Boost C++ Libraries

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.

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 '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*>

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.

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

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

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&>

make_tuple("Donald", "Daisy");

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

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.]

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:

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

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

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:

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..."

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');

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

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:

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.

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 [1, 2] for more in-depth discussions about efficiency.

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:

Advanced features (describes some metafunctions etc.).

Rationale behind some design/implementation decisions.

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::pairs.

[1] Järvi J.: Tuples and multiple return values in C++, TUCS Technical Report No 249, 1999.

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

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