Accessing vector over multiple threads?

Question

How can I protect the vector v from crashing? And another question, why is not it already crashed, shouldn't it?

#include <Windows.h> #include <thread> #include <vector> #include <algorithm> #include <iostream> using namespace std; vector<int> v; void a() { while (true) { v.push_back(1); Sleep(100); } } void b() { while (true) { if (!v.empty()) { v.erase(v.begin()); } Sleep(100); } } void c() { while (true) { v.push_back(1); Sleep(100); } } int main() { thread(&a).detach(); thread(&b).detach(); thread(&c).detach(); while (true) { for (int i = 0; i < v.size(); i++) { v[i]++; } cout << v.size() << endl; if (!v.empty()) v.erase(v.begin()); Sleep(100); } }

possible duplicate: stackoverflow.com/questions/12260946/… — default
– default, Commented Jan 8, 2013 at 9:27
Just to add to what others said, if you are only reading contents of a vector from multiple threads, it is thread-safe. — NeonGlow
– NeonGlow, Commented Jan 8, 2013 at 10:05

billz · Accepted Answer · 2013-01-08 09:35:49Z

To access one vector from multiple thread, you need to add std::mutex, idiomatic way is to implement RAII, see below demo code:

#include <mutex> #include <thread> class raii_vector { public: raii_vector() { } void Add() { std::lock_guard<std::mutex> lock(m_); v_.push_back(1); } void Remove() { std::lock_guard<std::mutex> lock(m_); if (!v_.empty()) { v_.erase(v_.begin()); } } private: std::mutex m_; std::vector<int> v_; }; raii_vector rv; void a() { while (true) { rv.Add(); std::cout << "adding " << std::endl; std::chrono::milliseconds dura( 100 ); std::this_thread::sleep_for( dura ); } } void b() { while (true) { std::cout << "removing " << std::endl; rv.Remove(); std::chrono::milliseconds dura( 100 ); std::this_thread::sleep_for( dura ); } } int main(int argc, char* argv[]) { std::thread t1(a); std::thread t2(b); t1.join(); t2.join(); return 0; }

dwxw · Accepted Answer · 2013-01-08 09:55:04Z

When wanting to use data structures such as vectors from multiple threads, I've found the Intel Threading Building Blocks library to be really useful.

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