doc/tuple_users_guide.qbk

[/
 /  Copyright (c) 2001 Jaakko Järvi
 /
 / 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)
 /]

[library Boost.Tuple
  [quickbook 1.6]
  [id tuple]
  [copyright 2001 Jaakko J\u00E4rvi]
  [dirname tuple]
  [license Distributed under the
    [@http://boost.org/LICENSE_1_0.txt Boost Software License,
      Version 1.0].
  ]
]

[include tuple_advanced_interface.qbk]
[include design_decisions_rationale.qbk]

[template simplesect[title]
[block '''<simplesect><title>'''[title]'''</title>''']]

[template endsimplesect[]
[block '''</simplesect>''']]

A tuple (or n-tuple) is a fixed size collection of elements. Pairs, triples,
quadruples etc. are tuples. In a programming language, a tuple is a data
object containing other objects as elements. These element objects may be of
different types.

Tuples are convenient in many circumstances. For instance, tuples make it easy
to define functions that return more than one value.

Some programming languages, such as ML, Python and Haskell, have built-in
tuple constructs. Unfortunately C++ does not. To compensate for this
"deficiency", the Boost Tuple Library implements a tuple construct using
templates.

[section:using_library Using the Library]

To use the library, just include:

    #include "boost/tuple/tuple.hpp"

Comparison operators can be included with:

    #include "boost/tuple/tuple_comparison.hpp"

To use tuple input and output operators,

    #include "boost/tuple/tuple_io.hpp"

Both `tuple_io.hpp` and `tuple_comparison.hpp` include `tuple.hpp`.

All definitions are in namespace `::boost::tuples`, but the most common names
are lifted to namespace `::boost` with using declarations. These names are:
`tuple`, `make_tuple`, `tie` and `get`. Further, `ref` and `cref` are defined
directly under the `::boost` namespace.

[endsect]

[section:tuple_types Tuple Types]

A tuple type is an instantiation of the `tuple` template. The template
parameters specify the types of the tuple elements. The current version
supports tuples with 0-10 elements. If necessary, the upper limit can be
increased up to, say, a few dozen elements. The data element can be any C++
type. Note that `void` and plain function types are valid C++ types, but
objects of such types cannot exist. Hence, if a tuple type contains such types
as elements, the tuple type can exist, but not an object of that type. There
are natural limitations for element types that cannot be copied, or that are
not default constructible (see [link tuple.constructing_tuples 'Constructing tuples']
below).

For example, the following definitions are valid tuple instantiations (`A`,
`B` and `C` are some user defined classes):

    tuple<int>
    tuple<double&, const double&, const double, double*, const double*>
    tuple<A, int(*)(char, int), B(A::*)(C&), C>
    tuple<std::string, std::pair<A, B> >
    tuple<A*, tuple<const A*, const B&, C>, bool, void*>

[endsect]

[section:constructing_tuples Constructing Tuples]

The tuple constructor takes the tuple elements as arguments. For an /n/-
element tuple, the constructor can be invoked with /k/ arguments, where
`0` <= /k/ <= /n/. For example:

    tuple<int, double>()
    tuple<int, double>(1)
    tuple<int, double>(1, 3.14)

If no initial value for an element is provided, it is default initialized
(and hence must be default initializable). For example:

    class X {
      X();
    public:
      X(std::string);
    };

    tuple<X,X,X>()                                              // error: no default constructor for X
    tuple<X,X,X>(string("Jaba"), string("Daba"), string("Duu")) // ok

In particular, reference types do not have a default initialization:

    tuple<double&>()                // error: reference must be
                                    // initialized explicitly

    double d = 5;
    tuple<double&>(d)               // ok

    tuple<double&>(d+3.14)          // error: cannot initialize
                                    // non-const reference with a temporary

    tuple<const double&>(d+3.14)    // ok, but dangerous:
                                    // the element becomes a dangling reference

Using an initial value for an element that cannot be copied, is a compile time
error:

    class Y {
      Y(const Y&);
    public:
      Y();
    };

    char a[10];

    tuple<char[10], Y>(a, Y()); // error, neither arrays nor Y can be copied
    tuple<char[10], Y>();       // ok

Note particularly that the following is perfectly ok:

    Y y;
    tuple<char(&)[10], Y&>(a, y);

It is possible to come up with a tuple type that cannot be constructed. This
occurs if an element that cannot be initialized has a lower index than an
element that requires initialization. For example: `tuple<char[10], int&>`.

In sum, the tuple construction is semantically just a group of individual
elementary constructions.

[section:make_tuple The `make_tuple` function]

Tuples can also be constructed using the `make_tuple` (cf. `std::make_pair`)
helper functions. This makes the construction more convenient, saving the
programmer from explicitly specifying the element types:

    tuple<int, int, double> add_multiply_divide(int a, int b) {
      return make_tuple(a+b, a*b, double(a)/double(b));
    }

By default, the element types are deduced to the plain non-reference types.
E.g.:

    void foo(const A& a, B& b) {
      ...
      make_tuple(a, b);

The `make_tuple` invocation results in a tuple of type `tuple<A, B>`.

Sometimes the plain non-reference type is not desired, e.g. if the element
type cannot be copied. Therefore, the programmer can control the type
deduction and state that a reference to const or reference to non-const type
should be used as the element type instead. This is accomplished with two
helper template functions: [@boost:/libs/core/doc/html/core/ref.html `boost::ref`]
and [@boost:/libs/core/doc/html/core/ref.html `boost::cref`]. Any argument can
be wrapped with these functions to get the desired type. The mechanism does
not compromise const correctness since a const object wrapped with ref results
in a tuple element with const reference type (see the fifth example below).
For example:

    A a; B b; const A ca = a;
    make_tuple(cref(a), b);      // creates tuple<const A&, B>
    make_tuple(ref(a), b);       // creates tuple<A&, B>
    make_tuple(ref(a), cref(b)); // creates tuple<A&, const B&>
    make_tuple(cref(ca));        // creates tuple<const A&>
    make_tuple(ref(ca));         // creates tuple<const A&>

Array arguments to `make_tuple` functions are deduced to reference to const
types by default; there is no need to wrap them with `cref`. For example:

    make_tuple("Donald", "Daisy");

This creates an object of type `tuple<const char (&)[7], const char (&)[6]>`
(note that the type of a string literal is an array of const characters, not
`const char*`). However, to get `make_tuple` to create a tuple with an element
of a non-const array type one must use the `ref` wrapper.

Function pointers are deduced to the plain non-reference type, that is, to
plain function pointer. A tuple can also hold a reference to a function, but
such a tuple cannot be constructed with `make_tuple` (a const qualified
function type would result, which is illegal):

    void f(int i);
      ...
    make_tuple(&f); // tuple<void (*)(int)>
      ...
    tuple<tuple<void (&)(int)> > a(f) // ok
    make_tuple(f);                    // not ok

[endsect]

[endsect]

[section:accessing_elements Accessing Tuple Elements]

Tuple elements are accessed with the expression:

    t.get<N>()

or

    get<N>(t)

where `t`  is a tuple object and `N` is a constant integral expression
specifying the index of the element to be accessed. Depending on whether `t`
is const or not, `get` returns the `N`-th element as a reference to const or
non-const type. The index of the first element is `0` and thus `N` must be
between `0` and /k/`-1`, where /k/ is the number of elements in the tuple.
Violations of these constraints are detected at compile time. Examples:

    double d = 2.7; A a;
    tuple<int, double&, const A&> t(1, d, a);
    const tuple<int, double&, const A&> ct = t;
      ...
    int i = get<0>(t); i = t.get<0>();        // ok
    int j = get<0>(ct);                       // ok
    get<0>(t) = 5;                            // ok
    get<0>(ct) = 5;                           // error, can't assign to const
      ...
    double e = get<1>(t); // ok
    get<1>(t) = 3.14;     // ok
    get<2>(t) = A();      // error, can't assign to const
    A aa = get<3>(t);     // error: index out of bounds
      ...
    ++get<0>(t);  // ok, can be used as any variable

/[Note:/ The member `get` functions are not supported with MS Visual C++
compiler. Further, the compiler has trouble with finding the non-member `get`
functions without an explicit namespace qualifier. Hence, all `get` calls
should be qualified as `tuples::get<N>(a_tuple)` when writing code that should
compile with MSVC++ 6.0./]/

[endsect]

[section:construction_and_assignment Copy Construction and Tuple Assignment]

A tuple can be copy constructed from another tuple, provided that the element
types are element-wise copy constructible. Analogously, a tuple can be
assigned to another tuple, provided that the element types are element-wise
assignable. For example:

    class A {};
    class B : public A {};
    struct C { C(); C(const B&); };
    struct D { operator C() const; };
    tuple<char, B*, B, D> t;
      ...
    tuple<int, A*, C, C> a(t); // ok
    a = t;                     // ok

In both cases, the conversions performed are:

* `char -> int`,
* `B* -> A*` (derived class pointer to base class pointer),
* `B -> C` (a user defined conversion), and
* `D -> C` (a user defined conversion).

Note that assignment is also defined from `std::pair` types:

    tuple<float, int> a = std::make_pair(1, 'a');

[endsect]

[section:relational_operators Relational Operators]

Tuples reduce the operators `==`, `!=`, `<`, `>`, `<=` and `>=` to the
corresponding elementary operators. This means, that if any of these operators
is defined between all elements of two tuples, then the same operator is
defined between the tuples as well. The equality operators for two tuples `a`
and `b` are defined as:

* `a == b` iff for each `i`: `a`'''<subscript>i</subscript>'''` == b`'''<subscript>i</subscript>'''
* `a != b` iff exists `i`: `a`'''<subscript>i</subscript>'''` != b`'''<subscript>i</subscript>'''

The operators `<`, `>`, `<=` and `>=` implement a lexicographical ordering.

Note that an attempt to compare two tuples of different lengths results in a
compile time error. Also, the comparison operators are /"short-circuited"/:
elementary comparisons start from the first elements and are performed only
until the result is clear.

Examples:

    tuple<std::string, int, A> t1(std::string("same?"), 2, A());
    tuple<std::string, long, A> t2(std::string("same?"), 2, A());
    tuple<std::string, long, A> t3(std::string("different"), 3, A());

    bool operator==(A, A) { std::cout << "All the same to me..."; return true; }

    t1 == t2;               // true
    t1 == t3;               // false, does not print "All the..."

[endsect]

[section:tiers Tiers]

/Tiers/ are tuples, where all elements are of non-const reference types. They
are constructed with a call to the `tie` function template (cf. `make_tuple`):

    int i; char c; double d;
      ...
    tie(i, c, a);

The above `tie` function creates a tuple of type `tuple<int&, char&, double&>`.
The same result could be achieved with the call `make_tuple(ref(i), ref(c), ref(a))`.

A tuple that contains non-const references as elements can be used to 'unpack'
another tuple into variables. E.g.:

    int i; char c; double d;
    tie(i, c, d) = make_tuple(1,'a', 5.5);
    std::cout << i << " " <<  c << " " << d;

This code prints `1 a 5.5` to the standard output stream. A tuple unpacking
operation like this is found for example in ML and Python. It is convenient
when calling functions which return tuples.

The tying mechanism works with `std::pair` templates as well:

    int i; char c;
    tie(i, c) = std::make_pair(1, 'a');

[section Ignore]

There is also an object called `ignore` which allows you to ignore an element
assigned by a tuple. The idea is that a function may return a tuple, only part
of which you are interested in. For example (note, that ignore is under the
`tuples` subnamespace):

    char c;
    tie(tuples::ignore, c) = std::make_pair(1, 'a');

[endsect]

[endsect]

[section:streaming Streaming]

The global `operator<<` has been overloaded for `std::ostream` such that
tuples are output by recursively calling `operator<<` for each element.

Analogously, the global `operator>>` has been overloaded to extract tuples
from `std::istream` by recursively calling `operator>>` for each element.

The default delimiter between the elements is space, and the tuple is enclosed
in parenthesis. For Example:

    tuple<float, int, std::string> a(1.0f,  2, std::string("Howdy folks!");

    cout << a;

outputs the tuple as: `(1.0 2 Howdy folks!)`

The library defines three manipulators for changing the default behavior:

* `set_open(char)` defines the character that is output before the first element.
* `set_close(char)` defines the character that is output after the last element.
* `set_delimiter(char)` defines the delimiter character between elements.

Note, that these manipulators are defined in the tuples subnamespace. For
example:

    cout << tuples::set_open('[') << tuples::set_close(']') << tuples::set_delimiter(',') << a;

outputs the same tuple `a` as: `[1.0,2,Howdy folks!]`

The same manipulators work with `operator>>` and `istream` as well. Suppose
the `cin` stream contains the following data:

    (1 2 3) [4:5]

The code:

    tuple<int, int, int> i;
    tuple<int, int> j;

    cin >> i;
    cin >> tuples::set_open('[') >> tuples::set_close(']') >> tuples::set_delimiter(':');
    cin >> j;

reads the data into the tuples `i` and `j`.

Note that extracting tuples with `std::string` or C-style string elements does
not generally work, since the streamed tuple representation may not be
unambiguously parseable.

[endsect]

[section:performance Performance]

All tuple access and construction functions are small inlined one-liners.
Therefore, a decent compiler can eliminate any extra cost of using tuples
compared to using hand-written tuple like classes. Particularly, with a decent
compiler there is no performance difference between this code:

    class hand_made_tuple {
      A a; B b; C c;
    public:
      hand_made_tuple(const A& aa, const B& bb, const C& cc)
        : a(aa), b(bb), c(cc) {};
      A& getA() { return a; };
      B& getB() { return b; };
      C& getC() { return c; };
    };

    hand_made_tuple hmt(A(), B(), C());
    hmt.getA(); hmt.getB(); hmt.getC();

and this code:

    tuple<A, B, C> t(A(), B(), C());
    t.get<0>(); t.get<1>(); t.get<2>();

Note, that there are widely used compilers (e.g. bcc 5.5.1) which fail to
optimize this kind of tuple usage.

Depending on the optimizing ability of the compiler, the tier mechanism may
have a small performance penalty compared to using non-const reference
parameters as a mechanism for returning multiple values from a function. For
example, suppose that the following functions `f1` and `f2` have equivalent
functionalities:

    void f1(int&, double&);
    tuple<int, double> f2();

Then, the call #1 may be slightly faster than #2 in the code below:

    int i; double d;
      ...
    f1(i,d);         // #1
    tie(i,d) = f2(); // #2

See [[link publ_1 1], [link publ_2 2]] for more in-depth discussions about 
efficiency.

[section Effect on Compile Time]

Compiling tuples can be slow due to the excessive amount of template
instantiations. Depending on the compiler and the tuple length, it may be more
than 10 times slower to compile a tuple construct, compared to compiling an
equivalent explicitly written class, such as the `hand_made_tuple` class above.
However, as a realistic program is likely to contain a lot of code in addition
to tuple definitions, the difference is probably unnoticeable. Compile time
increases between 5 and 10 percent were measured for programs which used tuples
very frequently. With the same test programs, memory consumption of compiling
increased between 22% to 27%. See [[link publ_1 1], [link publ_2 2]] for
details.

[endsect]

[endsect]

[section:portability Portability]

The library code is(?) standard C++ and thus the library works with a standard
conforming compiler. Below is a list of compilers and known problems with each
compiler:

[table
    [[Compiler]        [Problems]]
    [[gcc 2.95]        [-]]
    [[edg 2.44]        [-]]
    [[Borland 5.5]     [Can't use function pointers or member pointers as
                       tuple elements]]
    [[Metrowerks 6.2]  [Can't use `ref` and `cref` wrappers]]
    [[MS Visual C++]   [No reference elements (`tie` still works). Can't use
                       `ref` and `cref` wrappers]]
]

[endsect]

[section:more_details More Details]

[link tuple_advanced_interface Advanced features] (describes some metafunctions etc.).

[link design_decisions_rationale Rationale behind some design/implementation decisions].

[endsect]

[section:thanks Acknowledgements]

Gary Powell has been an indispensable helping hand. In particular, stream
manipulators for tuples were his idea. Doug Gregor came up with a working
version for MSVC, David Abrahams found a way to get rid of most of the
restrictions for compilers not supporting partial specialization. Thanks to
Jeremy Siek, William Kempf and Jens Maurer for their help and suggestions. The
comments by Vesa Karvonen, John Max Skaller, Ed Brey, Beman Dawes, David
Abrahams and Hartmut Kaiser helped to improve the library. The idea for the
`tie` mechanism came from an old usenet article by Ian McCulloch, where he
proposed something similar for `std::pair`s.

[endsect]

[section:references References]

[#publ_1]
[1] J\u00E4rvi J.: /Tuples and multiple return values in C++/, TUCS Technical Report No 249, 1999.

[#publ_2]
[2] J\u00E4rvi J.: /ML-Style Tuple Assignment in Standard C++ - Extending the Multiple Return Value Formalism/, TUCS Technical Report No 267, 1999.

[#publ_3]
[3] J\u00E4rvi J.: /Tuple Types and Multiple Return Values/, C/C++ Users Journal, August 2001.

[endsect]