Class BenchmarkFunction

test/expected/catch.hpp:6645–6706

We need to reinvent std::function because every piece of code that might add overhead in a measurement context needs to have consistent performance characteristics so that we can account for it in the measurement. Implementations of std::function with optimizations that aren't always applicable, like small buffer optimizations, are not uncommon. This is effectively an implementation of std::functi

            /// This is effectively an implementation of std::function without any such optimizations;
            /// it may be slow, but it is consistently slow.
            struct BenchmarkFunction {
            private:
                struct callable {
                    virtual void call(Chronometer meter) const = 0;
                    virtual callable* clone() const = 0;
                    virtual ~callable() = default;
                };
                template <typename Fun>
                struct model : public callable {
                    model(Fun&& fun) : fun(std::move(fun)) {}
                    model(Fun const& fun) : fun(fun) {}

                    model<Fun>* clone() const override { return new model<Fun>(*this); }

                    void call(Chronometer meter) const override {
                        call(meter, is_callable<Fun(Chronometer)>());
                    }
                    void call(Chronometer meter, std::true_type) const {
                        fun(meter);
                    }
                    void call(Chronometer meter, std::false_type) const {
                        meter.measure(fun);
                    }

                    Fun fun;
                };

                struct do_nothing { void operator()() const {} };

                template <typename T>
                BenchmarkFunction(model<T>* c) : f(c) {}

            public:
                BenchmarkFunction()
                    : f(new model<do_nothing>{ {} }) {}

                template <typename Fun,
                    typename std::enable_if<!is_related<Fun, BenchmarkFunction>::value, int>::type = 0>
                    BenchmarkFunction(Fun&& fun)
                    : f(new model<typename std::decay<Fun>::type>(std::forward<Fun>(fun))) {}

                BenchmarkFunction(BenchmarkFunction&& that)
                    : f(std::move(that.f)) {}

                BenchmarkFunction(BenchmarkFunction const& that)
                    : f(that.f->clone()) {}

                BenchmarkFunction& operator=(BenchmarkFunction&& that) {
                    f = std::move(that.f);
                    return *this;
                }

                BenchmarkFunction& operator=(BenchmarkFunction const& that) {
                    f.reset(that.f->clone());
                    return *this;
                }

                void operator()(Chronometer meter) const { f->call(meter); }