Chapter 39. Boost.Phoenix

Boost.Phoenix is the most important Boost library for functional programming. While libraries like Boost.Bind or Boost.Lambda provide some support for functional programming, Boost.Phoenix includes the features of these libraries and goes beyond them.

In functional programming, functions are objects and can be processed like objects. With Boost.Phoenix, it is possible for a function to return another function as a result. It is also possible to pass a function as a parameter to another function. Because functions are objects, it’s possible to distinguish between instantiation and execution. Accessing a function isn’t equal to executing it.

Boost.Phoenix supports functional programming with function objects: Functions are objects based on classes which overload the operator operator(). That way function objects behave like other objects in C++. For example, they can be copied and stored in a container. However, they also behave like functions because they can be called.

Functional programming isn’t new in C++. You can pass a function as a parameter to another function without using Boost.Phoenix.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

bool is_odd(int i) { return i % 2 == 1; }

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  std::cout << std::count_if(v.begin(), v.end(), is_odd) << '\n';

  auto lambda = [](int i){ return i % 2 == 1; };
  std::cout << std::count_if(v.begin(), v.end(), lambda) << '\n';

  using namespace boost::phoenix::placeholders;
  auto phoenix = arg1 % 2 == 1;
  std::cout << std::count_if(v.begin(), v.end(), phoenix) << '\n';
}

Example 39.1 uses the algorithm std::count_if() to count odd numbers in vector v. std::count_if() is called three times, once with a predicate as a free-standing function, once with a lambda function, and once with a Phoenix function.

The Phoenix function differs from free-standing and lambda functions because it has no frame. While the other two functions have a function header with a signature, the Phoenix function seems to consist of a function body only.

The crucial component of the Phoenix function is boost::phoenix::placeholders::arg1. arg1 is a global instance of a function object. You can use it like std::cout: These objects exist once the respective header file is included.

arg1 is used to define an unary function. The expression arg1 % 2 == 1 creates a new function that expects one parameter. The function isn’t executed immediately but stored in phoenix. phoenix is passed to std::count_if() which calls the predicate for every number in v.

arg1 is a placeholder for the value passed when the Phoenix function is called. Since only arg1 is used here, a unary function is created. Boost.Phoenix provides additional placeholders such as boost::phoenix::placeholders::arg2 and boost::phoenix::placeholders::arg3. A Phoenix function always expects as many parameters as the placeholder with the greatest number.

Example 39.1 writes 3 three times to standard output.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  auto lambda = [](int i){ return i % 2 == 1; };
  std::cout << std::count_if(v.begin(), v.end(), lambda) << '\n';

  std::vector<long> v2;
  v2.insert(v2.begin(), v.begin(), v.end());

  using namespace boost::phoenix::placeholders;
  auto phoenix = arg1 % 2 == 1;
  std::cout << std::count_if(v.begin(), v.end(), phoenix) << '\n';
  std::cout << std::count_if(v2.begin(), v2.end(), phoenix) << '\n';
}

Example 39.2 highlights a crucial difference between Phoenix and lambda functions. In addition to requiring no function header with a parameter list, Phoenix function parameters have no types. The lambda function lambda expects a parameter of type int. The Phoenix function phoenix will accept any type that the modulo operator can handle.

Think of Phoenix functions as function templates. Like function templates, Phoenix functions can accept any type. This makes it possible in Example 39.2 to use phoenix as a predicate for the containers v and v2 even though they store numbers of different types. If you try to use the predicate lambda with v2, you get a compiler error.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix::placeholders;
  auto phoenix = arg1 > 2 && arg1 % 2 == 1;
  std::cout << std::count_if(v.begin(), v.end(), phoenix) << '\n';
}

Example 39.3 uses a Phoenix function as a predicate with std::count_if() to count odd numbers greater than 2. The Phoenix function accesses arg1 twice: Once to test if the placeholder is greater than 2 and once to test whether it’s an odd number. The conditions are linked with &&.

You can think of Phoenix functions as C++ code that isn’t executed immediately. The Phoenix function in Example 39.3 looks like a condition that uses multiple logical and arithmetic operators. However, the condition isn’t executed immediately. It is only executed when it is accessed from within std::count_if(). The access in std::count_if() is a normal function call.

Example 39.3 writes 2 to standard output.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix;
  using namespace boost::phoenix::placeholders;
  auto phoenix = arg1 > val(2) && arg1 % val(2) == val(1);
  std::cout << std::count_if(v.begin(), v.end(), phoenix) << '\n';
}

Example 39.4 uses explicit types for all operands in the Phoenix function. Strictly speaking, you don’t see types, just the helper function boost::phoenix::val(). This function returns a function object initialized with the values passed to boost::phoenix::val(). The actual type of the function object doesn’t matter. What is important is that Boost.Phoenix overloads operators like >, &&, % and == for different types. Thus, conditions aren’t checked immediately. Instead, function objects are combined to create more powerful function objects. Depending on the operands they may be automatically used as function objects. Otherwise you can call helper functions like val().

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

int main()
{
  using namespace boost::phoenix::placeholders;
  std::cout << arg1(1, 2, 3, 4, 5) << '\n';

  auto v = boost::phoenix::val(2);
  std::cout << v() << '\n';
}

Example 39.5 illustrates how arg1 and val() work. arg1 is an instance of a function object. It can be used directly and called like a function. You can pass as many parameters as you like – arg1 returns the first one.

val() is a function to create an instance of a function object. The function object is initialized with the value passed as a parameter. If the instance is accessed like a function, the value is returned.

Example 39.5 writes 1 and 2 to standard output.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

struct is_odd_impl
{
    typedef bool result_type;

    template <typename T>
    bool operator()(T t) const { return t % 2 == 1; }
};

boost::phoenix::function<is_odd_impl> is_odd;

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix::placeholders;
  std::cout << std::count_if(v.begin(), v.end(), is_odd(arg1)) << '\n';
}

Example 39.6 explains how you can create your own Phoenix function. You pass a function object to the template boost::phoenix::function. The example passes the class is_odd_impl. This class overloads the operator operator(): when an odd number is passed in, the operator returns true. Otherwise, the operator returns false.

Please note that you must define the type result_type. Boost.Phoenix uses it to detect the type of the return value of the operator operator().

is_odd() is a function you can use like val(). Both functions return a function object. When called, parameters are forwarded to the operator operator(). For Example 39.6, this means that std::count_if() still counts odd numbers.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

bool is_odd_function(int i) { return i % 2 == 1; }

BOOST_PHOENIX_ADAPT_FUNCTION(bool, is_odd, is_odd_function, 1)

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix::placeholders;
  std::cout << std::count_if(v.begin(), v.end(), is_odd(arg1)) << '\n';
}

If you want to transform a free-standing function into a Phoenix function, you can proceed as in Example 39.7. You don’t necessarily have to define a function object as in the previous example.

You use the macro BOOST_PHOENIX_ADAPT_FUNCTION to turn a free-standing function into a Phoenix function. Pass the type of the return value, the name of the Phoenix function to define, the name of the free-standing function, and the number of parameters to the macro.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

bool is_odd(int i) { return i % 2 == 1; }

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix;
  using namespace boost::phoenix::placeholders;
  std::cout << std::count_if(v.begin(), v.end(), bind(is_odd, arg1)) << '\n';
}

To use a free-standing function as a Phoenix function, you can also use boost::phoenix::bind() as in Example 39.8. boost::phoenix::bind() works like std::bind(). The name of the free-standing function is passed as the first parameter. All further parameters are forwarded to the free-standing function.

#include <boost/phoenix/phoenix.hpp>
#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
  std::vector<int> v{1, 2, 3, 4, 5};

  using namespace boost::phoenix;
  using namespace boost::phoenix::placeholders;
  int count = 0;
  std::for_each(v.begin(), v.end(), if_(arg1 > 2 && arg1 % 2 == 1)
    [
      ++ref(count)
    ]);
  std::cout << count << '\n';
}

Boost.Phoenix provides some function objects that simulate C++ keywords. For example, you can use the function boost::phoenix::if_() (see Example 39.9) to create a function object that acts like if and tests a condition. If the condition is true, the code passed to the function object with operator[] will be executed. Of course, that code also has to be based on function objects. That way, you can create complex Phoenix functions.

Example 39.9 increments count for every odd number greater than 2. To use the increment operator on count, count is wrapped in a function object using boost::phoenix::ref(). In contrast to boost::phoenix::val(), no value is copied into the function object. The function object returned by boost::phoenix::ref() stores a reference – here a reference to count.