A C++ template library for embedded applications
MIT licensed
Join the ETL community
Designed and
maintained by
John Wellbelove
Like the ETL? Become a patron!


A helper class for templates that require constants and classes that declare value and value_type members.

The class allows constants to be wrapped in a template that defines value and value_type.
This can be particularly useful in recursively defined templates where the value is defined as either a constant or a
further invocation of the template.


Take the example of etl::sqrt.

template <const intmax_t VALUE, const intmax_t I = 1>
struct sqrt
  typedef typename etl::conditional<((I * I) < VALUE),
                                    etl::sqrt<VALUE, I + 1>,
                                    etl::constant<intmax_t, I> >::type type;

  enum value_type
    value = type::value

// Stop condition.
template <const intmax_t I>
struct sqrt<I, I>
  enum value_type
    value = I

etl::conditional will select one of the two types based on the boolean template parameter. The types are expected
to have a value member. This is achieved for the constant by wrapping it in etl::constant.