CODEWITHSUNDEEP
multithreadingc++11parallel-processingatomiccompare-and-swap
Erik Henriksson
Erik Henriksson

Reputation: 737

C++11 std::compare_exchange_strong does not compile when comparing with null pointer

I am working on a parallel avl tree and have ran into a problem. Here is the function that causes this problem:

template<typename T, int Threads>
bool PTreeSet<T, Threads>::tryInsert(Node* parent, Node* node) {
    if (parent->key > node->key) {
        return parent->left.compare_exchange_strong(nullptr, node); // <-- Error
    } else if (parent->key < node->key) {
        return parent->right.compare_exchange_strong(nullptr, node); // <-- Error
    } else {
        return false;
    }
    return true;
}

parent->lefthas type atomic<Node*>and I want to set that pointer to node if the current value is null. The compiler complains with error

error: no matching member function for call to 'compare_exchange_strong'
        return parent->left.compare_exchange_strong(nullptr, node);

Why is this not valid code?

Upvotes: 1

Views: 3369

Answers (1)

Igor Tandetnik
Igor Tandetnik

Reputation: 52471

The first parameter of atomic<T>::compare_exhange_strong is a T&. It requires an lvalue. That's one half of the "exchange": the current value of the atomic is loaded into the object referred to by the first parameter.

You need something like this:

Node* tmp = nullptr;
parent->left.compare_exchange_strong(tmp, node);

As a side effect, if parent->left is not in fact NULL, you get its current value in tmp (which you can ignore if you don't need it, of course).

Upvotes: 4

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