CODEWITHSUNDEEP
c++multithreadingc++11
kww
kww

Reputation: 541

C++11 std::thread std::move complaining attempt to use a deleted function

I am learning C++11 thread and tried to write a thread changing the shared memory. I used std::ref and std::move respectively. I ran the code using: g++ eg3.cpp -std=c++11 -pthread. However I found std::move does not work on my mac. I got error like this:

In file included from eg3.cpp:1: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/thread:337:5: error: 
      attempt to use a deleted function
    __invoke(_VSTD::move(_VSTD::get<0>(__t)), _VSTD::move(_VSTD::get<_Indices>(__t))...);
    ^
...

My code is as follows:

#include<thread>
#include<iostream>
#include<mutex>
#include<condition_variable>
#include<string>
#include<functional>
#include<utility>
using namespace std;
int main(){
  string s = "Hello!";
  cout << "Main before: " << s << endl;
  // thread t([](string& s){cout << s << endl; s = "Ni hao!";}, ref(s)); //// This works!
  // thread t([](string& s){cout << s << endl; s = "Ni hao!";}, move(s)); //// This does not work
  t.join();
  cout << "Main after: " << s << endl;
  return 0;
}

Upvotes: 5

Views: 3777

Answers (1)

Bryan Chen
Bryan Chen

Reputation: 46598

You just need to make the lambda takes string (by value) or string const & (by constant reference) or string && (rvalue reference) to support move. In this case, because you are modifying s, and you can't use string const &.

thread t([](string && s){cout << s << endl; s = "Ni hao!";}, move(s));

It failed because you can't pass rvalue reference (string &&) to a function/lambda that takes lvalue reference (string &).

A simplified example is

void test(std::string &) {}
void test2(std::string &&) {}
void test3(std::string const&) {}
void test4(std::string) {}

int main(){
  std::string s;
  test(std::move(s)); // fail
  test2(std::move(s)); // ok
  test3(std::move(s)); // ok
  test4(std::move(s)); // ok
}

Upvotes: 2

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