QPromise::QPromise

Since: 0.1.0

QPromise<T>::QPromise(Function resolver)

Creates a new promise that will be fulfilled or rejected by the given resolver lambda:

QPromise<int> promise([](const QPromiseResolve<int>& resolve, const QPromiseReject<int>& reject) {
    async_method([=](bool success, int result) {
        if (success) {
            resolve(result);
        } else {
            reject(customException{});
        }
    });
});

NOTE

QPromise<void> is specialized to not contain any value, meaning that the resolve callback takes no argument.

C++14 alternative:

QPromise<int> promise{[](const auto& resolve, const auto& reject) {
    // {...}
}};

Undefined rejection reason

Since: 0.5.0

While not recommended because it makes tracking errors more difficult, it's also possible to reject a promise without explicit reason, in which case, a built-in QPromiseUndefinedException is thrown:

QPromise<int> promise{[](const QPromiseResolve<int>& resolve, const QPromiseReject<int>& reject) {
    async_method([=](bool success, int result) {
        if (success) {
            resolve(result);
        } else {
            reject();
        }
    });
}};

// The exception can be caught explicitly
promise.fail([](const QPromiseUndefinedException&) {
    // { ... }
})

// ... or implicitly (since undefined)
promise.fail([]() {
    // { ... }
})