boost_1_45_0/libs/foreach/doc/foreach.qbk

[library Boost.Foreach
    [quickbook 1.3]
    [authors [Niebler, Eric]]
    [copyright 2004 Eric Niebler]
    [category algorithms]
    [purpose 
        foreach looping construct, for writing simple loops over STL containers,
        null-terminated strings, arrays, iterator pairs and user defined types.
    ]
    [id foreach]
    [dirname foreach]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        [@http://www.boost.org/LICENSE_1_0.txt])
    ]
]

[/  Images   ]

[def _note_                         [$images/note.png]]
[def _alert_                        [$images/caution.png]]
[def _detail_                       [$images/note.png]]
[def _tip_                          [$images/tip.png]]

[/  Links   ]

[def _foreach_                      [^BOOST_FOREACH]] 
[def _range_                        [@../../libs/range/index.html Boost.Range]]
[def _iterator_range_               [@boost:/libs/range/doc/html/range/reference/utilities/iterator_range.html `boost::iterator_range<>`]]
[def _sub_range_                    [@boost:/libs/range/doc/html/range/reference/utilities/sub_range.html `boost::sub_range<>`]]
[def _extending_range_              [@boost:/libs/range/doc/html/range/reference/extending.html Extending Boost.Range]]
[def _single_pass_range_concept_    [@boost:/libs/range/doc/html/range/concepts/single_pass_range.html Single Pass Range Concept]]
[def _range_portability_            [@boost:/libs/range/doc/html/range/portability.html Boost.Range Portability]]
[def _noncopyable_                  [@../../libs/utility/utility.htm#Class_noncopyable `boost::noncopyable`]]
[def _iterator_                     [@../../libs/iterator/doc/index.html Boost.Iterator]]

[section Introduction]

[:["Make simple things easy.]\n[*['-- Larry Wall]]]

[h2 What is _foreach_?]

In C++, writing a loop that iterates over a sequence is tedious. We can either
use iterators, which requires a considerable amount of boiler-plate, or we can
use the `std::for_each()` algorithm and move our loop body into a predicate, which
requires no less boiler-plate and forces us to move our logic far from where
it will be used. In contrast, some other languages, like Perl, provide a dedicated
"foreach" construct that automates this process. _foreach_ is just such a construct
for C++. It iterates over sequences for us, freeing us from having to deal directly
with iterators or write predicates.

_foreach_ is designed for ease-of-use and efficiency. It does no dynamic allocations,
makes no virtual function calls or calls through function pointers, and makes no calls
that are not transparent to the compiler's optimizer. This results in near-optimal code
generation; the performance of _foreach_ is usually within a few percent of the
equivalent hand-coded loop. And although _foreach_ is a macro, it is a remarkably
well-behaved one. It evaluates its arguments exactly once, leading to no nasty surprises.

[h2 Hello, world!]

Below is a sample program that uses _foreach_ to loop over the contents of
a `std::string`.

    #include <string>
    #include <iostream>
    #include <boost/foreach.hpp>

    int main()
    {
        std::string hello( "Hello, world!" );
        
        BOOST_FOREACH( char ch, hello )
        {
            std::cout << ch;
        }

        return 0;
    }

This program outputs the following:

[pre
Hello, world!
]

[h2 Supported Sequence Types]

_foreach_ iterates over sequences. But what qualifies as a sequence, exactly? Since
_foreach_ is built on top of _range_, it automatically supports those types which
_range_ recognizes as sequences. Specifically, _foreach_ works with types that satisfy
the _single_pass_range_concept_. For example, we can use _foreach_ with:

* STL containers
* arrays
* Null-terminated strings (`char` and `wchar_t`)
* std::pair of iterators

[note The support for STL containers is very general; anything that looks like
an STL container counts. If it has nested `iterator` and `const_iterator` types and `begin()`
and `end()` member functions, _foreach_ will automatically know how to iterate over
it. It is in this way that _iterator_range_ and _sub_range_ work with _foreach_.]

See the section on [link foreach.extensibility Extensibility] to find
out how to make _foreach_ work with other types.

[h2 Examples]

Below are some examples that demonstrate all the different ways we can use _foreach_.

Iterate over an STL container:

    std::list<int> list_int( /*...*/ );
    BOOST_FOREACH( int i, list_int )
    {
        // do something with i
    }

Iterate over an array, with covariance (i.e., the type of the iteration variable is
not exactly the same as the element type of the container):

    short array_short[] = {1,2,3};
    BOOST_FOREACH( int i, array_short )
    {
        // The short was implicitly converted to an int
    }

Predeclare the loop variable, and use `break`, `continue`, and `return` in the loop body:

    std::deque<int> deque_int( /*...*/ );
    int i = 0;
    BOOST_FOREACH( i, deque_int )
    {
        if( i == 0 ) return;
        if( i == 1 ) continue;
        if( i == 2 ) break;
    }

Iterate over a sequence by reference, and modify the underlying sequence:

    short array_short[] = { 1, 2, 3 };
    BOOST_FOREACH( short & i, array_short )
    {
        ++i;
    }
    // array_short contains {2,3,4} here

Iterate over a vector of vectors with nested _foreach_ loops. In this
example, notice that braces around the loop body are not necessary:

    std::vector<std::vector<int> > matrix_int;
    BOOST_FOREACH( std::vector<int> & row, matrix_int )
        BOOST_FOREACH( int & i, row )
            ++i;

Iterate over an expression that returns a sequence by value (i.e. an rvalue):

    extern std::vector<float> get_vector_float();
    BOOST_FOREACH( float f, get_vector_float() )
    {
        // Note: get_vector_float() will be called exactly once
    }    

Iterate in reverse:

    std::list<int> list_int( /*...*/ );
    BOOST_REVERSE_FOREACH( int i, list_int )
    {
        // do something with i
    }

Iterating over rvalues doesn't work on some older compilers. Check the 
[link foreach.portability Portability] section to see whether your
compiler supports this.

[h2 Making _foreach_ Prettier]

People have complained about the name _foreach_. It's too long. `ALL CAPS` can
get tiresome to look at. That may be true, but _foreach_ is merely following
the [@http://www.boost.org/more/lib_guide.htm Boost Naming Convention]. That
doesn't mean you're stuck with it, though. If you would like to use a different
identifier (`foreach`, perhaps), you can simply do:

    #define foreach         BOOST_FOREACH
    #define reverse_foreach BOOST_REVERSE_FOREACH

Only do this if you are sure that the identifier you choose will not cause
name conflicts in your code.

[note Do not use `#define foreach(x,y) BOOST_FOREACH(x,y)`.
 This can be problematic if the arguments are macros themselves. This would
 result in an additional expansion of these macros. Instead, use the 
 form shown above.]

[endsect]

[section Extensibility]

If we want to use _foreach_ to iterate over some new collection type, we must
"teach" _foreach_ how to interact with our type. Since _foreach_ is built on top
of _range_, we must extend _range_ in order to extend _foreach_. The section
_extending_range_ explores this topic in detail.

Below is an example for extending _foreach_ to iterate over a sub-string type,
which contains two iterators into a `std::string`.

    namespace my
    {
        // sub_string: part of a string, as delimited by a pair
        // of iterators
        struct sub_string
        {
            std::string::iterator begin;
            std::string::iterator end;
            
            /* ... implementation ... */
        };

        // Add overloads of range_begin() and range_end() in the
        // same namespace as sub_string, to be found by Argument-Dependent Lookup.

        inline std::string::iterator range_begin( sub_string & x )
        {
            return x.begin;
        }

        inline std::string::iterator range_end( sub_string & x )
        {
            return x.end;
        }

        // Also add overloads for const sub_strings. Note we use the conversion
        // from string::iterator to string::const_iterator here.

        inline std::string::const_iterator range_begin( sub_string const & x )
        {
            return x.begin;
        }

        inline std::string::const_iterator range_end( sub_string const & x )
        {
            return x.end;
        }
    }

    namespace boost
    {
        // specialize range_mutable_iterator and range_const_iterator in namespace boost
        template<>
        struct range_mutable_iterator< my::sub_string >
        {
            typedef std::string::iterator type;
        };

        template<>
        struct range_const_iterator< my::sub_string >
        {
            typedef std::string::const_iterator type;
        };
    }

Now that we have taught _range_ (and hence _foreach_) about our type, we
can now use _foreach_ to iterate over our sub_string type.

    my::sub_string substr;
    BOOST_FOREACH( char ch, substr )
    {
        // Woo-hoo!
    }

There are some portability issues we should be aware of when extending _foreach_. Be sure
to check out the [link foreach.portability Portability] section. In particular, if your
compiler does not support Argument-Dependent Lookup, the _range_portability_ section
offers some suggested work-arounds.

[h2 Making _foreach_ Work with Non-Copyable Sequence Types]

For sequence types that are non-copyable, we will need to tell _foreach_ to
not try to make copies. If our type inherits from _noncopyable_, no further action is
required. If not, we must specialize the `boost::foreach::is_noncopyable<>` template, as
follows:

    class noncopy_vector
    {
        // ...
    private:
        noncopy_vector( noncopy_vector const & ); // non-copyable!
    };
    
    namespace boost { namespace foreach
    {
        template<>
        struct is_noncopyable< noncopy_vector >
          : mpl::true_
        {
        };
    }}

Another way to achieve the same effect is to override the global `boost_foreach_is_noncopyable()`
function. Doing it this way has the advantage of being portable to older compilers.

    // At global scope...
    inline boost::mpl::true_ *
    boost_foreach_is_noncopyable( noncopy_vector *&, boost::foreach::tag )
    {
        return 0;
    }

[tip Even though we have to tell _foreach_ that our type is non-copyable, that
doesn't mean that _foreach_ always makes a copy of our sequence type. Obviously, doing so
would be expensive and even wrong in some cases. _foreach_ is quite smart about when to
make a copy and when not to. The `is_noncopyable<>` trait is needed to elide the copy, which
is on a branch that might never get taken.]

[h2 Optimizing _foreach_ for Lightweight Proxy Sequence Types]

On some compilers, _foreach_ must occasionally take a slightly slower code path to guarantee
correct handling of sequences stored in temporary objects. It asks itself, "Should I make
a copy of this object?" and later, "Did I make a copy or not?" For some types of sequences,
this is overkill. Consider a sequence which is a simple pair of iterators. Jumping through
hoops of fire to avoid copying it doesn't make sense because copying it is so cheap.

A pair of iterators is an example of a lightweight proxy. It does not store the values of
the sequence; rather, it stores iterators to them. This means that iterating over a copy of
the proxy object will give the same results as using the object itself. For such types,
_foreach_ provides a hook that lets us tell it not to worry about the expense of making a
copy. This can result in slightly faster loop execution. Simply specialize the
`boost::foreach::is_lightweight_proxy<>` trait, as follows:

    struct sub_string
      : boost::iterator_range< std::string::iterator >
    {
        // ...
    };
    
    namespace boost { namespace foreach
    {
        template<>
        struct is_lightweight_proxy< sub_string >
          : mpl::true_
        {
        };
    }}

Alternately, we could achieve the same effect by overriding the global
`boost_foreach_is_lightweight_proxy()` function, as follows:

    // At global scope...
    inline boost::mpl::true_ *
    boost_foreach_is_lightweight_proxy( sub_string *&, boost::foreach::tag )
    {
        return 0;
    }

This method is portable to older compilers.

[endsect]

[section Portability]

_foreach_ uses some fairly sophisticated techniques that not all compilers support. Depending
on how compliant your compiler is, you may not be able to use _foreach_ in some scenarios. Since
_foreach_ uses _range_, it inherits _range_'s portability issues. You can read about those
issues in the _range_portability_ section.

In addition to the demands placed on the compiler by _range_, _foreach_ places additional demands
in order to handle rvalue sequences properly. (Recall that an rvalue is an unnamed object, so
an example of an rvalue sequence would be a function that returns a `std::vector<>` by value.) Compilers
vary in their handling of rvalues and lvalues. To cope with the situation _foreach_ defines three
levels of compliance, described below:

[table BOOST_FOREACH Compliance Levels
  [[Level]     [Meaning]]
  [[*Level 0*] [['[_Highest level of compliance]]\n
                _foreach_ works with lvalues, rvalues and const-qualified rvalues.]]
  [[*Level 1*] [['[_Moderate level of compliance]]\n
                _foreach_ works with lvalues and plain rvalues, but not const-qualified rvalues.\n
                `BOOST_FOREACH_NO_CONST_RVALUE_DETECTION` is defined in this case.]]
  [[*Level 2*] [['[_Lowest level of compliance]]\n
                _foreach_ works with lvalues only, not rvalues.\n
                `BOOST_FOREACH_NO_RVALUE_DETECTION` is defined in this case.]]
]

Below are the compilers with which _foreach_ has been tested, and the compliance level _foreach_
provides for them.

[table Compiler Compliance Level
  [[Compiler]                [Compliance Level]]
  [[Visual C++ 8.0]          [Level 0]]
  [[Visual C++ 7.1]          [Level 0]]
  [[Visual C++ 7.0]          [Level 2]]
  [[Visual C++ 6.0]          [Level 2]]
  [[gcc 4.0]                 [Level 0]]
  [[gcc 3.4]                 [Level 0]]
  [[gcc 3.3]                 [Level 0]]
  [[mingw 3.4]               [Level 0]]
  [[Intel for Linux 9.0]     [Level 0]]
  [[Intel for Windows 9.0]   [Level 0]]
  [[Intel for Windows 8.0]   [Level 1]]
  [[Intel for Windows 7.0]   [Level 2]]
  [[Comeau 4.3.3]            [Level 0]]
  [[Borland 5.6.4]           [Level 2]]
  [[Metrowerks 9.5]          [Level 1]]
  [[Metrowerks 9.4]          [Level 1]]
  [[SunPro 5.8]              [Level 2]]
  [[qcc 3.3]                 [Level 0]]
  [[tru64cxx 65]             [Level 2]]
  [[tru64cxx 71]             [Level 2]]
]

[endsect]

[section Pitfalls]

This section describes some common pitfalls with _foreach_.

[h2 Types With Commas]

Since _foreach_ is a macro, it must have exactly two arguments, with exactly one
comma separating them. That's not always convenient, especially when the type of the
loop variable is a template. Consider trying to iterate over a `std::map`:

    std::map<int,int> m;

    // ERROR! Too many arguments to BOOST_FOREACH macro.
    BOOST_FOREACH(std::pair<int,int> p, m) // ...

One way to fix this is with a typedef.

    std::map<int,int> m;
    typedef std::pair<int,int> pair_t;

    BOOST_FOREACH(pair_t p, m) // ...

Another way to fix it is to predeclare the loop variable:

    std::map<int,int> m;
    std::pair<int,int> p;

    BOOST_FOREACH(p, m) // ...

[h2 Hoisting and Iterator Invalidation]

Under the covers, _foreach_ uses iterators to traverse the element
sequence. Before the loop is executed, the end iterator is cached
in a local variable. This is called ['hoisting], and it is an
important optimization. It assumes, however, that the end iterator
of the sequence is stable. It usually is, but if we modify the
sequence by adding or removing elements while we are iterating
over it, we may end up hoisting ourselves on our own petard.

Consider the following code:

    std::vector<int> vect(4, 4);
    BOOST_FOREACH(int i, vect)
    {
        vect.push_back(i + 1);
    }

This code will compile, but it has undefined behavior. That is because
it is logically equivalent to the following:

    std::vector<int> vect(4, 4);
    for(std::vector<int>::iterator it1 = vect.begin(), it2 = vect.end();
        it1 != it2; ++it1)
    {
        int i = *it1;
        vect.push_back(i + 1); // Oops! This invalidates it1 and it2!
    }

The call to `vect.push_back()` will cause all iterators into `vect` to
become invalid, including `it1` and `it2`. The next iteration through
the loop will cause the invalid iterators to be used. That's bad news.

The moral of the story is to think twice before adding and removing
elements from the sequence over which you are iterating. If doing
so could cause iterators to become invalid, don't do it. Use a regular
`for` loop instead.

[endsect]

[section History and Acknowledgements]

[h2 History]

The ideas for _foreach_ began life in the Visual C++ group at Microsoft during the early phases of
the design for C++\/CLI. Whether to add a dedicated "foreach" looping construct to the language was
an open question at the time. As a mental exercise, Anson Tsao sent around some proof-of-concept
code which demonstrated that a pure library solution might be possible. The code was written in the
proposed C++\/CLI dialect of the time, for which there was no compiler as of yet. I was intrigued by
the possibility, and I ported his code to Managed C++ and got it working. We worked together to
refine the idea and eventually published an article about it in the November 2003 issue of the
[@http://www.cuj.com CUJ].

After leaving Microsoft, I revisited the idea of a looping construct. I reimplemented the macro
from scratch in standard C++, corrected some shortcomings of the CUJ version and rechristened it
_foreach_. In October of 2003 I began a discussion about it on the Boost developers list, where
it met with a luke-warm reception. I dropped the issue until December 2004, when I reimplemented
_foreach_ yet again. The new version only evaluated its sequence expression once and correctly
handled both lvalue and rvalue sequence expressions. It was built on top of the recently
accepted _range_ library, which increased its portability. This was the version that, on Dec. 12 2004,
I finally submitted to Boost for review. It was accepted into Boost on May 5, 2005.

[h2 Acknowledgements]

Thanks go out to Anson Tsao of Microsoft for coming up with the idea and demonstrating its feasibility.
I would also like to thank [@http://boost.org/people/thorsten_ottosen.html Thorsten Ottosen] for
the _range_ library, on which the current version of _foreach_ is built. Finally, I'd like to thank
Russell Hind, Alisdair Meredith and Stefan Slapeta for their help porting to various compilers.

[h2 Further Reading]

For more information about how _foreach_ works, you may refer to the article
[@http://www.artima.com/cppsource/foreach.html ["Conditional Love]] at
[@http://www.artima.com/cppsource/ The C++ Source].

[endsect]