efficient thread-safe singleton in C++

Question

The usual pattern for a singleton class is something like

static Foo &getInst()
{
  static Foo *inst = NULL;
  if(inst == NULL)
    inst = new Foo(...);
         


        

Answer 1

Herb Sutter talks about the double-checked locking in CppCon 2014.

Below is the code I implemented in C++11 based on that:

class Foo {
public:
    static Foo* Instance();
private:
    Foo() {}
    static atomic pinstance;
    static mutex m_;
};

atomic Foo::pinstance { nullptr };
std::mutex Foo::m_;

Foo* Foo::Instance() {
  if(pinstance == nullptr) {
    lock_guard lock(m_);
    if(pinstance == nullptr) {
        pinstance = new Foo();
    }
  }
  return pinstance;
}

you can also check complete program here: http://ideone.com/olvK13