Namespaces
Variants
Views
Actions

Default comparisons (since C++20)

From cppreference.com
< cpp‎ | language
 
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
const/volatile
decltype (C++11)
auto (C++11)
constexpr (C++11)
consteval (C++20)
constinit (C++20)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Memory allocation
Classes
Class-specific function properties
explicit (C++11)
static

Special member functions
Templates
Miscellaneous
 
 

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

Contents

[edit] Definition

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

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.

[edit] Default comparison order

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 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:

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]

[edit] Three-way comparison

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

[edit] Comparison category types

There are three comparison category types:

Type  Equivalent values are..   Incomparable values are.. 
std::strong_ordering indistinguishable not allowed
std::weak_ordering distinguishable not allowed
 std::partial_ordering  distinguishable allowed

[edit] Synthesized three-way comparison

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.
a == b ? std::strong_ordering::equal :
a < b  ? std::strong_ordering::less :
         std::strong_ordering::greater
a == b ? std::weak_ordering::equivalent :
a < b  ? std::weak_ordering::less :
         std::weak_ordering::greater
a == b ? std::partial_ordering::equivalent :
a < b  ? std::partial_ordering::less :
b < a  ? std::partial_ordering::greater : 
         std::partial_ordering::unordered

[edit] Placeholder return type

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>.

[edit] Non-placeholder return type

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.

[edit] Comparison result

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
}

[edit] Equality comparison

[edit] Explicit declaration

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
}

[edit] Implicit declaration

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:

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;
};

[edit] Secondary comparison

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
};

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 2539 C++20 the synthesized three-way comparison would choose
static_cast even if the explicit conversion is not available
does not choose
static_cast in this case
CWG 2546 C++20 the defaulted secondary operator@ was not
defined as deleted if the overload resolution of
x @ y selects a non-usable rewritten candidate
defined as deleted
in this case
CWG 2547 C++20 it was unclear whether comparison operator
functions for non-classes can be defaulted
they cannot be defaulted
CWG 2568 C++20 the implicit definition of comparison operator
functions might violate member access rules
access checks are performed
from a context equivalent
to their function bodies

[edit] See also