Combining multiple values into one

There is a very nice present for those who like std::pair. Boost has a library called Boost.Tuple. It is just like std::pair, but it can also work with triples, quads, and even bigger collections of types.

Getting ready

Only basic knowledge of C++ and a standard library is required for this recipe.

How to do it...

Perform the following steps to combine multiple values into one:

1. To start working with tuples, you need to include a proper header and declare a variable:

#include <boost/tuple/tuple.hpp> 
#include <string> 
 
boost::tuple<int, std::string> almost_a_pair(10, "Hello");
boost::tuple<int, float, double, int> quad(10, 1.0f, 10.0, 1);

2. Getting a specific value is implemented via the boost::get<N>() function, where N is a zero-based index of a required value:

#include <boost/tuple/tuple.hpp>

void sample1() {
    const int i = boost::get<0>(almost_a_pair); 
    const std::string& str = boost::get<1>(almost_a_pair); 
    const double d = boost::get<2>(quad);
}

The boost::get<> function has many overloads and is used widely across Boost. We already saw how it can be used with other libraries in the Storing multiple chosen types in a container/variable recipe.

3. You can construct tuples using the boost::make_tuple() function, which is shorter to write, because you do not need to fully qualify the tuple type:

#include <boost/tuple/tuple.hpp>
#include <boost/tuple/tuple_comparison.hpp>
#include <set>

void sample2() {
    // Tuple comparison operators are
    // defined in header "boost/tuple/tuple_comparison.hpp"
    // Don't forget to include it!
    std::set<boost::tuple<int, double, int> > s;
    s.insert(boost::make_tuple(1, 1.0, 2));
    s.insert(boost::make_tuple(2, 10.0, 2));
    s.insert(boost::make_tuple(3, 100.0, 2));
 
    // Requires C++11
    const auto t = boost::make_tuple(0, -1.0, 2);
    assert(2 == boost::get<2>(t));
    // We can make a compile time assert for type
    // of t. Interested? See chapter 'Compile time tricks'
}

4. Another function that makes life easier is boost::tie(). It works almost as make_tuple, but adds a nonconst reference for each of the passed types. Such a tuple can be used to get values to a variable from another tuple. It can be better understood from the following example:

#include <boost/tuple/tuple.hpp>
#include <cassert>

void sample3() {
    boost::tuple<int, float, double, int> quad(10, 1.0f, 10.0, 1); 
    int i; 
    float f; 
    double d; 
    int i2; 
 
    // Passing values from 'quad' variables 
    // to variables 'i', 'f', 'd', 'i2'.
    boost::tie(i, f, d, i2) = quad; 
    assert(i == 10); 
    assert(i2 == 1); 
}

How it works...

Some readers may wonder why we need a tuple when we can always write our own structures with better names; for example, instead of writing boost::tuple<int, std::string>, we can create a structure:

struct id_name_pair { 
    int id; 
    std::string name; 
}; 

Well, this structure is definitely clearer than boost::tuple<int, std::string>. The main idea behind the tuple's library is to simplify template programming.

There's more...

A tuple works as fast as std::pair (it does not allocate memory on a heap and has no virtual functions). The C++ committee found this class to be very useful and it was included in the standard library. You can find it in a C++11 compatible implementation in the header file <tuple> (don't forget to replace all the boost:: namespaces with std::).

The standard library version of tuple must have multiple micro optimizations and typically provides a slightly better user experience. However, there is no guarantee on the order of construction of tuple elements, so, if you need a tuple that constructs its elements starting from the first, you have to use the boost::tuple:

#include <boost/tuple/tuple.hpp>
#include <iostream>

template <int I>
struct printer {
    printer() { std::cout << I; }
};

int main() {
    // Outputs 012
    boost::tuple<printer<0>, printer<1>, printer<2> > t;
}

The current Boost implementation of a tuple does not use variadic templates, does not support rvalue references, does not support C++17 structured bindings, and is not usable with constexpr.

See also

• Boost's official documentation contains more examples, information about performance, and abilities of Boost.Tuple. It is available at the link http://boost.org/libs/tuple.
• The Converting all tuple elements to a string recipe in Chapter 8, Metaprogramming, shows some advanced usages of tuples.