Difference between revisions of "cpp/language/default comparisons"

Revision as of 22:48, 28 March 2023

Provides a way to request the compiler to generate consistent comparison operators for a class.

Syntax

Explanation

The three-way comparison function (whether defaulted or not) is called whenever values are compared using <, >, <=, >=, or <=> and overload resolution selects this overload.

The equality comparison function (whether defaulted or not) is called whenever values are compared using == or != and overload resolution selects this overload.

Like defaulted special member functions, a defaulted comparison function is defined if odr-used or needed for constant evaluation.

Defaulted comparisons

Defaulted three-way comparison

The default operator<=> performs lexicographical comparison by successively comparing the base (left-to-right depth-first) and then non-static member (in declaration order) subobjects of T to compute <=>, recursively expanding array members (in order of increasing subscript), and stopping early when a not-equal result is found, that is:

for /*each base or member subobject o of T*/
    if (auto cmp = static_cast<R>(compare(lhs.o, rhs.o)); cmp != 0)
        return cmp;
return static_cast<R>(strong_ordering::equal);

It is unspecified whether virtual base subobjects are compared more than once.

If the declared return type is auto, then the actual return type is the common comparison category of the base and member subobject and member array elements to be compared (see std::common_comparison_category). This makes it easier to write cases where the return type non-trivially depends on the members, such as:

template<class T1, class T2>
struct P
{
    T1 x1;
    T2 x2;
    friend auto operator<=>(const P&, const P&) = default;
};

Let R be the return type, each pair of subobjects a, b is compared as follows:

• If a <=> b is usable and can be explicitly converted to R using static_cast, the result of comparison is static_cast<R>(a <=> b).
• Otherwise, if overload resolution for a <=> b is performed and finds at least one viable candidate, the comparison is not defined (operator<=> is defined as deleted).
• Otherwise, if R is not a comparison category type (see below), or either a == b or a < b is not usable, the comparison is not defined (operator<=> is defined as deleted).
• Otherwise, if R is std::strong_ordering, the result is

a == b ? R::equal :
a < b  ? R::less :
         R::greater

• Otherwise, if R is std::weak_ordering, the result is

a == b ? R::equivalent :
a < b  ? R::less :
         R::greater

• Otherwise (R is std::partial_ordering), the result is

a == b ? R::equivalent :
a < b  ? R::less :
b < a  ? R::greater : 
         R::unordered

Per the rules for any operator<=> overload, a defaulted <=> overload will also allow the type to be compared with <, <=, >, and >=.

If operator<=> is defaulted and operator== is not declared at all, then operator== is implicitly defaulted.

#include <compare>
#include <iostream>
#include <set>
 
struct Point
{
    int x;
    int y;
    auto operator<=>(const Point&) const = default;
    // ... non-comparison functions ...
};
// compiler generates all six two-way comparison operators
 
int main()
{
    Point pt1{1, 1}, pt2{1, 2};
    std::set<Point> s; // OK
    s.insert(pt1);     // OK
 
    std::cout << std::boolalpha
        << (pt1 == pt2) << ' '  // false; operator== is implicitly defaulted.
        << (pt1 != pt2) << ' '  // true
        << (pt1 <  pt2) << ' '  // true
        << (pt1 <= pt2) << ' '  // true
        << (pt1 >  pt2) << ' '  // false
        << (pt1 >= pt2) << ' '; // false
}

Defaulted equality comparison

A class can define operator== as defaulted, with a return value of bool. This will generate an equality comparison of each base class and member subobject, in their declaration order. Two objects are equal if the values of their base classes and members are equal. The test will short-circuit if an inequality is found in members or base classes earlier in declaration order.

Per the rules for operator==, this will also allow inequality testing:

#include <iostream>
 
struct Point
{
    int x;
    int y;
    bool operator==(const Point&) const = default;
    // ... non-comparison functions ...
};
// compiler generates element-wise equality testing
 
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
}

Other defaulted comparison operators

Any of the four relational operators can be explicitly defaulted. A defaulted relational operator must have the return type bool.

Such operator will be deleted if overload resolution over x <=> y (considering also operator<=> with reversed order of parameters) fails, or if this operator@ is not applicable to the result of that x <=> y. Otherwise, the defaulted operator@ calls x <=> y @ 0 if an operator<=> with the original order of parameters was selected by overload resolution, or 0 @ y <=> x otherwise:

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

Similarly, operator!= can be defaulted. It is deleted if overload resolution over x == y (considering also operator== with reversed order of parameters) fails, or if the result of x == y does not have type bool. The defaulted operator!= calls !(x == y) or !(y == x) as selected by overload resolution.

Defaulting the relational operators can be useful in order to create functions whose addresses may be taken. For other uses, it is sufficient to provide only operator<=> and operator==.

Custom comparisons and comparison categories

When the default semantics are not suitable, such as when the members must be compared out of order, or must use a comparison that's different from their natural comparison, then the programmer can write operator<=> and let the compiler generate the appropriate two-way comparison operators. The kind of two-way comparison operators generated depends on the return type of the user-defined operator<=>.

There are three available return types:

Strong ordering

An example of a custom operator<=> that returns std::strong_ordering is an operator that compares every member of a class, except in order that is different from the default (here: last name first).

#include <cassert>
#include <compare>
#include <set>
#include <string>
 
struct Base
{
    std::string zip;
    auto operator<=>(const Base&) const = default;
};
 
struct TotallyOrdered : Base
{
    std::string tax_id;
    std::string first_name;
    std::string last_name;
public:
    // custom operator<=> because we want to compare last names first:
    std::strong_ordering operator<=>(const TotallyOrdered& that) const
    {
        if (auto cmp = (Base&)(*this) <=> (Base&)that; cmp != 0)
            return cmp;
        if (auto cmp = last_name <=> that.last_name; cmp != 0)
            return cmp;
        if (auto cmp = first_name <=> that.first_name; cmp != 0)
            return cmp;
        return tax_id <=> that.tax_id;
    }
    // ... non-comparison functions ...
};
// compiler generates all four relational operators
 
int main()
{
    TotallyOrdered to1{"a", "b", "c", "d"}, to2{"a", "b", "d", "c"};
    std::set<TotallyOrdered> s; // OK
    s.insert(to1); // OK
    assert(to2 <= to1); // OK, single call to <=>
}

Note: an operator that returns a std::strong_ordering should compare every member, because if any member is left out, substitutability can be compromised: it becomes possible to distinguish two values that compare equal.

Weak ordering

An example of a custom operator<=> that returns std::weak_ordering is an operator that compares string members of a class in case-insensitive manner: this is different from the default comparison (so a custom operator is required) and it is possible to distinguish two strings that compare equal under this comparison:

class CaseInsensitiveString
{
    std::string s;
public:
    std::weak_ordering operator<=>(const CaseInsensitiveString& b) const
    {
        return case_insensitive_compare(s.c_str(), b.s.c_str());
    }
 
    std::weak_ordering operator<=>(const char* b) const
    {
        return case_insensitive_compare(s.c_str(), b);
    }
    // ... non-comparison functions ...
};
 
// Compiler generates all four relational operators
CaseInsensitiveString cis1, cis2;
std::set<CaseInsensitiveString> s; // OK
s.insert(/*...*/); // OK
if (cis1 <= cis2) { /*...*/ } // OK, performs one comparison operation
 
// Compiler also generates all eight heterogeneous relational operators
if (cis1 <= "xyzzy") { /*...*/ } // ok, performs one comparison operation
if ("xyzzy" >= cis1) { /*...*/ } // ok, identical semantics

Note that this example demonstrates the effect a heterogeneous operator<=> has: it generates heterogeneous comparisons in both directions.

Partial ordering

Partial ordering is an ordering that allows incomparable (unordered) values, such as NaN values in floating-point ordering, or, in this example, persons that are not related:

class PersonInFamilyTree // ...
{
public:
    std::partial_ordering operator<=>(const PersonInFamilyTree& that) const
    {
        if (this->is_the_same_person_as(that))
            return partial_ordering::equivalent;
        if (this->is_transitive_child_of(that))
            return partial_ordering::less;
        if (that. is_transitive_child_of(*this))
            return partial_ordering::greater;
        return partial_ordering::unordered;
    }
    // ... non-comparison functions ...
};
 
// compiler generates all four relational operators
PersonInFamilyTree per1, per2;
if (per1 < per2) { /*...*/ } // OK, per2 is an ancestor of per1
else if (per1 > per2) { /*...*/ } // OK, per1 is an ancestor of per2
else if (std::is_eq(per1 <=> per2)) { /*...*/ } // OK, per1 is per2
else { /*...*/ } // per1 and per2 are unrelated
 
if (per1 <= per2) { /*...*/ } // OK, per2 is per1 or an ancestor of per1
if (per1 >= per2) { /*...*/ } // OK, per1 is per2 or an ancestor of per2
if (std::is_neq(per1 <=> per2)) { /*...*/ } // OK, per1 is not per2

See also

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