boost/libs/bind/doc/bind/purpose.qbk

[/
 /  Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
 /  Copyright (c) 2003-2008 Peter Dimov
 /
 / 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)
 /]

[section:purpose Purpose]

`boost::bind` is a generalization of the standard functions `std::bind1st` and
`std::bind2nd`. It supports arbitrary function objects, functions, function
pointers, and member function pointers, and is able to bind any argument to a
specific value or route input arguments into arbitrary positions. `bind` does
not place any requirements on the function object; in particular, it does not
need the `result_type`, `first_argument_type` and `second_argument_type`
standard typedefs.

[section Using bind with functions and function pointers]

Given these definitions:

    int f(int a, int b)
    {
        return a + b;
    }

    int g(int a, int b, int c)
    {
        return a + b + c;
    }

`bind(f, 1, 2)` will produce a "nullary" function object that takes no
arguments and returns `f(1, 2)`. Similarly, `bind(g, 1, 2, 3)()` is equivalent
 `to g(1, 2, 3)`.

It is possible to selectively bind only some of the arguments.
`bind(f, _1, 5)(x)` is equivalent to `f(x, 5)`; here `_1` is a /placeholder/
argument that means "substitute with the first input argument."

For comparison, here is the same operation expressed with the standard library
primitives:

    std::bind2nd(std::ptr_fun(f), 5)(x);

`bind` covers the functionality of `std::bind1st` as well:

    std::bind1st(std::ptr_fun(f), 5)(x);   // f(5, x)
    bind(f, 5, _1)(x);                     // f(5, x)

`bind` can handle functions with more than two arguments, and its argument
substitution mechanism is more general:

    bind(f, _2, _1)(x, y);                 // f(y, x)
    bind(g, _1, 9, _1)(x);                 // g(x, 9, x)
    bind(g, _3, _3, _3)(x, y, z);          // g(z, z, z)
    bind(g, _1, _1, _1)(x, y, z);          // g(x, x, x)

Note that, in the last example, the function object produced by
`bind(g, _1, _1, _1)` does not contain references to any arguments beyond the
first, but it can still be used with more than one argument. Any extra
arguments are silently ignored, just like the first and the second argument
are ignored in the third example.

The arguments that `bind` takes are copied and held internally by the returned
function object. For example, in the following code:

    int i = 5;
    bind(f, i, _1);

a copy of the value of `i` is stored into the function object.
[@boost:/libs/core/doc/html/core/ref.html `boost::ref`] and
[@boost:/libs/core/doc/html/core/ref.html `boost::cref`] can be used to make the function
object store a reference to an object, rather than a copy:

    int i = 5;
    bind(f, ref(i), _1);
    bind(f, cref(i), _1);

[endsect]

[section:with_function_objects Using bind with function objects]

`bind` is not limited to functions; it accepts arbitrary function objects. In
the general case, the return type of the generated function object's
`operator()` has to be specified explicitly (without a `typeof` operator the
return type cannot be inferred):

    struct F
    {
        int operator()(int a, int b) { return a - b; }
        bool operator()(long a, long b) { return a == b; }
    };

    F f;
    int x = 104;
    bind<int>(f, _1, _1)(x);		// f(x, x), i.e. zero

Some compilers have trouble with the `bind<R>(f, ...)` syntax. For portability
reasons, an alternative way to express the above is supported:

    boost::bind(boost::type<int>(), f, _1, _1)(x);

Note, however, that the alternative syntax is provided only as a workaround.
It is not part of the interface.

When the function object exposes a nested type named `result_type`, the explicit
return type can be omitted:

    int x = 8;
    bind(std::less<int>(), _1, 9)(x);	// x < 9

/[Note:/ the ability to omit the return type is not available on all compilers./]/

By default, `bind` makes a copy of the provided function object. `boost::ref`
and `boost::cref` can be used to make it store a reference to the function
object, rather than a copy. This can be useful when the function object is
non-copyable, expensive to copy, or contains state; of course, in this case
the programmer is expected to ensure that the function object is not destroyed
while it's still being used.

    struct F2
    {
        int s;

        typedef void result_type;
        void operator()(int x) { s += x; }
    };

    F2 f2 = { 0 };
    int a[] = { 1, 2, 3 };

    std::for_each(a, a+3, bind(ref(f2), _1));

    assert(f2.s == 6);

[endsect]

[section Using bind with pointers to members]

Pointers to member functions and pointers to data members are not function
objects, because they do not support `operator()`. For convenience, `bind`
accepts member pointers as its first argument, and the behavior is as if
[@boost:/libs/bind/mem_fn.html `boost::mem_fn`] has been used to convert the
member pointer into a function object. In other words, the expression

    bind(&X::f, args)

is equivalent to

    bind<R>(``[@boost:/libs/bind/mem_fn.html `mem_fn`]``(&X::f), args)

where `R` is the return type of `X::f` (for member functions) or the type of
the member (for data members.)

/[Note:/ `mem_fn` creates function objects that are able to accept a pointer,
a reference, or a smart pointer to an object as its first argument; for
additional information, see the `mem_fn`
[@boost:/libs/bind/mem_fn.html documentation]./]/

Example:

    struct X
    {
        bool f(int a);
    };

    X x;
    shared_ptr<X> p(new X);
    int i = 5;

    bind(&X::f, ref(x), _1)(i);		// x.f(i)
    bind(&X::f, &x, _1)(i);			// (&x)->f(i)
    bind(&X::f, x, _1)(i);			// (internal copy of x).f(i)
    bind(&X::f, p, _1)(i);			// (internal copy of p)->f(i)

The last two examples are interesting in that they produce "self-contained"
function objects. `bind(&X::f, x, _1)` stores a copy of `x`.
`bind(&X::f, p, _1)` stores a copy of `p`, and since `p` is a
[@boost:/libs/smart_ptr/shared_ptr.htm `boost::shared_ptr`], the function
object retains a reference to its instance of `X` and will remain valid even
when `p` goes out of scope or is `reset()`.

[endsect]

[section Using nested binds for function composition]

Some of the arguments passed to `bind` may be nested /bind expressions/
themselves:

    bind(f, bind(g, _1))(x);               // f(g(x))

The inner /bind expressions/ are evaluated, in unspecified order, before the
outer `bind` when the function object is called; the results of the evaluation
are then substituted in their place when the outer `bind` is evaluated. In the
example above, when the function object is called with the argument list `(x)`,
`bind(g, _1)(x)` is evaluated first, yielding `g(x)`, and then
`bind(f, g(x))(x)` is evaluated, yielding the final result `f(g(x))`.

This feature of `bind` can be used to perform function composition. See
[@../../bind_as_compose.cpp bind_as_compose.cpp] for an example that
demonstrates how to use `bind` to achieve similar functionality to
[@http://www.boost.org/doc/libs/1_31_0/libs/compose/index.htm Boost.Compose].

Note that the first argument - the bound function object - is not evaluated,
even when it's a function object that is produced by `bind` or a /placeholder/
argument, so the example below does not work as expected:

    typedef void (*pf)(int);

    std::vector<pf> v;
    std::for_each(v.begin(), v.end(), bind(_1, 5));

The desired effect can be achieved via a helper function object `apply` that
applies its first argument, as a function object, to the rest of its argument
list. For convenience, an implementation of `apply` is provided in the
[@../../include/boost/bind/apply.hpp apply.hpp] header file. Here is how the
modified version of the previous example looks like:

    typedef void (*pf)(int);

    std::vector<pf> v;
    std::for_each(v.begin(), v.end(), bind(apply<void>(), _1, 5));

Although the first argument is, by default, not evaluated, all other arguments
are. Sometimes it is necessary not to evaluate arguments subsequent to the
first, even when they are nested /bind subexpressions/. This can be achieved
with the help of another function object, `protect`, that masks the type so
that `bind` does not recognize and evaluate it. When called, protect simply
forwards the argument list to the other function object unmodified.

The header [@../../include/boost/bind/protect.hpp protect.hpp] contains an
implementation of `protect`. To `protect` a bind function object from
evaluation, use `protect(bind(f, ...))`.

[endsect]

[section Overloaded operators (new in Boost 1.33)]

For convenience, the function objects produced by `bind` overload the logical
not operator `!` and the relational and logical operators `==, !=, <, <=, >,
>=, &&, ||`.

`!bind(f, ...)` is equivalent to `bind(logical_not(), bind(f, ...))`, where
`logical_not` is a function object that takes one argument `x` and returns
`!x`.

`bind(f, ...) op x`, where _op_ is a relational or logical operator, is
equivalent to `bind(relation(), bind(f, ...), x)`, where `relation` is a
function object that takes two arguments `a` and `b` and returns `a op b`.

What this means in practice is that you can conveniently negate the result of
`bind`:

    std::remove_if(first, last, !bind(&X::visible, _1)); // remove invisible objects

and compare the result of `bind` against a value:

    std::find_if(first, last, bind(&X::name, _1) == "Peter");
    std::find_if(first, last, bind(&X::name, _1) == "Peter" || bind(&X::name, _1) == "Paul");

against a /placeholder/:

    bind(&X::name, _1) == _2

or against another /bind expression/:

    std::sort(first, last, bind(&X::name, _1) < bind(&X::name, _2)); // sort by name

[endsect]

[endsect]