# QPromise::tap

Since: 0.2.0

QPromise<T>::tap(Function handler) -> QPromise<T>

This handler allows to observe the value of the input promise, without changing the propagated value. The output promise will be resolved with the same value as the input promise (the handler returned value will be ignored). However, if handler throws, output is rejected with the new exception. Unlike finally, this handler is not called for rejections.

QPromise<int> input = {...}
auto output = input.tap([](int res) {
    log(res);
}).then([](int res) {
    // {...}
});

If handler returns a promise (or QFuture), the output promise is delayed until the returned promise is resolved and under the same conditions: the delayed value is ignored, the error transmitted to the output promise.