error: no type named ‘type’ in ‘class std::result_of<std::launch(bool (*)(diffpng::CompareArgs&), diffpng::CompareArgs)>’

Question

my code is like:

    std::vector<bool> LevelClimberCompareMulti(std::vector<CompareArgs> &args) {
    std::vector<bool> ret(args.size());
    std::vector<std::shared_future<bool>> fts(args.size());
    //using ArgIter = std::vector<CompareArgs>::iterator;
    //using FtIter = std::vector<std::shared_future<bool>>::iterator;
    std::transform(args.begin(), args.end(), fts.begin(),
            [](CompareArgs& a) -> std::shared_future<bool> {return std::async(std::launch::async, &diffpng::LevelClimberCompare, a).share();});
    std::transform(fts.begin(), fts.end(), ret.begin(),
            [](std::shared_future<bool> ft) -> bool {return ft.get();});
    return ret;
}

example:

#include <assert.h>
#include <cstdlib>
#include <cassert>
#include <cmath>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <algorithm>
#include <chrono>
#include <iostream>
#include <future>
#include <mutex>

using CompareArgs = int;

namespace diffpng {
    bool LevelClimberCompare(CompareArgs &args) {
        return true;
    }

    std::vector<bool> LevelClimberCompareMulti(std::vector<CompareArgs> &args) {
        std::vector<bool> ret(args.size());
        std::vector<std::shared_future<bool>> fts(args.size());
        //using ArgIter = std::vector<CompareArgs>::iterator;
        //using FtIter = std::vector<std::shared_future<bool>>::iterator;
        std::transform(args.begin(), args.end(), fts.begin(),
                [](CompareArgs& a) -> std::shared_future<bool> {return std::async(std::launch::async, &diffpng::LevelClimberCompare, a).share();});
        std::transform(fts.begin(), fts.end(), ret.begin(),
                [](std::shared_future<bool> ft) -> bool {return ft.get();});
        return ret;
    }
};

int main(int argc, char** argv) {
    std::vector<CompareArgs> args{1, 2, 3, 4, 5};
    auto res = diffpng::LevelClimberCompareMulti(args);
    for (int i = 0; i < res.size(); ++i) {
        std::cout << "res[" << i << "]: " << res[i] << std::endl;
    }
    return 0;
}

and I get errors like that: I wonder what the error is about and how can I fix it up? btw, I am currently run it on a Win10 WSL ubuntu shell, but I dont think that has something to do with the error here. gcc version:

gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

compile it simply with the cmd: g++ -std=c++14 example.cpp you can see the error.

Answer 1

The arguments in std::async are passed as temporary copies (rvalues). You can't assign an rvalue to a reference. To fix your problem you could use const ref or copy

bool LevelClimberCompare(const CompareArgs &args)

or

bool LevelClimberCompare(CompareArgs args)

You can use std::ref or std::cref to pass a reference

std::async(std::launch::async, &diffpng::LevelClimberCompare, std::ref(a))