C++ determine if class is comparable

If you want to avoid cryptic errors (as you often get when the lack of comparability occurred deeply in the template instantiation tree), just use enable_if:

• If you have C++98 or C++03 compiler, boost::enable_if: http://www.boost.org/doc/libs/release/libs/utility/enable_if.html

• If you have C++11 compiler, std::enable_if: http://en.cppreference.com/w/cpp/types/enable_if

In particular, take a look at "Enabling template class specializations" in the docs of boost::enable_if.

You often use enable_if with type_traits: http://www.boost.org/doc/libs/release/libs/type_traits/doc/html/index.html

Of particular interest in your case might be the following ones:

http://www.boost.org/doc/libs/release/libs/type_traits/doc/html/boost_typetraits/reference:/has_equal_to.html

http://www.boost.org/doc/libs/release/libs/type_traits/doc/html/boost_typetraits/reference/has_not_equal_to.html

But see also has_greater, has_greater_equal, has_less, has_less_equal, etc. // I'm actually somewhat surprised that there isn't a straightforward is_equality_comparable type-trait.

// EDIT: it appears I've found it, it's ::boost::is_equality_comparable::value in the Concept Traits Library: http://neoscientists.org/~tschwinger/boostdev/concept_traits/libs/concept_traits/doc/

http://neoscientists.org/~tschwinger/boostdev/concept_traits/libs/concept_traits/doc/#StandardConceptTraits

However, it appears to be abandoned: https://svn.boost.org/trac/boost/wiki/LibrariesUnderConstruction#Boost.ConceptTraits

An alternative solution is to use the Boost Concept Checking Library (BCCL), in particular applying the EqualityComparableConcept:

http://www.boost.org/doc/libs/release/libs/concept_check/using_concept_check.htm

Yet another alternative: Boost.Generic -- https://svn.boost.org/trac/boost/wiki/LibrariesUnderConstruction#Boost.Generic

Prensentation: http://github.com/boostcon/2011_presentations/raw/master/thu/Boost.Generic.pdf

Yet another alternative: http://code.google.com/p/origin/source/browse/trunk/core/tests/concepts/equality_comparable.cpp

This is now possible in C++20 Concepts library.

class Node<T> requires Compare<T> {...}

Though this will still throw an error if the concept is not satisfied, it makes it easier to understand what caused the error. You can write a wrapper around it to prevent errors.

C++ templates are duck-typed, so no interface or constraint is necessary, the compiler will use the comparison operators if they exist, and generate an error if not.

See also this more detailed answer.