strict weak ordering
This is a mathematical term to define a relationship between two objects.
Its definition is:
Two objects x and y are equivalent if both f(x, y) and f(y, x) are false. Note that an object is always (by the irreflexivity invariant) equivalent to itself.
In terms of C++ this means if you have two objects of a given type, you should return the following values when compared with the operator <.
X a;
X b;
Condition: Test: Result
a is equivalent to b: a < b false
a is equivalent to b b < a false
a is less than b a < b true
a is less than b b < a false
b is less than a a < b false
b is less than a b < a true
How you define equivalent/less is totally dependent on the type of your object.
Formal Definition:
Strict Weak ordering
Computer Science:
Strict Weak Ordering
How it relates to operators:
Comparator
As a side note we can implement strict weak ordering manually. But we can do it simply using the std::tuple which has implemented it for you. You simply need to create a tuple without copying the objects.
struct S
{
ThingA a;
ThingB b;
};
bool operator<(S const& lhs, S const& rhs)
{
return std::tie(lhs.a, lhs.b) < std::tie(rhs.a, rhs.b);
}
Note: This assumes that thingA and thingB already implement strict weak ordering themselves.
We can also implement equality the same way:
bool operator==(S const& lhs, S const& rhs)
{
return std::tie(lhs.a, lhs.b) == std::tie(rhs.a, rhs.b);
}
Note again: This assumes that thingA and thingB already implement equality.