Can not pass 1D array in std::thread

Question

This is for my actual project, but I made a minimal example (code is based on an example of Lightness Races in Orbit).

#include <thread>
#include <iostream>

class Foo
{
    Foo(int n = 10)
    {
        size_t a[n];
        constexpr int p = 5;
        std::thread threads[p];
        for (int i = 0; i < p; ++i)
            threads[i] = std::thread(std::bind(&Foo::bar, this, a, n));

        for (auto& th : threads) th.join();
    }

    void bar(size_t* a, int n) {}
};

int main() {std::cout << "ok\n";}

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The error comes by the fact that I am using an array that has n as its size. However, it will be very difficult for me -in the real project- to change that, since many lines of code are based on that.

Hopefully, you learned your lesson about using variable-length arrays in C++... — Brian Bi
– Brian Bi, Commented Jan 14, 2015 at 21:06
How about doing explicit dynamic allocation with the new operator instead of using the variable-length array... those things always have acted funny for me. — Rubix Rechvin
– Rubix Rechvin, Commented Jan 14, 2015 at 21:07

sehe · Accepted Answer · 2015-01-14 21:09:50Z

7

Use a vector

Or solve the problem by decaying the array into a pointer before type deduction takes place:

std::bind(&Foo::bar, this, +a, n)

The problem is, bind is deducing an array reference and then tries to copy it /by value/. Array copy is not specified by the language.

answered Jan 14, 2015 at 21:09

sehe

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7 Comments

juanchopanza Over a year ago

Nice trick! BTW "use a vector" would have different semantics, because the thread would have its own copy. Worth pointing that out.

gsamaras Over a year ago

Thank you sehe, I am trying to make the concepts you and another user introduced to me in another post if you remember. :) However, what about &a[0]? It works too. Which one should I prefer? I also do not understand what we did in your answer, +a is something new to me.

juanchopanza Over a year ago

@G.Samaras The unary operator + applied to the array makes it decay to a pointer. See this example. It is a nice trick, but I would probably do something more explicit (such as &a[0]).

Praetorian Over a year ago

@G.Samaras Applying unary + causes the array to decay into a pointer to the first element. If I had to use a C-style array, I'd use &a[0] because the former would confuse a lot of people. Also, I don't see why you need bind at all to solve your problem, passing those argument to the thread constructor should be sufficient.

gsamaras Over a year ago

You are right Praetorian, bind is redundant. Thanks, I will use &a[0] for readability. Thanks for the example juanchopanza, really straightforward.

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