Passing a vector element reference to a stream function

Question

I am trying to do something like this:

#include <thread>
#include <vector>

void foo(bool &check){

}

int main(){
    std::vector<bool> vec(1);
    std::thread T(foo, std::ref(vec[0]));
}

Unfortunately, gcc causes an error:

prog.cpp: In function 'int main()':
prog.cpp:10:34: error: use of deleted function 'void std::ref(const _Tp&&) [with _Tp = std::_Bit_reference]'
  std::thread(foo, std::ref(vec[1]))
                                  ^
In file included from /usr/include/c++/4.9/thread:39:0,
                 from prog.cpp:1:
/usr/include/c++/4.9/functional:453:10: note: declared here
     void ref(const _Tp&&) = delete;

However, it works with a normal variable:

bool var;
std::thread(foo, std::ref(var));

I have no idea why I cannot pass a link to the vec element. Can someone explain why? Is there any workaround?

Answer 1

The problem is what you are using std::vector<bool>. operator []for vector<bool>returns not bool, but std::vector<bool>::reference, that is, a proxy class. You can use something like:

bool value = vec[0];
std::thread T(foo, std::ref(value));
T.join();
vec[0] = value;

Answer 2

Another workaround (not verified)

#include <thread>
#include <vector>

void foo(vector& v){
  //here access v[0] and assign whatever value you'd like
  //ideally you could pass an index as well
  // if you want to access the Nth element
}

int main(){
    std::vector<bool> vec(1);
    std::thread T(foo, std::ref(vec));
}