Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://en.cppreference.com/w/cpp/language/../language/operator_comparison.html below:

Comparison operators - cppreference.com

Compares the arguments.

Notes

• Where built-in operators return bool, most user-defined overloads 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.

1. ↑ ^{1.0 1.1} R is the return type of operator<=> (see below)

The two-way comparison operator expressions have the form

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.

For built-in two-way comparison operators, lvalue-to-rvalue conversions, array-to-pointer conversions(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.

For built-in relational operators, if one of the operands is a pointer, the array-to-pointer conversion is performed on the other operand.

For built-in equality operators, if one of the operands is a pointer or a null pointer constant, the array-to-pointer conversion is performed on the other operand.

For built-in two-way comparison operators, the result is a bool prvalue.

If the converted operands both have arithmetic or enumeration type (scoped or unscoped), usual arithmetic conversions are performed on both operands. The values are compared after conversions:

#include <iostream>
 
int main()
{
    static_assert(sizeof(unsigned char) < sizeof(int),
                  "Cannot compare signed and smaller unsigned properly");
    int a = -1;
    int b = 1;
    unsigned int c = 1;
    unsigned char d = 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

The converted operands of equality operators == and != can also have the type std::nullptr_t,(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:

If at least one of converted lhs and rhs is a pointer, pointer conversions, function pointer conversions(since C++17) and qualification conversions are performed on both converted operands to bring them to their composite pointer type. The two pointers of the composite pointer type are compared as follows:

• If one pointer represents the address 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, pointer-to-member conversions, function pointer conversions(since C++17) and qualification conversions are performed on both converted operands to bring them to their composite pointer type. 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 virtual member function, 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 union, they compare equal.
• Otherwise, two pointers to members compare equal if they would refer to the same member of the same most derived object or the same subobject if indirection with a hypothetical object of the associated class type were performed, otherwise they compare unequal.

struct P {};
struct Q : P { int x; };
struct R : P { int x; };
 
int P::*bx = (int(P::*)) &Q::x;
int P::*cx = (int(P::*)) &R::x;
 
bool b1 = (bx == cx); // unspecified
 
struct B
{
    int f();
};
struct L : B {};
struct R : B {};
struct D : L, R {};
 
int (B::*pb)() = &B::f;
int (L::*pl)() = pb;
int (R::*pr)() = pb;
int (D::*pdl)() = pl;
int (D::*pdr)() = pr;
 
bool x = (pdl == pdr); // false
bool y = (pb == pl);   // true

Two operands of type std::nullptr_t or one operand of type std::nullptr_t and the other a null pointer constant compare equal.

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:

If converted lhs and rhs are both pointers, pointer conversions, function pointer conversions(since C++17) and qualification conversions are performed on both converted operands to bring them to their composite pointer type. 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 non-zero-sized(since C++20) non-static data members with the same member access(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.

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 std::less, std::greater, std::less_equal, and std::greater_equal.

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:

bool operator<(L, R);

bool operator>(L, R);

bool operator<=(L, R);

bool operator>=(L, R);

bool operator==(L, R);

bool operator!=(L, R);

For every type P which is either pointer to object or pointer to function, the following function signatures participate in overload resolution:

bool operator<(P, P);

bool operator>(P, P);

bool operator<=(P, P);

bool operator>=(P, P);

bool operator==(P, P);

bool operator!=(P, P);

For every type MP that is a pointer to member object or pointer to member function or std::nullptr_t(since C++11), the following function signatures participate in overload resolution:

bool operator==(MP, MP);

bool operator!=(MP, MP);

#include <iostream>
 
struct Foo
{
    int n1;
    int n2;
};
 
union Union
{
    int n;
    double d;
};
 
int main()
{
    std::cout << std::boolalpha;
 
    char a[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';
 
    Foo f;
    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';
 
    Union u;
    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

The three-way comparison operator expressions have the form

The 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:

• 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 std::partial_ordering. 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, array-to-pointer conversions, pointer conversions and qualification conversions are applied to both operands to bring them to their composite pointer type.

For converted pointer operands p and q, p <=> q returns a prvalue of type std::strong_ordering:

• std::strong_ordering::equal if they compare equal,
• std::strong_ordering::less if q compares greater than 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.

In overload resolution against user-defined operators, for pointer or enumeration type T, the following function signature participates in overload resolution:

Where R is the ordering category type defined above.

#include <compare>
#include <iostream>
 
int main()
{
    double foo = -0.0;
    double bar = 0.0;
 
    auto res = foo <=> bar;
 
    if (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:

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).

#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 user-defined operator< is strict weak ordering. In particular, this is required by the standard algorithms and containers that work with Compare types: std::sort, std::max_element, std::map, etc.

The comparison result of pointers to different non-static data members of the same class implies that non-static data members in each of the three member access modes(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 strict total ordering 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 std::less 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 std::set or std::map.

For the types that are both EqualityComparable and LessThanComparable, 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 CWG issue 583 included in N3624:

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 default comparisons.

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

Comparison operators are overloaded for many classes in the standard library.

The namespace std::rel_ops provides generic operators !=, >, <=, and >=:

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

• Operator precedence
• Operator overloading
• Compare (named requirements)

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 || b

a == b
a != b
a < b
a > b
a <= b
a >= b
a <=> b

a[...]
*a
&a
a->b
a.b
a->*b
a.*b

a(...)

a, b

a ? b : c

static_cast converts one type to another related type
dynamic_cast converts within inheritance hierarchies
const_cast adds or removes cv-qualifiers
reinterpret_cast converts type to unrelated type
C-style cast converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast
new creates objects with dynamic storage duration
delete destructs objects previously created by the new expression and releases obtained memory area
sizeof queries the size of a type
sizeof... queries the size of a pack (since C++11)
typeid queries the type information of a type
noexcept checks if an expression can throw an exception (since C++11)
alignof queries alignment requirements of a type (since C++11)

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4