C++ thread with a recursive function

Question

void merge_sort_thread(vector<int>& array) {
if (1 < array.size()) {
    vector<int> array1(array.begin(), array.begin() + array.size() / 2);
    thread first= thread(merge_sort_thread,array1);

    vector<int> array2(array.begin() + array.size() / 2, array.end());
    thread second = thread(merge_sort_thread,array2);
    first.join(); //wait for both ended
    second.join();
    merge (array1.begin(),array1.end(),array2.begin(),array2.end(),array.begin());
}

I am using Xcode to build and run, and it is a build failure. With prompt:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/thread:332:5: Attempt to use a deleted function

I know threading here is not efficient, but I want to know why this not work.

Answer 1

std::thread deduces the type of bound arguments and stores a copy of them in the thread object. In your case the the argument is deduced to be a reference. However references, as you know, cannot be copied. If you want to pass a reference to a function inside std::thread , then you can use std::ref which creates a reference wrapper that is copyable:

thread first(merge_sort_thread,std::ref(array1));

Answer 2

You're doing copy initialization (see here ), and threads are not allowed to be copyable objects for obvious reasons.

Instead, replace this kind of code

 thread foo = thread(...);

With this

thread foo(...);

Or, if you don't have phobias (like me...) to ugly code, and you believe in the promises of the 11th C++...

thread foo{...};