cpplinq: Range Generators and Conversion Operators

In my previous post, I introduced cpplinq, a C++ template library that provides .NET-like query operators for sequences of objects in C++11. In this second installment, I will discuss about two things: range generators and range conversion operators. These two sets of operators are ubiquitous in queries.

Range Generators

A range generator builds an object that represents the range on which query operators are applied. The library provides several such generators:

• from_iterators: constructs a range from a pair of iterators
• from: constructs a range from an STL-like container that provides begin() and end() methods (representing the first and past-the-end elements). This is basically a wrapper on from_iterators operator.
  C++

  std::vector<int> numbers;
  numbers.push_back(1);
  numbers.push_back(2);
  numbers.push_back(3);
  numbers.push_back(4);
  numbers.push_back(5);
  
  auto result = from(numbers);
  result >> for_each([](int i) {std::cout << i << std::endl;});

  This is similar to:

  C++

  auto result = from_iterators(numbers.begin(), numbers.end());

• from_array: constructs a range from an array.
  C++

  int numbers[] = {1,2,3,4,5};
  auto result = from_array(numbers);
  result >> for_each([](int i) {std::cout << i << std::endl;});

  This is similar to:

  C++

  auto result = from_iterators(arrnumbers, arrnumbers + 5);

In addition to the "from" operators, the library also provides several .NET like range generators:

• range: generates a range of integral, consecutive numbers, starting with an initial seed and having a specified number of elements.
  C++

  auto result = range(10, 90); // creates a range of numbers in the interval [10, 100)

• repeat: generates a range by repeating a value a given number of times
  C++

  auto result = repeat("cpplinq", 10); // creates a range with 10 strings with the value "cpplinq"

• empty: returns an empty range of a given type
  C++

  auto result = empty<customer>(); // creates an empty range of customers

Range Conversion Operators

A conversion operator folds a range into a container that holds the values of the range. There are several such conversion operators that the library provides.

• to_vector: creates a std::vector<TValue> from a range, where TValue is the type of the elements of the range.
  C++

  auto result = range(1, 10) >> to_vector();
  
  std::list<int> numbers;
  auto result = from(numbers) >> to_vector(); // transforms a list into a vector

• to_list: creates a std::list<TValue> from a range, where TValue is the type of the elements of the range.
  C++

  auto result = range(1, 10) >> to_list();

• to_map: creates a std::map<TKey, TValue> from a range. It takes a predicate that selects the value to use as the key for each element of the range. It implements a one-to-one dictionary that maps keys to single values.
  C++

  // creates a map where key is the customer ID, and the value is the customer object
  auto result = from_array (customers) >> to_map ([](customer const & c){return c.id;});

• to_lookup: creates a cpplinq::lookup<TKey, TElement> from a sequence. It implements a one-to-many dictionary that maps keys to sequences of values.
  C++

  customer_address customer_addresses[] =
  {
     customer_address (2, 4, "Finland"   ),
     customer_address (3, 4, "USA"       ),
     customer_address (1, 1, "USA"       ),
  };
  
  auto lookup = from_array (customer_addresses) 
             >> to_lookup ([] (customer_address const & ca){return ca.customer_id;}); 
  
  auto countries = lookup[4] 
                >> select([](customer_address const & ca) {return ca.country;}) 
                >> to_vector();  // yields {"Finland", "USA"}

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