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C++ promise/A+ library in Javascript style.

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What is promise-cpp ?

Promise-cpp is library that implements promise/A+ standard, which can be the base component in event-looped asynchronized programming. It is NOT std::promise.

Features

Similar to Javascript Promise API.

Type safety: the resolved/rejected arguments can be captured by the "then" function with same arguments type.

Exceptions supports: cpp exception will be received by the "on_rejected" function.

Optional header-only configuration enabled with the PROMISE_HEADONLY macro

Easy to use, just #include "promise-cpp/promise.hpp" is enough, code based on standard c++11 syntax, no external dependencies required.

Easy to integrate with other libararies (see examples of asio, qt and mfc).

Useful extended functions on promise object: doWhile, raceAndResolve, raceAndReject

Examples

Examples list

Please use cmake to build from CMakeLists.txt.

Compiler required

The library has passed test on these compilers --

Usage

Used as header only library

To use as header only library, just define macro PROMISE_HEADONLY when compiling.

Used as static library

cmake /path/to/promise_source

Used as shared library

cmake -DPROMISE_BUILD_SHARED=ON /path/to/promise_source

Build tips about asio examples

Some of the examples use boost::asio as io service, and use boost::beast as http service. You need to boost_1_66 or higher to build these examples.

For examples, you can build with boost library --

> cmake -DBOOST_ROOT=/path/to/boost_source /path/to/promise_source

Sample code 1

Example of asio http client. (full code here)

int main(int argc, char** argv) {
    // The io_context is required for all I/O
    asio::io_context ioc;

    // Launch the asynchronous operation
    download(ioc, "http://www.163.com/")
    .then([&]() {
        return download(ioc, "http://baidu.com/");
    }).then([&]() {
        return download(ioc, "http://qq.com");
    }).then([&]() {
        return download(ioc, "http://github.com/xhawk18");
    });

    // Run the I/O service. The call will return when
    // the get operation is complete.
    ioc.run();

    return 0;
}

Sample code 2

This sample code shows converting a timer callback to promise object.

#include <stdio.h>
#include <boost/asio.hpp>
#include "add_ons/asio/timer.hpp"

using namespace promise;
using namespace boost::asio;

/* Convert callback to a promise */
Promise myDelay(boost::asio::io_service &io, uint64_t time_ms) {
    return newPromise([&io, time_ms](Defer &d) {
        setTimeout(io, [d](bool cancelled) {
            if (cancelled)
                d.reject();
            else
                d.resolve();
        }, time_ms);
    });
}


Promise testTimer(io_service &io) {

    return myDelay(io, 3000).then([&] {
        printf("timer after 3000 ms!\n");
        return myDelay(io, 1000);
    }).then([&] {
        printf("timer after 1000 ms!\n");
        return myDelay(io, 2000);
    }).then([] {
        printf("timer after 2000 ms!\n");
    }).fail([] {
        printf("timer cancelled!\n");
    });
}

int main() {
    io_service io;

    Promise timer = testTimer(io);

    delay(io, 4500).then([=] {
        printf("clearTimeout\n");
        clearTimeout(timer);
    });

    io.run();
    return 0;
}

Global functions

Promise newPromise(FUNC func);

Creates a new promise object with a user-defined function. The user-defined functions, used as parameters by newPromise, must have a parameter Defer d. for example --

return newPromise([](Defer d){
})

Promise resolve(const RET_ARG... &ret_arg);

Returns a promise that is resolved with the given value. for example --

return resolve(3, '2');

Promise reject(const RET_ARG... &ret_arg);

Returns a promise that is rejected with the given arguments. for example --

return reject("some_error");

Promise all(const PROMISE_LIST &promise_list);

Wait until all promise objects in "promise_list" are resolved or one of which is rejected. The "promise_list" can be any container that has promise object as element type.

for (Promise &promise : promise_list) { ... }

for example --

Promise d0 = newPromise([](Defer d){ /* ... */ });
Promise d1 = newPromise([](Defer d){ /* ... */ });
std::vector<Promise> promise_list = { d0, d1 };

all(promise_list).then([](){
    /* code here for all promise objects are resolved */
}).fail([](){
    /* code here for one of the promise objects is rejected */
});

Promise race(const PROMISE_LIST &promise_list);

Returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves or rejects, with the value or reason from that promise. The "promise_list" can be any container that has promise object as element type.

for (Promise &promise : promise_list) { ... }

for example --

Promise d0 = newPromise([](Defer d){ /* ... */ });
Promise d1 = newPromise([](Defer d){ /* ... */ });
std::vector<Promise> promise_list = { d0, d1 };

race(promise_list).then([](){
    /* code here for one of the promise objects is resolved */
}).fail([](){
    /* code here for one of the promise objects is rejected */
});

Promise raceAndReject(const PROMISE_LIST &promise_list);

Same as function race(), and reject all depending promises object in the list.

Promise raceAndResolve(const PROMISE_LIST &promise_list);

Same as function race(), and resove all depending promises object in the list.

Promise doWhile(FUNC func);

"While loop" for promisied task. A promise object will passed as parameter when call func, which can be resolved to continue with the "while loop", or be rejected to break from the "while loop".

for example --

doWhile([](DeferLoop d){
    // Add code here for your task in "while loop"
    
    // Call "d.doContinue()" to continue with the "while loop",
    
    // or call "d.doBreak()" to break from the "while loop", in this case,
    // the returned promise object will be in resolved status.
});

Class Promise - type of promise object

Promise::then(FUNC_ON_RESOLVED on_resolved, FUNC_ON_REJECTED on_rejected)

Return the chaining promise object, where on_resolved is the function to be called when previous promise object was resolved, on_rejected is the function to be called when previous promise object was rejected. for example --

return newPromise([](Defer d){
    d.resolve(9567, 'A');
}).then(

    /* function on_resolved */ [](int n, char ch){
        printf("%d %c\n", n, ch);   //will print 9567 here
    },

    /* function on_rejected */ [](){
        printf("promise rejected\n"); //will not run to here in this code 
    }
);

Promise::then(FUNC_ON_RESOLVED on_resolved)

Return the chaining promise object, where on_resolved is the function to be called when previous promise object was resolved. for example --

return newPromise([](Defer d){
    d.resolve(9567);
}).then([](int n){
    printf("%d\n", n);  b //will print 9567 here
});

Promise::then(Defer d)

Return the chaining promise object, where d is the callback function be called when previous promise object was resolved or rejected.

Promise::then(DeferLoop d)

Return the chaining promise object, where d is the callback function be called when previous promise object was resolved or rejected.

Promise::then(Promise promise)

Return the chaining promise object, where "promise" is the promise object be called when previous promise object was resolved or rejected.

Promise::fail(FUNC_ON_REJECTED on_rejected)

Return the chaining promise object, where on_rejected is the function to be called when previous promise object was rejected.

This function is usually named "catch" in most implements of Promise library. https://www.promisejs.org/api/

In promise_cpp, function name "fail" is used instead of "catch", since "catch" is a keyword of c++.

for example --

return newPromise([](Defer d){
    d.reject(-1, std::string("oh, no!"));
}).fail([](int err, string &str){
    printf("%d, %s\n", err, str.c_str());   //will print "-1, oh, no!" here
});

Promise::finally(FUNC_ON_FINALLY on_finally)

Return the chaining promise object, where on_finally is the function to be called whenever the previous promise object was resolved or rejected.

The returned promise object will keeps the resolved/rejected state of current promise object.

for example --

return newPromise([](Defer d){
    d.reject(std::string("oh, no!"));
}).finally([](){
    printf("in finally\n");   //will print "in finally" here
});

Promise::always(FUNC_ON_ALWAYS on_always)

Return the chaining promise object, where on_always is the function to be called whenever the previous promise object was resolved or rejected.

The returned promise object will be in resolved state whenever current promise object is resolved or rejected.

for example --

return newPromise([](Defer d){
    d.reject(std::string("oh, no!"));
}).always([](){
    printf("in always\n");   //will print "in always" here
});

Class Defer - type of callback object for promise object.

Defer::resolve(const RET_ARG... &ret_arg);

Resolve the promise object with arguments, where you can put any number of ret_arg with any type. (Please be noted that it is a method of Defer object, which is different from the global resolve function.) for example --

return newPromise([](Defer d){
    //d.resolve();
    //d.resolve(3, '2', std::string("abcd"));
    d.resolve(9567);
})

Defer::reject(const RET_ARG... &ret_arg);

Reject the promise object with arguments, where you can put any number of ret_arg with any type. (Please be noted that it is a method of Defer object, which is different from the global reject function.) for example --

return newPromise([](Defer d){
    //d.reject();
    //d.reject(std::string("oh, no!"));
    d.reject(-1, std::string("oh, no!"))
})

Class DeferLoop - type of callback object for doWhile.

DeferLoop::doContinue();

Continue the doWhile loop.

for example --

static int *i = new int(0);
doWhile([i](DeferLoop d) {
    if(*i < 10) {
        ++ (*i);
        d.doContinue();
    }
    else {
        d.doBreak(*i);
    }

}).then([](int result) {
    printf("result = %d\n", result);
}).finally([i]() {
    delete i;
})

DeferLoop::doBreak(const RET_ARG... &ret_arg);

Break the doWhile loop (ret_arg will be transferred).

(please see the example above)

And more ...

About exceptions

To throw any object in the callback functions above, including on_resolved, on_rejected, on_always, will same as d.reject(the_throwed_object) and returns immediately. for example --

return newPromise([](Defer d){
    throw std::string("oh, no!");
}).fail([](string &str){
    printf("%s\n", str.c_str());   //will print "oh, no!" here
});

For the better performance, we suggest to use function reject instead of throw.

About the chaining parameter

Any type of parameter can be used when call resolve, reject or throw, except that the plain string or array. To use plain string or array as chaining parameters, we may wrap it into an object.

newPromise([](Defer d){
    // d.resolve("ok"); may cause a compiling error, use the following code instead.
    d.resolve(std::string("ok"));
})

Match rule for chaining parameters

"then" and "fail" function can accept multiple promise parameters and they follows the below rule --

Resolved parameters

Resolved parameters must match the next "then" function, otherwise it will throw an exception and can be caught by the following "fail" function.

Rejected parameters

First let's take a look at the rule of c++ try/catch, in which the thrown value will be caught in the block where value type is matched. If type in the catch block can not be matched, it will run into the default block catch(...) { }.

try{
    throw (short)1;
}catch(int a){
    // will not go to here
}catch(short b){
    // (short)1 will be caught here
}catch(...){
    // will not go to here
}

"Promise-cpp" implement "fail" chain as the match style of try/catch.

newPromise([](Defer d){
    d.reject(3, 5, 6);
}).fail([](std::string str){
    // will not go to here since parameter types are not match
}).fail([](const int &a, int b, int c) {
    // d.reject(3, 5, 6) will be caught here
}).fail([](){
    // Will not go to here sinace previous rejected promise was caught.
});

Omit parameters

The number of parameters in "then" or "fail" chain can be lesser than that's in resolve function.

newPromise([](Defer d){
    d.resolve(3, 5, 6);
}).then([](int a, int b) {
    // d.resolve(3, 5, 6) will be caught here since a, b matched with the resolved parameters and ignore the 3rd parameter.
});

A function in "then" chain without any parameters can be used as default promise caught function.

newPromise([](Defer d){
    d.resolve(3, 5, 6);
}).then([]() {
    // Function without parameters will be matched with any resolved values,
    // so d.resolve(3, 5, 6) will be caught here.
});

The reject parameters follows the the same omit rule as resolved parameters.

Copy the promise object

To copy the promise object is allowed and effective, please do that when you need.

Promise promise = newPromise([](Defer d){});
Promise promise2 = promise;  //It's safe and effective

Life time of the internal storage inside a promise chain

The library uses std::shared_ptr to maintain the internal object of task and task chain. Resources of a task will be released after the task is finished (in resolved or rejected status) and not obtained by Defer or DeferLoop objects. Resources of a promise chain will be released when it is not obtained by any Promise, Defer or DeferLoop objects.

lifetime

Handle uncaught exceptional or rejected parameters

The uncaught exceptional or rejected parameters are ignored by default. We can specify a handler function to do with these parameters --

handleUncaughtException([](Promise &d) {
    d.fail([](int n, int m) {
        //go here if the uncaught parameters match types "int n, int m".
    }).fail([](char c) {
        //go here if the uncaught parameters match type "char c".
    }).fail([]() {
        //go here for all other uncaught parameters.
    });
});

about multithread

This library is thread safe by default. However, it is strongly recommented to use this library on single thread, especially when you don't clearly know about which thread will runs the chain tasks.

For better performance, we can also disable multithread by adding macro PROMISE_MULTITHREAD=0