transacted<T> - Stores a copy of a type and writes the result back to the original unless an exception is thrown.

main.cpp

#include "transacted.h"
#include <iostream>
#include <stdexcept>
using namespace std;

int x = 1;
long y = 2;
float z = 3;

void doSomething()
{
	TRANSACTED(x, y, z);
	++x; ++y; ++z;
}


void doSomethingButThrow()
{
	TRANSACTED(x, y, z);
	++x; ++y; ++z;
	throw runtime_error("Transaction not applied!");
}


int main()
{
	cout << x << y << z << "\n";

	doSomething();

	cout << x << y << z << "\n";
	try {
		doSomethingButThrow();
	} catch (const exception& ex) {
		cout << ex.what() << "\n";
	}

	cout << x << y << z << "\n";
}

transacted.h

#pragma once

#include <exception>

template <typename T>
class transacted {
public:
	explicit transacted(T& t)
		: copy_(t), original_(t), uncaughtExceptions_(std::uncaught_exceptions())
	{}

	~transacted()
	{
		if (uncaughtExceptions_ == std::uncaught_exceptions())
			commit();
	}

	void commit() const
	{
		original_ = copy_;
	}

	operator T&() { return copy_; }
	operator const T&() const { return copy_; }

	T& get() { return copy_; }
	const T& get() const { return copy_; }


private:
	T copy_;
	T& original_;
	int uncaughtExceptions_;
};

#define TRANSACTED1(VAR1) auto& transacted_ref_ ## VAR1 = VAR1; transacted<decltype(VAR1)> VAR1{transacted_ref_ ## VAR1};
#define TRANSACTED2(VAR1, VAR2) TRANSACTED1(VAR1) TRANSACTED1(VAR2)
#define TRANSACTED3(VAR1, VAR2, VAR3) TRANSACTED2(VAR1, VAR2) TRANSACTED1(VAR3)
#define TRANSACTED4(VAR1, VAR2, VAR3, VAR4) TRANSACTED2(VAR1, VAR2) TRANSACTED2(VAR3, VAR4)
#define TRANSACTED5(VAR1, VAR2, VAR3, VAR4, VAR5) TRANSACTED4(VAR1, VAR2, VAR3, VAR4) TRANSACTED1(VAR5)

#define EXPAND(x) x
#define GET_NTH_ARG(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
#define VA_NUM_ARGS(...) EXPAND(GET_NTH_ARG(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1))

#define CONCATENATE(_1, _2) CONCATENATE_IMPL(_1, _2)
#define CONCATENATE_IMPL(_1, _2) _1##_2

#define TRANSACTEDN(N, ...) CONCATENATE(TRANSACTED, N)(__VA_ARGS__)
#define TRANSACTED(...) TRANSACTEDN(VA_NUM_ARGS(__VA_ARGS__), __VA_ARGS__)