promise.hpp
C++ asynchronous promises like a Promises/A+
Requirements
Installation
promise.hpp is a header-only library. All you need to do is copy the headers files from headers directory into your project and include them:
#include "promise.hpp/promise.hpp"
Also, you can add the root repository directory to your cmake project:
add_subdirectory(external/promise.hpp)
target_link_libraries(your_project_target PUBLIC promise.hpp::promise.hpp)
Examples
Creating a promise
#include "promise.hpp/promise.hpp"
using promise_hpp;
promise<std::string> download(const std::string& url)
{
promise<std::string> result;
web_client.download_with_callbacks(
[result](const std::string& html) mutable
{
result.resolve(html);
},
[result](int error_code) mutable
{
result.reject(std::runtime_error("error code: " + std::to_string(error_code)));
});
return result;
}
Alternative way to create a promise
promise<std::string> p = make_promise<std::string>(
[](auto&& resolver, auto&& rejector)
{
web_client.download_with_callbacks(
[resolver](const std::string& html) mutable
{
resolver(html);
},
[rejector](int error_code) mutable
{
rejector(std::runtime_error("error code: " + std::to_string(error_code)));
});
});
Waiting for an async operation
download("http://www.google.com")
.then([](const std::string& html)
{
std::cout << html << std::endl;
})
.except([](std::exception_ptr e)
{
try {
std::rethrow_exception(e);
} catch (const std::exception& ee) {
std::cerr << ee.what() << std::endl;
}
});
Chaining async operations
download("http://www.google.com")
.then([](const std::string& html)
{
return download(extract_first_link(html));
})
.then([](const std::string& first_link_html)
{
std::cout << first_link_html << std::endl;
})
.except([](std::exception_ptr e)
{
try {
std::rethrow_exception(e);
} catch (const std::exception& ee) {
std::cerr << ee.what() << std::endl;
}
});
Transforming promise results
download("http://www.google.com")
.then([](const std::string& html)
{
return extract_all_links(html);
})
.then([](const std::vector<std::string>& links)
{
for ( const std::string& link : links ) {
std::cout << link << std::endl;
}
});
Combining multiple async operations
std::vector<std::string> urls{
"http://www.google.com",
"http://www.yandex.ru"};
std::vector<promise<std::string>> url_promises;
std::transform(urls.begin(), urls.end(), url_promises.begin(), download);
make_all_promise(url_promises)
.then([](const std::vector<std::string>& pages)
{
std::vector<std::string> all_links;
for ( const std::string& page : pages ) {
std::vector<std::string> page_links = extract_all_links(page);
all_links.insert(
all_links.end(),
std::make_move_iterator(page_links.begin()),
std::make_move_iterator(page_links.end()));
}
return all_links;
})
.then([](const std::vector<std::string>& all_links)
{
for ( const std::string& link : all_links ) {
std::cout << link << std::endl;
}
});
Chaining multiple async operations
download("http://www.google.com")
.then_all([](const std::string& html)
{
std::vector<promise<std::string>> url_promises;
std::vector<std::string> links = extract_all_links(html);
std::transform(links.begin(), links.end(), url_promises.begin(), download);
return url_promises;
})
.then([](const std::vector<std::string>& all_link_page_htmls)
{
// ...
})
.except([](std::exception_ptr e)
{
// ...
});