STEP 1

• Add a free operator<<, reusing str(), and simplify main() first lines.
• Replace equal() with operator==(), and upgrade tests.
• Add operator!=(), reusing operator==(), and upgrade tests.
• Replace compare() with operator<=>(), reusing <=> between doubles, and upgrade tests.
• Replace multiply() with operator*(), and upgrade tests.

STEP 2

• Replace TestResultPrinter::process() with operator()(), and upgrade CHECK().

OPTIONAL STEP 3

• Add an inplace multiplication operator*=(), and add tests.
• Review operator*() so to reuse operator*=().
• Ensure calls to operator*=() can be chained, the same as operator<<().

• Do not confuse equality and equivalence.
• We can very often implement an arithmetic operator@ in terms of operator@=.
• When implementing <=>, you get <, >, <=, >= for free.
• Object-functions are very used with standard algorithms, yet tend to be often replaced by lambdas in modern C++.