Compares the arguments.
Operator name Syntax
Prototype examples (for classT) Inside class definition Outside class definition Equal to a == bYes boolT::operator==(constU&b)const;booloperator==(constT&a,constU&b);Not equal to a != bYes boolT::operator!=(constU&b)const;booloperator!=(constT&a,constU&b);Less than a < bYes boolT::operator<(constU&b)const;booloperator<(constT&a,constU&b);Greater than a > bYes boolT::operator>(constU&b)const;booloperator>(constT&a,constU&b);Less than or equal to a <= bYes boolT::operator<=(constU&b)const;booloperator<=(constT&a,constU&b);Greater than or equal to a >= bYes boolT::operator>=(constU&b)const;booloperator>=(constT&a,constU&b);Three-way comparison (C++20)a <=> bYes R T::operator<=>(constU&b)const;
R operator<=>(constT&a,constU&b);
Notes
Where built-in operators return bool, most
also return bool so that the user-defined operators can be used in the same manner as the built-ins. However, in a user-defined operator overload, any type can be used as return type (including void).
U can be any type including T.
↑
R is the return type of operator<=> (
)
Two-way comparison
The two-way comparison operator expressions have the form
Relational operatorslhs<rhs(1) lhs>rhs(2) lhs<=rhs(3) lhs>=rhs(4) Equality operatorslhs==rhs(5) lhs!=rhs(6) 1) Returns true if lhs is less than rhs, false otherwise.
2) Returns true if lhs is greater than rhs, false otherwise.
3) Returns true if lhs is less than or equal to rhs, false otherwise.
4) Returns true if lhs is greater than or equal to rhs, false otherwise.
5) Returns true if lhs is equal to rhs, false otherwise.
6) Returns true if lhs is not equal to rhs, false otherwise.
Built-in two-way comparison operators
For built-in two-way comparison operators,
,
(until C++26) and
function-to-pointer conversions
are applied to lhs and rhs .
The comparison is deprecated if both lhs and rhs have array type prior to the application of these conversions.
(since C++20)
(until C++26)For built-in relational operators, if one of the operands is a pointer, the
is performed on the other operand.
For built-in equality operators, if one of the operands is a pointer or a
, the array-to-pointer conversion is performed on the other operand.
(since C++26)For built-in two-way comparison operators, the result is a bool prvalue.
Built-in arithmetic comparison
If the converted operands both have arithmetic or enumeration type (scoped or unscoped),
are performed on both operands. The values are compared after conversions:
Run this code
#include<iostream>intmain(){static_assert(sizeof(unsignedchar)<sizeof(int),"Cannot compare signed and smaller unsigned properly");inta=-1;intb=1;unsignedintc=1;unsignedchard=1;std::cout<<std::boolalpha<<"Comparing two signed values:\n"" -1 == 1 ? "<<(a==b)<<"\n"" -1 < 1 ? "<<(a<b)<<"\n"" -1 > 1 ? "<<(a>b)<<"\n""Comparing signed and unsigned:\n"// may issue different-signedness warning:" -1 == 1 ? "<<(a==c)<<"\n"// may issue different-signedness warning:" -1 < 1 ? "<<(a<c)<<"\n"// may issue different-signedness warning:" -1 > 1 ? "<<(a>c)<<"\n""Comparing signed and smaller unsigned:\n"" -1 == 1 ? "<<(a==d)<<"\n"" -1 < 1 ? "<<(a<d)<<"\n"" -1 > 1 ? "<<(a>d)<<'\n';}Output:
Comparing two signed values: -1 == 1 ? false -1 < 1 ? true -1 > 1 ? false Comparing signed and unsigned: -1 == 1 ? false -1 < 1 ? false -1 > 1 ? true Comparing signed and smaller unsigned: -1 == 1 ? false -1 < 1 ? true -1 > 1 ? false Built-in pointer equality comparison
The converted operands of equality operators == and != can also have the type
,(since C++11) pointer type or pointer-to-member type.
Built-in pointer equality comparison has three possible results: equal, unequal and unspecified. The values yielded by equality operators for built-in pointer equality comparison is listed below:
Comparison result
of p and qValue yielded by p==qp!=qequal truefalseunequal falsetrueunspecified unspecified bool value If at least one of converted lhs and rhs is a pointer,
,
(since C++17) and
are performed on both converted operands to bring them to their
. The two pointers of the composite pointer type are compared as follows:
If one pointer
of a complete object, and another pointer
represents the address past the end of a different complete non-array object, or
represents the address one past the last element of a different complete array object,
the result of the comparison is unspecified.Otherwise, if the pointers are both null, both point to the same function, or both represent the same address (i.e., they point to or are past the end of the same object), they compare equal.
Otherwise, the pointers compare unequal.
If at least one of converted lhs and rhs is a pointer to member,
,
(since C++17) and
are performed on both converted operands to bring them to their
. The two pointers to members of the composite pointer type are compared as follows:
If two pointers to members are both the null member pointer value, they compare equal.
If only one of two pointers to members is the null member pointer value, they compare unequal.
If either is a pointer to a
, the result is unspecified.
If one refers to a member of class C1 and the other refers to a member of a different class C2, where neither is a base class of the other, the result is unspecified.
If both refer to (possibly different) members of the same
, they compare equal.
Otherwise, two pointers to members compare equal if they would refer to the same member of the same
or the same subobject if indirection with a hypothetical object of the associated class type were performed, otherwise they compare unequal.
structP{};structQ:P{intx;};structR:P{intx;};intP::*bx=(int(P::*))&Q::x;intP::*cx=(int(P::*))&R::x;boolb1=(bx==cx);// unspecifiedstructB{intf();};structL:B{};structR:B{};structD:L,R{};int(B::*pb)()=&B::f;int(L::*pl)()=pb;int(R::*pr)()=pb;int(D::*pdl)()=pl;int(D::*pdr)()=pr;boolx=(pdl==pdr);// falsebooly=(pb==pl);// trueTwo operands of type
or one operand of type
and the other a null pointer constant compare equal.
(since C++11)Built-in pointer relational comparison
The converted operands of relational operators >, <, >= and <= can also have pointer type.
Built-in pointer relational comparison on unequal pointers p and q has three possible results: p is greater, q is greater and unspecified. The values yielded by relational operators for built-in pointer relational comparison is listed below:
Comparison result
of p and qValue yielded by p>qp<qp>=qp<=qequal falsefalsetruetruep is greater truefalsetruefalseq is greater falsetruefalsetrueunspecified unspecified bool value If converted lhs and rhs are both pointers,
,
(since C++17) and
are performed on both converted operands to bring them to their
. The two pointers of the composite pointer type are compared as follows:
If the pointers compare equal or the equality comparison result is unspecified, the relational comparison result falls into the same category.
Otherwise (the pointers compare unequal), if any of the pointers is not a pointer to object, the result is unspecified.
Otherwise (both pointers point to objects), the result is defined in terms of a partial order consistent with the following rules:
Given two different elements high and low of an array such than high has higher subscript than low, if one pointer points to high (or a subobject of high) and the other pointer points to low (or a subobject of low), the former compares greater than the latter.
If one pointer points to an element elem (or to a subobject of elem) of an array, and the other pointer is past the end of the same array, the past-the-end pointer compares greater than the other pointer.
If one pointer points to a complete object, a base class subobject or a member subobject obj (or to a subobject of obj), and the other pointer is past the end of obj, the past-the-end pointer compares greater than the other pointer.
If the pointers point to different
(since C++20) non-static data members with the same
(until C++23) of the same object of a non-union class type, or to subobjects of such members, recursively, the pointer to the later declared member compares greater than the other pointer.
Otherwise, the result is unspecified.
Pointer total order
There exists an implementation-defined strict total order over pointers in each program. The strict total order is consistent with the partial order described above: unspecified results become implementation-defined, while other results stay the same.
Pointer comparison with the strict total order is applied in the following cases:
Calling the operator() of the pointer type specializations of
,
,
, and
.
Calling built-in operators comparing pointers from the operator() of specializations
,
,
, and
.
(since C++14)Calling built-in operator<=> comparing pointers from the operator() of
.
Calling built-in operator== comparing pointers from the operator() of
and
.
Calling built-in operator< comparing pointers from the operator() of
,
,
, and
.
(since C++20)Overloads
In
overload resolution against user-defined operators
, for every pair of promoted arithmetic types L and R, including enumeration types, the following function signatures participate in overload resolution:
booloperator<(L,R);booloperator>(L,R);booloperator<=(L,R);booloperator>=(L,R);booloperator==(L,R);booloperator!=(L,R);For every type P which is either pointer to object or pointer to function, the following function signatures participate in overload resolution:
booloperator<(P,P);booloperator>(P,P);booloperator<=(P,P);booloperator>=(P,P);booloperator==(P,P);booloperator!=(P,P);For every type MP that is a pointer to member object or pointer to member function or
(since C++11) or std::meta::info(since C++26), the following function signatures participate in overload resolution:
booloperator==(MP,MP);booloperator!=(MP,MP);Run this code
#include<iostream>structFoo{intn1;intn2;};unionUnion{intn;doubled;};intmain(){std::cout<<std::boolalpha;chara[4]="abc";char*p1=&a[1];char*p2=&a[2];std::cout<<"Pointers to array elements:\n"<<"p1 == p2? "<<(p1==p2)<<'\n'<<"p1 < p2? "<<(p1<p2)<<'\n';Foof;int*p3=&f.n1;int*p4=&f.n2;std::cout<<"Pointers to members of a class:\n"<<"p3 == p4? "<<(p3==p4)<<'\n'<<"p3 < p4? "<<(p3<p4)<<'\n';Unionu;int*p5=&u.n;double*p6=&u.d;std::cout<<"Pointers to members of a union:\n"<<"p5 == (void*)p6? "<<(p5==(void*)p6)<<'\n'<<"p5 < (void*)p6? "<<(p5<(void*)p6)<<'\n';}Output:
Pointers to array elements: p1 == p2? false p1 < p2? true Pointers to members of a class: p3 == p4? false p3 < p4? true Pointers to members of a union: p5 == (void*)p6? true p5 < (void*)p6? false Three-way comparison
The three-way comparison operator expressions have the form
a<=>bThe expression returns an object such that
(a<=>b)<0 if a<b,
(a<=>b)>0 if a>b,
(a<=>b)==0 if a and b are equal/equivalent.
If one of the operands is of type bool and the other is not, the program is ill-formed.
If both operands have arithmetic types, or if one operand has unscoped enumeration type and the other has integral type, the usual arithmetic conversions are applied to the operands, and then
If a narrowing conversion is required, other than from an integral type to a floating point type, the program is ill-formed.
Otherwise, if the operands have integral type, the operator yields a prvalue of type
:
std::strong_ordering::equal if both operands are arithmetically equal,
std::strong_ordering::less if the first operand is arithmetically less than the second,
std::strong_ordering::greater otherwise.
Otherwise, the operands have floating-point type, and the operator yields a prvalue of type
. The expression a<=>b yields
std::partial_ordering::less if a is less than b,
std::partial_ordering::greater if a is greater than b,
std::partial_ordering::equivalent if a is equivalent to b (-0<=>+0 is equivalent),
std::partial_ordering::unordered (NaN<=>anything is unordered).
If both operands have the same enumeration type E, the operator yields the result of converting the operands to the underlying type of E and applying <=> to the converted operands.
If at least one of the operands is a pointer to object or pointer to member,
,
and
are applied to both operands to bring them to their
.
For converted pointer operands p and q, p<=>q returns a prvalue of type
:
std::strong_ordering::equal if they
,
std::strong_ordering::less if q
p,
std::strong_ordering::greater if p compares greater than q,
unspecified result if the two-way comparison result is unspecified.
Otherwise, the program is ill-formed.
Overloads
In
overload resolution against user-defined operators
, for pointer or enumeration type T, the following function signature participates in overload resolution:
R operator<=>(T, T);Where R is the ordering category type defined above.
Run this code
#include <compare> #include <iostream> int main() { double foo = -0.0; double bar = 0.0; if (const auto res = foo <=> bar; res < 0) std::cout << "-0 is less than 0"; else if (res > 0) std::cout << "-0 is greater than 0"; else if (res == 0) std::cout << "-0 and 0 are equal"; else std::cout << "-0 and 0 are unordered"; }Output:
-0 and 0 are equal(since C++20)Notes
Because comparison operators group left-to-right, the expression a < b < c is parsed (a < b) < c, and not a < (b < c) or (a < b) && (b < c).
Run this code
#include <iostream> int main() { int a = 3, b = 2, c = 1; std::cout << std::boolalpha << (a < b < c) << '\n' // true; maybe warning << ((a < b) < c) << '\n' // true << (a < (b < c)) << '\n' // false << ((a < b) && (b < c)) << '\n'; // false }A common requirement for
is
. In particular, this is required by the standard algorithms and containers that work with
types:
,
,
, etc.
The
of pointers to different non-static data members of the same class implies that non-static data members in each of the three
(until C++23) are positioned in memory in order of declaration.
Although the results of comparing pointers of random origin (e.g. not all pointing to members of the same array) is unspecified, many implementations provide
of pointers, e.g. if they are implemented as addresses within continuous virtual address space. Those implementations that do not (e.g. where not all bits of the pointer are part of a memory address and have to be ignored for comparison, or an additional calculation is required or otherwise pointer and integer is not a 1 to 1 relationship), provide a specialization of
for pointers that has that guarantee. This makes it possible to use all pointers of random origin as keys in standard associative containers such as
or
.
For the types that are both
and
, the C++ standard library makes a distinction between equality, which is the value of the expression a == b and equivalence, which is the value of the expression !(a < b) && !(b < a).
Comparison between pointers and null pointer constants was removed by the resolution of
included in
:
Run this code
void f(char* p) { if (p > 0) { /*...*/ } // Error with N3624, compiled before N3624 if (p > nullptr) { /*...*/ } // Error with N3624, compiled before N3624 } int main() {}Three-way comparison can be automatically generated for class types, see
.
If both of the operands are arrays, three-way comparison is ill-formed.
unsigned int i = 1; auto r = -1 < i; // existing pitfall: returns ‘false’ auto r2 = -1 <=> i; // Error: narrowing conversion required
macro ValueStdFeature
__cpp_impl_three_way_comparison
(C++20)Three-way comparison (compiler support)
__cpp_lib_three_way_comparison
(C++20)Three-way comparison (library support); adding three-way comparison to the library Standard library
Comparison operators are overloaded for many classes in the standard library.
(removed in C++20)
checks whether the objects refer to the same type
(public member function of std::type_info)
operator==operator!=operator<operator<=>
(removed in C++20)(removed in C++20)(C++20)
compares two error_codes
(function)
operator==operator!=operator<operator<=>
(removed in C++20)(removed in C++20)(C++20)
compares error_conditions and error_codes
(function)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values in the pair
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values in the tuple
(function template)
(removed in C++20)
compares the contents
(public member function of std::bitset<N>)
(removed in C++20)
compares two allocator instances
(public member function of std::allocator<T>)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(C++20)
compares to another unique_ptr or with nullptr
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
compares with another shared_ptr or with nullptr
(function template)
(removed in C++20)
compares a
with nullptr
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++11)(C++11)(removed in C++20)(C++11)(C++11)(C++11)(C++11)(C++20)
compares two durations
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++11)(C++11)(removed in C++20)(C++11)(C++11)(C++11)(C++11)(C++20)
compares two time points
(function template)
(removed in C++20)
compares two scoped_allocator_adaptor objects
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(C++20)
compares the underlying
objects
(public member function of std::type_index)
operator==operator!=operator<operator>operator<=operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares two strings
(function template)
(removed in C++20)
equality comparison between locale objects
(public member function of std::locale)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++11)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++20)
lexicographically compares the values of two arrays
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two deques
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++11)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++20)
lexicographically compares the values of two forward_lists
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two lists
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two vectors
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two maps
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two multimaps
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two sets
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
lexicographically compares the values of two multisets
(function template)
(C++11)(C++11)(removed in C++20)
compares the values in the unordered_map
(function template)
(C++11)(C++11)(removed in C++20)
compares the values in the unordered_multimap
(function template)
(C++11)(C++11)(removed in C++20)
compares the values in the unordered_set
(function template)
(C++11)(C++11)(removed in C++20)
compares the values in the unordered_multiset
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++20)
lexicographically compares the values of two queues
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++20)
lexicographically compares the values of two stacks
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++20)
compares the underlying iterators
(function template)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(C++11)(C++11)(removed in C++20)(C++11)(C++11)(C++11)(C++11)(C++20)
compares the underlying iterators
(function template)
(removed in C++20)
compares two istream_iterators
(function template)
(removed in C++20)
compares two istreambuf_iterators
(function template)
(removed in C++20)
compares two complex numbers or a complex and a scalar
(function template)
operator==operator!=operator<operator<=operator>operator>=
compares two valarrays or a valarray with a value
(function template)
(C++11)(C++11)(removed in C++20)
compares the internal states of two pseudo-random number engines
(function)
(C++11)(C++11)(removed in C++20)
compares two distribution objects
(function)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
compares a sub_match with another sub_match, a string, or a character
(function template)
(removed in C++20)
lexicographically compares the values in the two match result
(function template)
(removed in C++20)
compares two regex_iterators
(public member function of std::regex_iterator<BidirIt,CharT,Traits>)
(removed in C++20)
compares two regex_token_iterators
(public member function of std::regex_token_iterator<BidirIt,CharT,Traits>)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
compares two thread::id objects
(function)
The namespace
provides generic operators !=, >, <=, and >=:
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
(
) C++98 all six comparison operators could be used to
compare a pointer with a null pointer constant only equality operators
are allowed
C++98 the actual semantics of arithmetic comparisons (e.g.
whether 1 < 2 yields true or false) were unspecified specification added
C++98 pointers to function types and pointers
to void did not have built-in comparisons added comparison
specification for these pointers
C++98 non-array objects were considered to belong to arrays with
one element only for the purpose of pointer arithmetic the rule is also
applied to comparison
C++98 two pointers to members of classes that are different and
neither is the base class of the other did not compare equal
even if the offsets of the pointed members can be the same the result is
unspecified
in this case
C++98 it was not clear whether two pointers to members
that refer to different members of the same union
compare equal as if they refer to the same member they compare
equal in this case
C++98 a pointer to non-array object was only treated as a
pointer to the first element of an array with size 1
in pointer comparison if the pointer is obtained by &applies to all pointers
to non-array objects
C++98 the definition of relational comparison (>, >=, < and <=) of
pointers to void and function pointers were removed by
restored
C++17 function pointer conversions were not performed on the converted
pointer operands during built-in pointer relational comparisons performs these
conversions in this case See also
(named requirements)
Common operators
a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b++a --a a++ a--+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b!a a && b a || ba == b a != b a < b a > b a <= b a >= b a <=> ba[...] *a &a a->b a.b a->*b a.*bfunction calla(...)
commaa, b
conditionala ? b : c
Special operators
converts one type to another related type
converts within inheritance hierarchies
adds or removes
-qualifiers
converts type to unrelated type
converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast
creates objects with dynamic storage duration
destructs objects previously created by the new expression and releases obtained memory area
queries the size of a type
queries the size of a
(since C++11)
queries the type information of a type
checks if an expression can throw an exception (since C++11)
queries alignment requirements of a type (since C++11)
produces a reflection value from a grammatical construct (since C++26)