Why are there errors when creating thread from this function?

Question

I have a function:

void reader (std::istream *in, std::string& out)
{
    (*in) >> out;
}

I can easily call it with either:

reader(&std::cin, out);

or

std::function<void(std::istream*, std::string&)> r = reader;
r(&std::cin, out);

However, as soon as i attempt to create the thread from this function with either:

std::thread *reader = new std::thread(reader, &std::cin, out);

or

std::thread *reader = new std::thread(r, &std::cin, out);

I get some weird errors along the lines of:

/usr/include/c++/4.8/functional: In instantiation of ‘struct std::_Bind_simple<std::function<void(std::basic_istream<char>*, std::basic_string<char>&)>(std::basic_istream<char>*, std::basic_string<char>)>’: /usr/include/c++/4.8/thread:137:47:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with
_Callable = std::function<void(std::basic_istream<char>*, std::basic_string<char>&)>&; _Args = {std::basic_istream<char, std::char_traits<char> >*&, std::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]’ asyncinput.cpp:39:56:   required from here /usr/include/c++/4.8/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<std::function<void(std::basic_istream<char>*, std::basic_string<char>&)>(std::basic_istream<char>*, std::basic_string<char>)>’
       typedef typename result_of<_Callable(_Args...)>::type result_type;
                                                             ^ /usr/include/c++/4.8/functional:1727:9: error: no type named ‘type’ in ‘class std::result_of<std::function<void(std::basic_istream<char>*, std::basic_string<char>&)>(std::basic_istream<char>*, std::basic_string<char>)>’
         _M_invoke(_Index_tuple<_Indices...>)

I have attempted to turn the function into a lambda, but that didn't help. I don't know what i'm doing wrong.

Lightness Races in Orbit · Accepted Answer · 2014-12-20 15:59:17Z

4

The error message is incredibly hard to parse, but here's a testcase:

#include <sstream>
#include <string>
#include <thread>
#include <iostream>

void reader (std::istream* in, std::string& out)
{
    (*in) >> out;
}

int main()
{
    std::function<void(std::istream*, std::string&)> r = reader;
    std::istringstream ss("lol");
    std::string out;

    std::thread t(reader, &ss, out);
    t.join();
    std::cout << out << '\n';
}

(live demo)

And the problem is that you're passing out, but arguments to std::thread are copied and the copy (a temporary) cannot be bound to that std::string&.

You have to explicitly indicate that you wish to pass by reference:

std::thread t(reader, &ss, std::ref(out));
//                         ^^^^^^^^^   ^

Fixed testcase:

#include <sstream>
#include <string>
#include <thread>
#include <iostream>

void reader (std::istream* in, std::string& out)
{
    (*in) >> out;
}

int main()
{
    std::function<void(std::istream*, std::string&)> r = reader;
    std::istringstream ss("lol");
    std::string out;

    std::thread t(reader, &ss, std::ref(out));
    t.join();
    std::cout << out << '\n';
}

Output:

lol

(live demo)

(Note that I've dropped a ton of needless pointer usage from your example.)

Alternatively, drop the whole thing and use a lambda instead:

#include <sstream>
#include <string>
#include <thread>
#include <iostream>

int main()
{
    std::istringstream ss("lol");
    std::string out;

    std::thread t([&]() {
        ss >> out;
    });
    t.join();
    std::cout << out << '\n';
}

(live demo)

edited Dec 20, 2014 at 15:59

answered Dec 20, 2014 at 15:49

Lightness Races in Orbit

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5 Comments

v010dya Over a year ago

I guess i'll have to sit down and read up on std::ref and other stuff like that. Didn't know about it.

Puppy Over a year ago

@Volodya: What would be much superior would be to use a lambda instead.

v010dya Over a year ago

I just did turn it into lambda, it helps with readability, but you still need the std::ref bit.

Lightness Races in Orbit Over a year ago

@Volodya: BTW please try to construct testcases like those in my answer in the future!

Lightness Races in Orbit Over a year ago

@Volodya: You would capture out and not have to pass it explicitly at all. I've added an example to the answer. Whether this is an appropriate technique for you depends on the rest of your program.

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