Option/Sum Types for Qt Projects

A set of simple(ish) C++ templates which implement sum and option types. They serve a similar purpose to boost::variant and boost::optional but are implemented on top of Qt's QVariant container.

Update (2017-09-23): A variant class has been added to the C++ standard library for C++17 (std::variant) and you can find polyfills for compilers that don't ship with it yet. Consider using that instead of this code in your project.

Templates

Either

Either<T1,T2> represents either a value of type T1 or a value of type T2. You can think of it as a QVariant which is restricted to being either a T1 or T2 rather than any type which QVariant supports.

Either<T1,T2> instances can be constructed directly from an instance of T1 or T2 or by using the some(T) function which returns a type that can be implicitly cast to any Either type that has T as one of its types.

Simple example:

// parse a value which might either be a
// string or an integer

Either<QString,int> value = parseValue(data);
if (value.is1st()) {
	qDebug() << "Read string " << value.as1st();
} else {
	qDebug() << "Read integer " << value.as2nd();
}

Maybe

Maybe<T> represents either a value of type T or no value. You can think of it as a QVariant which is restricted to either being a T or a null type.

Maybe<T> instances are constructed using the just(T) or nothing() functions.

just(T) handles pointers specially. If passed a null pointer it returns a Maybe representing no value. This is useful when working with existing code which uses a null pointer to represent no value.

Simple example:

// try to read the size of a file, which may fail if
// the file is not readable

Maybe<int> size = fileSize(path);
if (size) {
	std::cout << "file size is " << size.value() << " bytes" << std::endl;
} else {
	std::cout << "could not read file size" << std::endl;
}

Examples

See TestMaybe.cpp for unit tests which show example usage of the Maybe and Either templates.

Performance

The templates use QVariant as the underlying storage and the same performance considerations apply. A QVariant in Qt 4 is 12 bytes in size on most platforms - 8 bytes for the data itself or a pointer to the data, plus 4 bytes for the type tag and flags. Pointers and primitive types of 8 bytes in size or less are stored within the QVariant. Larger types are stored in heap-allocated storage.

Exceptions

Unlike boost::variant, no attempt is made to handle exceptions that may be thrown when constructing or copying types. The state of a Maybe or Either after a failed copy constructor call is undefined.