How do I return a class object from a thread and create a vector of returned objects?

问题

I want to use C++11 std::thread. I am writing a code to parse JSON files and the parsing is not dependent on each other. I have 900 files and to increase the speed of execution I want to create as many threads and make the execution parallel.

My problem is at the moment the function I want to pass to the std::thread is a function of a class and it returns the parsed object as a JSON object from which I create a std::vector.

I googled 'returning values from threads' and I am getting the tutorials of using async, is there no other way other than async? I went through the async tutorials I am not able to understand how it's launching new threads.

Following is my code:

for (auto it = jsonFilesList.begin(); it != jsonFilesList.end(); it++) {
        std::ifstream jsonFile;
        jsonFile.open(*it);
        jf = json::parse(jsonFile);
        jsonFile.close();
        jsonObjects.push_back(jf);
    }

I want to pass the function parse(jsonFile) to my threads numbering the count of total JSON files, and return the json object, which I will use to create a std::vector of jsonObjects.

How can I do this?

回答1:

Using std::async seems like a good approach, since it return a std::future from which you can get the returned value.

Here is a short example that you could use as an idea to get going.

#include <iostream>
#include <future>
#include <vector>

int doWork(int i) {
    return i*2;
}

int main()
{
    std::vector<int> results;
    std::vector<std::future<int>> work;

    for (int i = 0; i < 10; ++i) {
        work.emplace_back(std::async(std::launch::async, &doWork, i));
    }

    // do some other stuff, or nothing
    // before getting the results from all the futures

    for (auto& future : work) {
        results.emplace_back(future.get());
    }

    for(auto i : results) {
        std::cout << i << '\n';
    }
    
    return 0;
}

std::future also has member functions like valid, wait, wait_for and wait_until if you want more flexibility in how you retrieve the value.

来源：https://stackoverflow.com/questions/65828746/how-do-i-return-a-class-object-from-a-thread-and-create-a-vector-of-returned-obj

标签

c++

multithreading

c++11

return-value