CODEWITHSUNDEEP
c++multithreadingc++11vectorreference
Tomasz Kasperczyk
Tomasz Kasperczyk

Reputation: 2093

Passing a reference of a vector element to a threaded function

I'm 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 throws 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 can't pass a reference to the vec element. Can someone explain why? Is there any workaround?

Upvotes: 5

Views: 2730

Answers (2)

dau_sama
dau_sama

Reputation: 4367

Another workaround (not tested)

#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));
}

Upvotes: -1

ForEveR
ForEveR

Reputation: 55897

Problem is, that you use std::vector<bool>. operator [] for vector<bool> returns not bool, but std::vector<bool>::reference, that is proxy class. You can use something like:

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

Upvotes: 7

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