io/doc/ios_state.adoc

////
Copyright 2019 Glen Joseph Fernandes
(glenjofe@gmail.com)

Distributed under the Boost Software License, Version 1.0.
(http://www.boost.org/LICENSE_1_0.txt)
////

# IO State Savers, <boost/io/ios_state.hpp>
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## Description

The header `<boost/io/ios_state.hpp>` covers saving the stream state of objects
in the {cpp} IOStreams system.

## Rationale

Sometimes a certain value has to change only for a limited scope. Saver classes
save a copy of the current state of some object (or an aspect of an object),
and reset the object's state at destruction time, undoing any change the object
may have gone through.

The saver class strategy is helpful when using I/O stream objects. Manipulator
objects can change some aspect of a stream during input or output. The state
changed by the manipulator usually sticks to its new value after the I/O
transaction. This can be a problem if manipulators are used in a function that
is not supposed to externally change a stream's state.

```
#include <ostream>
#include <ios>

void hex_my_byte(std::ostream& os, char byte)
{
    os << std::hex << static_cast<unsigned>(byte);
}
```

The `os` stream will retain its new hexadecimal printing mode after the call to
`hex_my_byte`. The stream's printing mode can be saved and restored with manual
calls to the stream's state inspecting and mutating member functions. The
manual method becomes unwieldy if the main functionality is complex and/or
needs to be exception safe. A saver class can implement the better
"resource acquisition is initialization" strategy.

See the example below for better code, using saver classes.

## Header Synopsis

```
namespace boost {
namespace io {

class ios_flags_saver;
class ios_precision_saver;
class ios_width_saver;
class ios_base_all_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_iostate_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_exception_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_tie_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_rdbuf_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_fill_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_locale_saver;

template<class Ch, class Tr = std::char_traits<Ch> >
class basic_ios_all_saver;

typedef basic_ios_iostate_saver<char>      ios_iostate_saver;
typedef basic_ios_iostate_saver<wchar_t>   wios_iostate_saver;
typedef basic_ios_exception_saver<char>    ios_exception_saver;
typedef basic_ios_exception_saver<wchar_t> wios_exception_saver;
typedef basic_ios_tie_saver<char>          ios_tie_saver;
typedef basic_ios_tie_saver<wchar_t>       wios_tie_saver;
typedef basic_ios_rdbuf_saver<char>        ios_rdbuf_saver;
typedef basic_ios_rdbuf_saver<wchar_t>     wios_rdbuf_saver;
typedef basic_ios_fill_saver<char>         ios_fill_saver;
typedef basic_ios_fill_saver<wchar_t>      wios_fill_saver;
typedef basic_ios_locale_saver<char>       ios_locale_saver;
typedef basic_ios_locale_saver<wchar_t>    wios_locale_saver;
typedef basic_ios_all_saver<char>          ios_all_saver;
typedef basic_ios_all_saver<wchar_t>       wios_all_saver;

class ios_iword_saver;
class ios_pword_saver;
class ios_all_word_saver;

} // io
} // boost
```

## Savers for Basic Standard Attributes

The basic saver classes have this format:

[subs=+quotes]
```
class saver {
public:
    typedef std::ios_base state_type;
    typedef `implementation_defined` aspect_type;

    explicit saver(state_type& s);
    saver(state_type& s, const aspect_type& new_value);
    ~saver();

    void restore();
};
```

The `state_type` is the IOStreams base class `std::ios_base`. The user would
usually place an actual input, output, or combined stream object for the
state-type parameter, and not a base class object. The first constructor takes
a stream object and saves a reference to the stream and the current value of a
particular stream attribute. The second constructor works like the first, and
uses its second argument to change the stream's attribute to the new
`aspect_type` value given. The destructor restores the stream's attribute to
the saved value. The restoration can be activated early (and often) with the
`restore` member function.

.Basic IOStreams State Saver Classes
[%header,cols=5*]
|===
|Class |Saved Attribute |Attribute Type |Reading Method |Writing Method
|`ios_flags_saver`
|Format control flags
|`std::ios_base::fmtflags`
|`flags`
|`flags`
|`ios_precision_saver`
|Number of digits to print after decimal point
|`std::streamsize`
|`precision`
|`precision`
|`ios_width_saver`
|Minimum field width for printing objects
|`std::streamsize`
|`width`
|`width`
|===

## Savers for Advanced Standard Attributes

The saver class templates have this format:

[subs=+quotes]
```
template<class Ch, class Tr>
class saver {
public:
    typedef std::basic_ios<Ch, Tr> state_type;
    typedef `implementation-defined` aspect_type;

    explicit saver(state_type& s);
    saver(state_type& s, const aspect_type& new_value);
    ~saver();

    void restore();
};
```

The `state_type` is a version of the IOStreams base class template
`std::basic_ios<Ch, Tr>`, where `Ch` is a character type and `Tr` is a
character traits class. The user would usually place an actual input,
output, or combined stream object for the state-type parameter, and not a base
class object. The first constructor takes a stream object and saves a reference
to the stream and the current value of a particular stream attribute. The
second constructor works like the first, and uses its second argument to change
the stream's attribute to the new `aspect_type` value given. The destructor
restores the stream's attribute to the saved value. The restoration can be
activated early (and often) with the `restore` member function.

.Advanced IOStreams State Saver Class Templates
[%header,cols=5*]
|===
|Class |Saved Attribute |Attribute Type |Reading Method |Writing Method
|`basic_ios_iostate_saver<Ch, Tr>`
|Failure state of the stream [1], [2]
|`std::ios_base::iostate`
|`rdstate`
|`clear`
|`basic_ios_exception_saver<Ch, Tr>`
|Which failure states trigger an exception [1]
|`std::ios_base::iostate`
|`exceptions`
|`exceptions`
|`basic_ios_tie_saver<Ch, Tr>`
|Output stream synchronized with the stream
|`std::basic_ostream<Ch, Tr>*`
|`tie`
|`tie`
|`basic_ios_rdbuf_saver<Ch, Tr>`
|Stream buffer associated with the stream [2]
|`std::basic_streambuf<Ch, Tr>*`
|`rdbuf`
|`rdbuf`
|`basic_ios_fill_saver<Ch, Tr>`
|Character used to pad oversized field widths
|`Ch`
|`fill`
|`fill`
|`basic_ios_locale_saver<Ch, Tr>`
|Locale information associated with the stream [3]
|`std::locale`
|`getloc` (from `std::ios_base`)
|`imbue` (from `std::basic_ios<Ch, Tr>`)
|===

### Notes

1. When the failure state flags and/or the failure state exception watching
flags are changed, an exception is thrown if a match occurs among the two sets
of flags. This could mean that the constructor or destructor of these class
templates may throw.
2. When the associated stream buffer is changed, the stream's failure state set
is reset to "good" if the given stream buffer's address is non-NULL, but the
"bad" failure state is set if that address is NULL. This means that a saved
failure state of "good" may be restored as "bad" if the stream is stripped of
an associated stream buffer. Worse, given a NULL stream buffer address, an
exception is thrown if the "bad" failure state is being watched. This could
mean that the constructor or destructor of these class templates may throw.
3. The saver for the locale uses the `std::basic_ios<Ch, Tr>` class to extract
their information, although it could have used the functionality in
`std::ios_base`. The problem is that the versions of the needed member
functions in `ios_base` are not polymorphically related to the ones in
`basic_ios`. The stream classes that will be used with the saver classes should
use the versions of the member functions closest to them by inheritance, which
means the ones in `basic_ios`.

## Savers for User-Defined Attributes

There are three class (templates) for combined attribute savers. The
`ios_base_all_saver` saver class combines the functionality of all the basic
attribute saver classes. It has a constructor that takes the stream to have its
state preserved. The `basic_ios_all_saver` combines the functionality of all
the advanced attribute saver class templates and the combined basic attribute
saver class. It has a constructor that takes the stream to have its state
preserved. The `ios_all_word_saver` saver class combines the saver classes that
preserve user-defined formatting information. Its constructor takes the stream
to have its attributes saved and the index of the user-defined attributes. The
destructor for each class restores the saved state. Restoration can be
activated early (and often) for a class with the restore member function.

## Example

The code used in the rationale can be improved at two places. The printing
function could use a saver around the code that changes the formatting state.
Or the calling function can surround the call with a saver. Or both can be
done, especially if the user does not know if the printing function uses a
state saver. If the user wants a series of changes back and forth, without
surrounding each change within a separate block, the restore member function
can be called between each trial.

```
#include <boost/io/ios_state.hpp>
#include <ios>
#include <iostream>
#include <ostream>

void new_hex_my_byte(std::ostream& os, char byte)
{
    boost::io::ios_flags_saver ifs(os);
    os << std::hex << static_cast<unsigned>(byte);
}

int main()
{
    // ...
    {
        boost::io::ios_all_saver ias(std::cout);
        new_hex_my_byte(std::cout, 'A');
    }
    // ...
    {
        boost::io::ios_all_saver ias(std::cerr);
        new_hex_my_byte(std::cerr, 'b');
        ias.restore();
        new_hex_my_byte(std::cerr, 'C');
    }
    // ...
}
```

## Credits

### Daryle Walker

Started the library. Contributed the initial versions of the format flags,
precision, width, and user-defined format flags saver classes. Contributed the
initial versions of the success state, success state exception flags, output
stream tie, stream buffer, character fill, and locale saver class templates.
Contributed the combined attribute classes and class template. Contributed the
test file `ios_state_test.cpp`.

## History

### 20 Dec 2019

Glen Fernandes made all the saver classes non-copyable.

### 28 Feb 2005

Daryle Walker added the restore member functions, based on suggestions by
Gennadiy Rozental and Rob Stewart.

### 13 Mar 2002

Daryle Walker implemented the initial version.