std::move

seastar::future<int> slow_do_something(std::unique_ptr<T> obj) {
    using namespace std::chrono_literals;
    return seastar::sleep(10ms).then([obj = std::move(obj)] () mutable {
        return do_something(std::move(obj));
    });
}

• mutable :

  • The [obj = ...] capture syntax we used here is new to C++14. This is the main reason why Seastar requires C++14, and does not support older C++11 compilers.

    The extra () mutable syntax was needed here because by default when C++ captures a value (in this case, the value of std::move(obj)) into a lambda, it makes this value read-only, so our lambda cannot, in this example, move it again. Adding mutable removes this artificial restriction.

异常和fail

#include "core/future.hh"
#include <iostream>
#include <exception>

class my_exception : public std::exception {
    virtual const char* what() const noexcept override { return "my exception"; }
};

seastar::future<> fail() {
    return seastar::make_exception_future<>(my_exception());
}

seastar::future<> f() {
    return fail().finally([] {
        std::cout << "cleaning up\n";
    });
}

抛出异常会导致整个 f() 执行失败，不建议抛出异常，应该返回fail future：

void inner() {
    throw my_exception();
}
seastar::future<> fail() {
    try {
        inner();
    } catch(...) {
        return seastar::make_exception_future(std::current_exception());
    }
    return seastar::make_ready_future<>();
}

或者使用futurize_apply：

seastar::future<> fail() {
    throw my_exception();
}
seastar::future<> f() {
    return seastar::futurize_apply(fail).finally([] {
        std::cout << "cleaning up\n";
    });
}