How to Define the `

Defining Operator '<' for Strict Weak Ordering on Tuples

For an n-tuple to satisfy strict weak ordering, it must fulfill the following criteria:

• Equivalence: Two tuples x and y are equivalent if neither x < y nor y < x holds true.
• Asymmetry: If x < y, then y < x cannot be true.
• Transitivity: If x < y and y < z, then x < z.

While boost library offers a pre-defined tuple class with operator '<' adhering to strict weak ordering, you may wish to define your own operator for a specific context.

Here's how you can implement operator '<' manually:

struct S
{
     ThingA   a;
     ThingB   b;
};

bool operator<(S const& lhs, S const& rhs)
{
    return std::make_tuple(lhs.a, lhs.b) < std::make_tuple(rhs.a, rhs.b);
}

In this example, we leverage the std::make_tuple function to create a tuple from the individual elements of S without copying them. Then, we compare the tuples using the generic tuple comparison operator.

For operator '==', you can follow a similar approach:

bool operator==(S const& lhs, S const& rhs)
{
    return std::make_tuple(lhs.a, lhs.b) == std::make_tuple(rhs.a, rhs.b);
}

Remember that these implementations assume that ThingA and ThingB have their own operator '<' and operator '==' defined for strict weak ordering.

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