How to check whether operator== exists?
C++SfinaeC++ Problem Overview
I am trying to create an example, which would check the existence of the operator== (member or, non-member function). To check whether a class has a member operator== is easy, but how to check whether it has a non-member operator==?
This is what I have to far :
#include <iostream>
struct A
{
int a;
#if 0
bool operator==( const A& rhs ) const
{
return ( a==rhs.a);
}
#endif
};
#if 1
bool operator==( const A &l,const A &r )
{
return ( l.a==r.a);
}
#endif
template < typename T >
struct opEqualExists
{
struct yes{ char a[1]; };
struct no { char a[2]; };
template <typename C> static yes test( typeof(&C::operator==) );
//template <typename C> static yes test( ???? );
template <typename C> static no test(...);
enum { value = (sizeof(test<T>(0)) == sizeof(yes)) };
};
int main()
{
std::cout<<(int)opEqualExists<A>::value<<std::endl;
}
Is it possible to write a test function to test the existence of non-member operator==?
If yes, how?
btw I have checked similar questions, but haven't found a proper solution :
https://stackoverflow.com/questions/4434569/is-it-possible-to-use-sfinae-templates-to-check-if-an-operator-exists
This is what I tried :
template <typename C> static yes test( const C*,bool(*)(const C&,constC&) = &operator== );
but the compilation fails if the non-member operator== is removed
C++ Solutions
Solution 1 - C++
C++03
The following trick works and it can be used for all such operators:
namespace CHECK
{
class No { bool b[2]; };
template<typename T, typename Arg> No operator== (const T&, const Arg&);
bool Check (...);
No& Check (const No&);
template <typename T, typename Arg = T>
struct EqualExists
{
enum { value = (sizeof(Check(*(T*)(0) == *(Arg*)(0))) != sizeof(No)) };
};
}
Usage:
CHECK::EqualExists<A>::value;
The 2nd template typename Arg is useful for some special cases like A::operator==(short), where it's not similar to class itself. In such cases the usage is:
CHECK::EqualExists<A, short>::value
// ^^^^^ argument of `operator==`
Demo.
C++11
We need not use sizeof and null reference trick when we have decltype and std::declval
namespace CHECK
{
struct No {};
template<typename T, typename Arg> No operator== (const T&, const Arg&);
template<typename T, typename Arg = T>
struct EqualExists
{
enum { value = !std::is_same<decltype(std::declval<T>() < std::declval<Arg>()), No>::value };
};
}
Solution 2 - C++
Have a look at Boost's Concept Check Library (BCCL) http://www.boost.org/doc/libs/1_46_1/libs/concept_check/concept_check.htm.
It enables you to write requirements that a class must match in order for the program to compile. You're relatively free with what you can check. For example, verifying the presence of operator== of a class Foo would write as follow:
#include <boost/concept_check.hpp>
template <class T>
struct opEqualExists;
class Foo {
public:
bool operator==(const Foo& f) {
return true;
}
bool operator!=(const Foo& f) {
return !(*this == f);
}
// friend bool operator==(const Foo&, const Foo&);
// friend bool operator!=(const Foo&, const Foo&);
};
template <class T>
struct opEqualExists {
T a;
T b;
// concept requirements
BOOST_CONCEPT_USAGE(opEqualExists) {
a == b;
}
};
/*
bool operator==(const Foo& a, const Foo& b) {
return true; // or whatever
}
*/
/*
bool operator!=(const Foo& a, const Foo& b) {
return ! (a == b); // or whatever
}
*/
int main() {
// no need to declare foo for interface to be checked
// declare that class Foo models the opEqualExists concept
// BOOST_CONCEPT_ASSERT((opEqualExists<Foo>));
BOOST_CONCEPT_ASSERT((boost::EqualityComparable<Foo>)); // need operator!= too
}
This code compiles fine as long as one of the two implementations of operator== is available.
Following @Matthieu M. and @Luc Touraille advice, I updated the code snippet to provide an example of boost::EqualityComparable usage. Once again, please note that EqualityComparable forces you to declare operator!= too.
Solution 3 - C++
It's also possible to use only c++11 type traits to check the existence of the member:
#include <type_traits>
#include <utility>
template<class T, class EqualTo>
struct has_operator_equal_impl
{
template<class U, class V>
static auto test(U*) -> decltype(std::declval<U>() == std::declval<V>());
template<typename, typename>
static auto test(...) -> std::false_type;
using type = typename std::is_same<bool, decltype(test<T, EqualTo>(0))>::type;
};
template<class T, class EqualTo = T>
struct has_operator_equal : has_operator_equal_impl<T, EqualTo>::type {};
You can use the trait like so:
bool test = has_operator_equal<MyClass>::value;
The resulting type of has_operator_equal will either be std::true_type or std::false_type (because it inherits from an alias of std::is_same::type), and both define a static value member which is a boolean.
If you want to be able to test whether your class defines operator==(someOtherType), you can set the second template argument:
bool test = has_operator_equal<MyClass, long>::value;
where the template parameter MyClass is still the class that you are testing for the presence of operator==, and long is the type you want to be able to compare to, e.g. to test that MyClass has operator==(long).
if EqualTo (like it was in the first example) is left unspecified, it will default to T, result in the normal definition of operator==(MyClass).
Note of caution: This trait in the case of operator==(long) will be true for long, or any value implicitly convertible to long, e.g. double, int, etc.
You can also define checks for other operators and functions, just by replacing what's inside the decltype. To check for !=, simply replace
static auto test(U*) -> decltype(std::declval<U>() == std::declval<V>());
with
static auto test(U*) -> decltype(std::declval<U>() != std::declval<V>());
Solution 4 - C++
As of c++14, the standard binary functions do most of the work for us for the majority of operators.
#include <utility>
#include <iostream>
#include <string>
#include <algorithm>
#include <cassert>
template<class X, class Y, class Op>
struct op_valid_impl
{
template<class U, class L, class R>
static auto test(int) -> decltype(std::declval<U>()(std::declval<L>(), std::declval<R>()),
void(), std::true_type());
template<class U, class L, class R>
static auto test(...) -> std::false_type;
using type = decltype(test<Op, X, Y>(0));
};
template<class X, class Y, class Op> using op_valid = typename op_valid_impl<X, Y, Op>::type;
namespace notstd {
struct left_shift {
template <class L, class R>
constexpr auto operator()(L&& l, R&& r) const
noexcept(noexcept(std::forward<L>(l) << std::forward<R>(r)))
-> decltype(std::forward<L>(l) << std::forward<R>(r))
{
return std::forward<L>(l) << std::forward<R>(r);
}
};
struct right_shift {
template <class L, class R>
constexpr auto operator()(L&& l, R&& r) const
noexcept(noexcept(std::forward<L>(l) >> std::forward<R>(r)))
-> decltype(std::forward<L>(l) >> std::forward<R>(r))
{
return std::forward<L>(l) >> std::forward<R>(r);
}
};
}
template<class X, class Y> using has_equality = op_valid<X, Y, std::equal_to<>>;
template<class X, class Y> using has_inequality = op_valid<X, Y, std::not_equal_to<>>;
template<class X, class Y> using has_less_than = op_valid<X, Y, std::less<>>;
template<class X, class Y> using has_less_equal = op_valid<X, Y, std::less_equal<>>;
template<class X, class Y> using has_greater_than = op_valid<X, Y, std::greater<>>;
template<class X, class Y> using has_greater_equal = op_valid<X, Y, std::greater_equal<>>;
template<class X, class Y> using has_bit_xor = op_valid<X, Y, std::bit_xor<>>;
template<class X, class Y> using has_bit_or = op_valid<X, Y, std::bit_or<>>;
template<class X, class Y> using has_left_shift = op_valid<X, Y, notstd::left_shift>;
template<class X, class Y> using has_right_shift = op_valid<X, Y, notstd::right_shift>;
int main()
{
assert(( has_equality<int, int>() ));
assert((not has_equality<std::string&, int const&>()()));
assert((has_equality<std::string&, std::string const&>()()));
assert(( has_inequality<int, int>() ));
assert(( has_less_than<int, int>() ));
assert(( has_greater_than<int, int>() ));
assert(( has_left_shift<std::ostream&, int>() ));
assert(( has_left_shift<std::ostream&, int&>() ));
assert(( has_left_shift<std::ostream&, int const&>() ));
assert((not has_right_shift<std::istream&, int>()()));
assert((has_right_shift<std::istream&, int&>()()));
assert((not has_right_shift<std::istream&, int const&>()()));
}
Solution 5 - C++
C++20
I guess you want to check if user provide Template type has equality operator or not, if that is the case Concepts are here to help.
#include<concepts>
struct S{
int x;
};
template<class T>
requires std::EqualityComparable<T>
void do_magic(T a, T b){
return a == b;
}
int main(){
// do_magic(S{}, S{}); Compile time error
do_magic(56, 46); // Okay int has == and !=
return 0;
}
If you pass any type that does not have == and != overload the compiler just errors out with message:
> EqualityComparable concept not satisfied by type
You can also use std::EqualityComparableWith<T, U> concept can check for those overload between two different types.
There are many more concepts that have been added to standards like Incrementable etc.. Have a look here
Solution 6 - C++
I know this question has long since been answered but I thought it might be worth noting for anyone who finds this question in the future that Boost just added a bunch of "has operator" traits to their type_traits library, and among them is has_equal_to, which does what OP was asking for.
Solution 7 - C++
This question has already been answered several times, but there is a simpler way to check for the existence of operator== or basically any other operation (e.g., testing for a member function with a certain name), by using decltype together with the , operator:
namespace detail
{
template<typename L, typename R>
struct has_operator_equals_impl
{
template<typename T = L, typename U = R> // template parameters here to enable SFINAE
static auto test(T &&t, U &&u) -> decltype(t == u, void(), std::true_type{});
static auto test(...) -> std::false_type;
using type = decltype(test(std::declval<L>(), std::declval<R>()));
};
} // namespace detail
template<typename L, typename R = L>
struct has_operator_equals : detail::has_operator_equals_impl<L, R>::type {};
You can use this same approach to check if a type T has a member function foo which is invocable with a certain argument list:
namespace detail
{
template<typename T, typename ...Args>
struct has_member_foo_impl
{
template<typename T_ = T>
static auto test(T_ &&t, Args &&...args) -> decltype(t.foo(std::forward<Args>(args)...), void(), std::true_type{});
static auto test(...) -> std::false_type;
using type = decltype(test(std::declval<T>(), std::declval<Args>()...));
};
} // namespace detail
template<typename T, typename ...Args>
struct has_member_foo : detail::has_member_foo_impl<T, Args...>::type {};
I think this makes the intent of the code much clearer. In addition to that, this is a C++11 solution, so it doesn't depend on any newer C++14 or C++17 features. The end result is the same, of course, but this has become my preferred idiom for testing these kinds of things.
Edit: Fixed the insane case of the overloaded comma operator, I always miss that.
Solution 8 - C++
Lets consider a meta-function of the following form, which checks for the existence of equality operator (i.e ==) for the given type:
template<typename T>
struct equality { .... };
However, that might not be good enough for some corner cases. For example, say your class X does define operator== but it doesn't return bool, instead it returns Y. So in this case, what should equality<X>::value return? true or false? Well, that depends on the specific use case which we dont know now, and it doesn't seem to be a good idea to assume anything and force it on the users. However, in general we can assume that the return type should be bool, so lets express this in the interface itself:
template<typename T, typename R = bool>
struct equality { .... };
The default value for R is bool which indicates it is the general case. In cases, where the return type of operator== is different, say Y, then you can say this:
equality<X, Y> //return type = Y
which checks for the given return-type as well. By default,
equality<X> //return type = bool
Here is one implementation of this meta-function:
namespace details
{
template <typename T, typename R, typename = R>
struct equality : std::false_type {};
template <typename T, typename R>
struct equality<T,R,decltype(std::declval<T>()==std::declval<T>())>
: std::true_type {};
}
template<typename T, typename R = bool>
struct equality : details::equality<T, R> {};
Test:
struct A {};
struct B { bool operator == (B const &); };
struct C { short operator == (C const &); };
int main()
{
std::cout<< "equality<A>::value = " << equality<A>::value << std::endl;
std::cout<< "equality<B>::value = " << equality<B>::value << std::endl;
std::cout<< "equality<C>::value = " << equality<C>::value << std::endl;
std::cout<< "equality<B,short>::value = " << equality<B,short>::value << std::endl;
std::cout<< "equality<C,short>::value = " << equality<C,short>::value << std::endl;
}
Output:
equality<A>::value = 0
equality<B>::value = 1
equality<C>::value = 0
equality<B,short>::value = 0
equality<C,short>::value = 1
Hope that helps.
Solution 9 - C++
c++17 slightly modified version of Richard Hodges godbolt
#include <functional>
#include <type_traits>
template<class T, class R, class ... Args>
std::is_convertible<std::invoke_result_t<T, Args...>, R> is_invokable_test(int);
template<class T, class R, class ... Args>
std::false_type is_invokable_test(...);
template<class T, class R, class ... Args>
using is_invokable = decltype(is_invokable_test<T, R, Args...>(0));
template<class T, class R, class ... Args>
constexpr auto is_invokable_v = is_invokable<T, R, Args...>::value;
template<class L, class R = L>
using has_equality = is_invokable<std::equal_to<>, bool, L, R>;
template<class L, class R = L>
constexpr auto has_equality_v = has_equality<L, R>::value;
struct L{};
int operator ==(int, L&&);
static_assert(has_equality_v<int>);
static_assert(!has_equality_v<L>);
static_assert(!has_equality_v<L, int>);
static_assert(has_equality_v<int, L>);
Solution 10 - C++
We can use std::equal_to<Type> (or any other overloaded struct members) to make a more generic solution if we want to test binary operators (or other binary functors).
struct No {};
template<class T, class BinaryOperator>
struct ExistsBinaryOperator>
{
enum { value = !std::is_same<decltype(std::declval<BinaryOperator>()(std::declval<T>(), std::declval<T>())), No>::value };
};
Usage:
using Type = int;
constexpr bool hasEqual = ExistsBinaryOperator<Type, std::equal_to<Type>>::value;
Solution 11 - C++
IMO, this must be part of the class itself as it's deals with the private attributes of the class. The templates are interpreted at compile time. By default it generates operator==,constructor, destructor and copy constructor which do bit-wise copy (shallow copy) or bit-wise comparisons for the object of same type. The special cases (different types) must be overloaded. If you use global operator function you will have to declare the function as friend to access the private part or else you've to expose the interfaces required. Sometimes this is really ugly which may cause an unnecessary expose of a function.
Solution 12 - C++
Just for a reference, I am posting how I solved my problem, without a need to check if the operator== exists :
#include <iostream>
#include <cstring>
struct A
{
int a;
char b;
#if 0
bool operator==( const A& r ) const
{
std::cout<<"calling member function"<<std::endl;
return ( ( a==r.a ) && ( b==r.b ) );
}
#endif
};
#if 1
bool operator==( const A &l,const A &r )
{
std::cout<<"calling NON-member function"<<std::endl;
return ( ( l.a==r.a ) &&( l.b==r.b ) );
}
#endif
namespace details
{
struct anyType
{
template < class S >
anyType( const S &s ) :
p(&s),
sz(sizeof(s))
{
}
const void *p;
int sz;
};
bool operator==( const anyType &l, const anyType &r )
{
std::cout<<"anyType::operator=="<<std::endl;
return ( 0 == std::memcmp( l.p, r.p, l.sz ) );
}
} // namespace details
int main()
{
A a1;
a1.a=3;a1.b=0x12;
A a2;
a2.a=3;a2.b=0x12;
using details::operator==;
std::cout<< std::boolalpha << "numbers are equals : " << ( a1 == a2 ) <<std::endl;
}
Solution 13 - C++
In addition to @coder3101 answer, concepts can help you implement any function existence tests you want to. For example, std::equality_comparable is implemented using 4 simple tests, that check the following scenarios:
For A and B variables, make sure that the following expressions are valid:
A == B, returns bool
A != B, returns bool
B == A, returns bool
B != A, returns bool
If any one of them is illegal at compile time, the program won't compile. The implementation of this test (simplified from the standard):
template <typename T> concept equality_comparable
= requires(T t, T u) {
{ t == u } -> std::convertible_to<bool>;
{ t != u } -> std::convertible_to<bool>;
{ u == t } -> std::convertible_to<bool>;
{ u != t } -> std::convertible_to<bool>;
};
As you can see, you can customize this concept and create your own concept the fulfill your conditions. For example, if you want to force only the existence of operator==, you can do something like this:
template <typename T> concept my_equality_comparable
= requires(T t, T u) {
{ t == u } -> std::convertible_to<bool>;
{ u == t } -> std::convertible_to<bool>;
};