Chapter 48. Boost.TypeTraits

Types have different properties that generic programming takes advantage of. The Boost.TypeTraits library provides the tools needed to determine a type’s properties and change them.

Since C++11, some functions provided by Boost.TypeTraits can be found in the standard library. You can access those functions through the header file type_traits. However, Boost.TypeTraits provides additional functions.

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

using namespace boost;

int main()
{
  std::cout.setf(std::ios::boolalpha);
  std::cout << is_integral<int>::value << '\n';
  std::cout << is_floating_point<int>::value << '\n';
  std::cout << is_arithmetic<int>::value << '\n';
  std::cout << is_reference<int>::value << '\n';
}

Example 48.1 calls several functions to determine type categories. boost::is_integral checks whether a type is integral – whether it can store integers. boost::is_floating_point checks whether a type stores floating point numbers. boost::is_arithmetic checks whether a type supports arithmetic operators. And boost::is_reference can be used to determine whether a type is a reference.

boost::is_integral and boost::is_floating_point are mutually exclusive. A type either stores an integer or a floating point number. However, boost::is_arithmetic and boost::is_reference can apply to multiple categories. For example, both integer and floating point types support arithmetic operations.

All functions from Boost.TypeTraits provide a result in value that is either true or false. Example 48.1 outputs true for is_integral<int> and is_arithmetic<int> and outputs false for is_floating_point<int> and is_reference<int>. Because all of these functions are templates, nothing is processed at run time. The example behaves at run time as though the values true and false were directly used in the code.

In Example 48.1, the result of the various functions is a value of type bool, which can be written directly to standard output. If the result is to be processed by a function template, it should be forwarded as a type, not as a bool value.

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

using namespace boost;

int main()
{
  std::cout.setf(std::ios::boolalpha);
  std::cout << is_same<is_integral<int>::type, true_type>::value << '\n';
  std::cout << is_same<is_floating_point<int>::type, false_type>::value <<
    '\n';
  std::cout << is_same<is_arithmetic<int>::type, true_type>::value << '\n';
  std::cout << is_same<is_reference<int>::type, false_type>::value << '\n';
}

Besides value, functions from Boost.TypeTraits also provide the result in type. While value is a bool value, type is a type. Just like value, which can only be set to true or false, type can only be set to one of two types: boost::true_type or boost::false_type. type lets you pass the result of a function as a type to another function.

Example 48.2 uses another function from Boost.TypeTraits called boost::is_same. This function expects two types as parameters and checks whether they are the same. To pass the results of boost::is_integral, boost::is_floating_point, boost::is_arithmetic, and boost::is_reference to boost::is_same, type must be accessed. type is then compared with boost::true_type or boost::false_type. The results from boost::is_same are then read through value again. Because this is a bool value, it can be written to standard output.

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

using namespace boost;

int main()
{
  std::cout.setf(std::ios::boolalpha);
  std::cout << has_plus<int>::value << '\n';
  std::cout << has_pre_increment<int>::value << '\n';
  std::cout << has_trivial_copy<int>::value << '\n';
  std::cout << has_virtual_destructor<int>::value << '\n';
}

Example 48.3 introduces functions that check properties of types. boost::has_plus checks whether a type supports the operator operator+ and whether two objects of the same type can be concatenated. boost::has_pre_increment checks whether a type supports the pre-increment operator operator++. boost::has_trivial_copy checks whether a type has a trivial copy constructor. And boost::has_virtual_destructor checks whether a type has a virtual destructor.

Example 48.3 displays true three times and false once.

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

using namespace boost;

int main()
{
  std::cout.setf(std::ios::boolalpha);
  std::cout << is_const<add_const<int>::type>::value << '\n';
  std::cout << is_same<remove_pointer<int*>::type, int>::value << '\n';
  std::cout << is_same<make_unsigned<int>::type, unsigned int>::value <<
    '\n';
  std::cout << is_same<add_rvalue_reference<int>::type, int&&>::value <<
    '\n';
}

Example 48.4 illustrates how type properties can be changed. boost::add_const adds const to a type. If the type is already constant, nothing changes. The code compiles without problems, and the type remains constant.

boost::remove_pointer removes the asterisk from a pointer type and returns the type the pointer refers to. boost::make_unsigned turns a type with a sign into a type without a sign. And boost::add_rvalue_reference transforms a type into a rvalue reference.

Example 48.4 writes true four times to standard output.