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Default comparisons (since C++20) - cppreference.com

Comparison operator functions can be explicitly defaulted to request the compiler to generate the corresponding default comparison for a class.

A defaulted comparison operator function is a non-template comparison operator function (i.e. <=>, ==, !=, <, >, <=, or >=) satisfying all following conditions:

• It is a non-static member or friend of some class C.
• It is defined as defaulted in C or in a context where C is complete.
• It has two parameters of type const C& or two parameters of type C, where the implicit object parameter (if any) is considered to be the first parameter.

Such a comparison operator function is termed a defaulted comparison operator function for class C.

struct X
{
    bool operator==(const X&) const = default; // OK
    bool operator==(const X&) = default;       // Error: the implicit object
                                               //        parameter type is X&
    bool operator==(this X, X) = default;      // OK
};
 
struct Y
{
    friend bool operator==(Y, Y) = default;        // OK
    friend bool operator==(Y, const Y&) = default; // Error: different parameter types
};
 
bool operator==(const Y&, const Y&) = default;     // Error: not a friend of Y

Name lookups and access checks in the implicit definition of a comparison operator function are performed from a context equivalent to its function body. A definition of a comparison operator function as defaulted that appears in a class must be the first declaration of that function.

Given a class C, a subobject list is formed by the following subjects in order:

• The direct base class subobjects of C, in declaration order.
• The non-static data members of C, in declaration order.

• If any member subobject is of array type, it is expanded to the sequence of its elements, in the order of increasing subscript. The expansion is recursive: array elements of array types will be expanded again until there is no subobject of array type.

For any object x of type C, in the following descriptions:

• Let n be the number of subobjects in the (expanded) subobject list for x.
• Let x_i be the ith subobject in the (expanded) subobject list for x, where x_i is formed by a sequence of derived-to-base conversions, class member access expressions, and array subscript expressions applied to x.

struct S {};
 
struct T : S
{
    int arr[2][2];
} t;
 
// The subobject list for “t” consists of the following 5 subobjects in order:
// (S)t → t[0][0] → t[0][1] → t[1][0] → t[1][1]

An operator<=> for a class type can be defined as defaulted with any return type.

There are three comparison category types:

• std::strong_ordering
• std::weak_ordering
• std::partial_ordering

The synthesized three-way comparison of type T between glvalues a and b of the same type is defined as follows:

• If the overload resolution for a <=> b results in a usable candidate, and can be explicitly converted to T using static_cast, the synthesized comparison is static_cast<T>(a <=> b).
• Otherwise, if any of the following condition is satisfied, the synthesized comparison is not defined:

• The overload resolution for a <=> b finds at least one viable candidate.
• T is not a comparison category type.
• The overload resolution for a == b does not result in a usable candidate.
• The overload resolution for a < b does not result in a usable candidate.

• Otherwise, if T is std::strong_ordering, the synthesized comparison is

a == b ? std::strong_ordering::equal :
a < b  ? std::strong_ordering::less :
         std::strong_ordering::greater

• Otherwise, if T is std::weak_ordering, the synthesized comparison is

a == b ? std::weak_ordering::equivalent :
a < b  ? std::weak_ordering::less :
         std::weak_ordering::greater

• Otherwise (T is std::partial_ordering), the synthesized comparison is

a == b ? std::partial_ordering::equivalent :
a < b  ? std::partial_ordering::less :
b < a  ? std::partial_ordering::greater : 
         std::partial_ordering::unordered

If the declared return type of a defaulted three-way comparison operator function (operator<=>) for a class type C is auto, the return type is deduced from the return types of the three-way comparisons between the corresponding subobjects of an object x of type C.

For each subobject x_i in the (expanded) subobject list for x:

1. Perform overload resolution for x_i <=> x_i, if the overload resolution does not result in a usable candidate, the defaulted operator<=> is defined as deleted.
2. Denote the cv-unqualified version of the type of x_i <=> x_i as R_i, if R_i is not a comparison category type, the defaulted operator<=> is defined as deleted.

If the defaulted operator<=> is not defined as deleted, its return type is deduced as std::common_comparison_category_t<R_1, R_2, ..., R_n>.

If the declared return type of the defaulted operator<=> is not auto, it cannot contain any placeholder type (e.g. decltype(auto)).

If there is a subobject x_i in the (expanded) subobject list for x such that the synthesized three-way comparison of the declared return type between x_i and x_i is not defined, the defaulted operator<=> is defined as deleted.

Let x and y be the parameters of a defaulted operator<=>, denote each subobject in the (expanded) subobject list for x and y as x_i and y_i respectively. The default three-way comparison between x and y is performed by comparing corresponding subobjects x_i and y_i with increasing i order.

Let R be the (possibly-deduced) return type, the comparison result between x_i and y_i is the result of the synthesized three-way comparison of type R between x_i and y_i.

• During the default three-way comparison between x and y, if a subobject-wise comparison between x_i and y_i generates a result v_i such that contextually converting v_i != 0 to bool yields true, the return value is a copy of v_i (the remaining subobjects will not be compared).
• Otherwise, the return value is static_cast<R>(std::strong_ordering::equal).

#include <compare>
#include <iostream>
#include <set>
 
struct Point
{
    int x;
    int y;
    auto operator<=>(const Point&) const = default;
    /* non-comparison functions */
};
 
int main()
{
    Point pt1{1, 1}, pt2{1, 2};
    std::set<Point> s; // OK
    s.insert(pt1);     // OK
 
    // two-way comparison operator functions are not required to be explicitly defined:
    // operator== is implicitly declared (see below)
    // the overload resolutions of other candidates will select rewritten candidates 
    std::cout << std::boolalpha
        << (pt1 == pt2) << ' '  // false
        << (pt1 != pt2) << ' '  // true
        << (pt1 <  pt2) << ' '  // true
        << (pt1 <= pt2) << ' '  // true
        << (pt1 >  pt2) << ' '  // false
        << (pt1 >= pt2) << ' '; // false
}

An operator== for a class type can be defined as defaulted with return type bool.

Given a class C and an object x of type C, if there is a subobject x_i in the (expanded) subobject list for x such that the overload resolution for x_i == x_i does not result in a usable candidate, the defaulted operator== is defined as deleted.

Let x and y be the parameters of a defaulted operator==, denote each subobject in the (expanded) subobject list for x and y as x_i and y_i respectively. The default equality comparison between x and y is performed by comparing corresponding subobjects x_i and y_i with increasing i order.

The comparison result between x_i and y_i is the result of x_i == y_i.

• During the default equality comparison between x and y, if a subobject-wise comparison between x_i and y_i generates a result v_i such that contextually converting v_i to bool yields false, the return value is false (the remaining subobjects will not be compared).
• Otherwise, the return value is true.

#include <iostream>
 
struct Point
{
    int x;
    int y;
    bool operator==(const Point&) const = default;
    /* non-comparison functions */
};
 
int main()
{
    Point pt1{3, 5}, pt2{2, 5};
    std::cout << std::boolalpha
        << (pt1 != pt2) << '\n'  // true
        << (pt1 == pt1) << '\n'; // true
 
    struct [[maybe_unused]] { int x{}, y{}; } p, q;
    // if (p == q) {} // Error: operator== is not defined
}

If a class C does not explicitly declare any member or friend named operator==, an == operator function is declared implicitly for each operator<=> defined as defaulted. Each implicity-declared operator== have the same access and function definition and in the same class scope as the respective defaulted operator<=>, with the following changes:

• The declarator identifier is replaced with operator==.
• The return type is replaced with bool.

template<typename T>
struct X
{
    friend constexpr std::partial_ordering operator<=>(X, X)
        requires (sizeof(T) != 1) = default;
    // implicitly declares: friend constexpr bool operator==(X, X)
    //                          requires (sizeof(T) != 1) = default;
 
    [[nodiscard]] virtual std::strong_ordering operator<=>(const X&) const = default;
    // implicitly declares: [[nodiscard]] virtual bool
    //                          operator==(const X&) const = default;
};

A secondary comparison operator function (!=, <, >, <=, or >=) for a class type can be defined as defaulted with return type bool.

Let @ be one of the five secondary comparison operators, for each defaulted operator@ with parameters x and y, up to two overloads resolutions are performed (not considering the defaulted operator@ as a candidate) to determine whether it is defined as deleted.

• The first overload resolution is performed for x @ y. If the overload resolution does not result in a usable candidate, or the selected candidate is not a rewritten candidate, the defaulted operator@ is defined as deleted. There is no second overload resolution in these cases.
• The second overload resolution is performed for the selected rewritten candidate of x @ y. If the overload resolution does not result in a usable candidate, the defaulted operator@ is defined as deleted.

If is x @ y cannot be implicitly converted to bool, the defaulted operator@ is defined as deleted.

If the defaulted operator@ is not defined as deleted, it yields x @ y.

struct HasNoRelational {};
 
struct C
{
    friend HasNoRelational operator<=>(const C&, const C&);
    bool operator<(const C&) const = default; // OK, function is defaulted
};

default

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• overload resolution in a call to an overloaded operator
• Built-in three-way comparison operator
• Operator overloading for comparison operators

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