked to perform I/O. (Or in the case of stringstream , the buffer provides the ultimate storage behind the stream.)

Answer 4

So a stream has state flags , but the stream buffer it uses doesn't .

A stream buffer goes inside a stream.

The buffer holds some amount of bytes that the stream is reading/writing before sending/recieving it to whatever the stream is talking to (file/stdin/tcpsocket/etc.).

Stream reference: http://www.cplusplus.com/reference/istream/iostream/

Stream Buffer Reference: http://www.cplusplus.com/reference/streambuf/streambuf/

By default a stream will usually create it's own stream buffer, but you can tell it to use one of your choosing in the constructor: http://www.cplusplus.com/reference/istream/iostream/iostream/

Or you can get/set the buffer with the rdbuf method.

Answer 5

The "stream buffer" is an object of class basic_streambuf. That class doesn't have state flags. Every stream (basic_istream or basic_ostream) has a pointer to a basic_streambuf, but the flags are a property of the stream, not of the stream buffer.