c++ - Thread-safe lock-free array

Question

I have a C++ library, which supposed to do some computations on multiple threads. I made independent threads code (i.e. there are no shared variables between them), except for one array. The problem is, I don't know how to make it thread-safe.

I looked at mutex lock/unlock (QMutex, as I'm using Qt), but it doesn't fit for my task - while one thread will lock the mutex, other threads will wait!

Then I read about std::atomic, which looked like exactly what I needed. Nevertheless, I tried to use it in the following way:

std::vector<std::atomic<uint64_t>> *myVector;

And it produced compiler error (use of deleted function 'std::atomic::atomic(const std::atomic&)'). Then I found the solution - use special wrapper for std::atomic. I tried this:

struct AtomicUInt64
{
    std::atomic<uint64_t> atomic;

    AtomicUInt64() : atomic() {}

    AtomicUInt64 ( std::atomic<uint64_t> a ) : atomic ( atomic.load() ) {}

    AtomicUInt64 ( AtomicUInt64 &auint64 ) : atomic ( auint64.atomic.load() ) {}

    AtomicUInt64 &operator= ( AtomicUInt64 &auint64 )
    {
                atomic.store ( auint64.atomic.load() );
    }
};

std::vector<AtomicUInt64> *myVector;

This thing compiles succesfully, but when I can't fill the vector:

myVector = new std::vector<AtomicUInt64>();

for ( int x = 0; x < 100; ++x )
{
    /* This approach produces compiler error:
     * use of deleted function 'std::atomic<long long unsigned int>::atomic(const std::atomic<long long unsigned int>&)'
     */
    AtomicUInt64 value( std::atomic<uint64_t>( 0 ) ) ;
    myVector->push_back ( value );

    /* And this one produces the same error: */
    std::atomic<uint64_t> value1 ( 0 );
    myVector->push_back ( value1 );
}

What am I doing wrong? I assume I tried everything (maybe not, anyway) and nothing helped. Are there any other ways for thread-safe array sharing in C++?

By the way, I use MinGW 32bit 4.7 compiler on Windows.

Answer 1

Here is a cleaned up version of your AtomicUInt64 type:

template<typename T>
struct MobileAtomic
{
  std::atomic<T> atomic;

  MobileAtomic() : atomic(T()) {}

  explicit MobileAtomic ( T const& v ) : atomic ( v ) {}
  explicit MobileAtomic ( std::atomic<T> const& a ) : atomic ( a.load() ) {}

  MobileAtomic ( MobileAtomic const&other ) : atomic( other.atomic.load() ) {}

  MobileAtomic& operator=( MobileAtomic const &other )
  {
    atomic.store( other.atomic.load() );
    return *this;
  }
};

typedef MobileAtomic<uint64_t> AtomicUInt64;

and use:

AtomicUInt64 value;
myVector->push_back ( value );

or:

AtomicUInt64 value(x);
myVector->push_back ( value );

your problem was you took a std::atomic by value, which causes a copy, which is blocked. Oh, and you failed to return from operator=. I also made some constructors explicit, probably needlessly. And I added const to your copy constructor.

I would also be tempted to add store and load methods to MobileAtomic that forwards to atomic.store and atomic.load.