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| Category: utilities | | Component type: function |
Prototype
template <class T> bool operator!=(const T& x, const T& y);
template <class T> bool operator>(const T& x, const T& y);
template <class T> bool operator<=(const T& x, const T& y);
template <class T> bool operator>=(const T& x, const T& y);
Description
The EqualityComparable requirements specify that it must be possible to compare objects using operator!= as well as operator==; similarly, the LessThanComparable requirements include operator>, operator<= and operator>= as well as operator<. Logically, however, most of these operators are redundant: all of them can be defined in terms of operator== and operator<.
These four templates use operator== and operator< to define the other four relational operators. They exist purely for the sake of convenience: they make it possible to write algorithms in terms of the operators !=, >, <=, and >=, without requiring that those operators be explicitly defined for every type.
As specified in the EqualityComparable requirements, x != y is equivalent to !(x == y). As specified in the LessThanComparable requirements, x > y is equivalent to y < x, x >= y is equivalent to !(x < y), and x <= y is equivalent to !(y < x).
Definition
Defined in the standard header utility, and in the nonstandard backward-compatibility header function.h.
Requirements on types
The requirement for operator!= is that x == y is a valid expression for objects x and y of type T.
The requirement for operator> is that y < x is a valid expression for objects x and y of type T.
The requirement for operator<= is that y < x is a valid expression for objects x and y of type T.
The requirement for operator>= is that x < y is a valid expression for objects x and y of type T.
Preconditions
The precondition for operator!= is that x and y are in the domain of operator==.
The precondition for operator>, operator<=, and operator>= is that x and y are in the domain of operator<.
Complexity
Example
template <class T> void relations(T x, T y)
{
if (x == y) assert(!(x != y));
else assert(x != y);
if (x < y) {
assert(x <= y);
assert(y > x);
assert(y >= x);
}
else if (y < x) {
assert(y <= x);
assert(x < y);
assert(x <= y);
}
else {
assert(x <= y);
assert(x >= y);
}
}
Notes
See also
EqualityComparable, LessThanComparable
