Why priority_queue<> template self-defined compare function need a wrapper struct

  • 0

    Simple C++ with overloaded priority_queue and BFS, the tricky part is wrapper struct for dvComp, we need an object/class type here, and need to overload operator ().

    Should we use function object here?
    Does any compiler support lambda in priority_queue template?



    	vector<int> closestKValues(TreeNode* root, double target, int k) {
    	typedef struct {
    		double dist;
    		int val;
    	} DistVal;
    	struct dvComp {
    		bool operator() (const DistVal& a, const DistVal& b) const {
    			return a.dist < b.dist;
    	assert(root != NULL);
    	priority_queue < DistVal, vector<DistVal>, dvComp> topk;
    	DistVal closestVal = { abs(root->val - target), root->val };
    	queue<TreeNode*> qTN;
    	// BFS All
    	while (qTN.size()) {
    		TreeNode *rt = qTN.front();
    		if (rt) {
    			DistVal tmp = { abs(rt->val - target), rt->val };
    			if ((int)topk.size()>k) topk.pop();
    	vector<int> ret;
    	for (int i = 0; i<k; i++) {
    	return ret;

  • 1

    You could use function object here, as shown below. It is supposed to be supported in both the latest GCC and VC++ compiler.

    auto lessDistVal = [](DistVal const &d1, DistVal const &d2) {return d1.dist < d2.dist;};
    priority_queue<DistVal, vector<DistVal>, decltype(lessDistVal)> topk(lessDistVal);

    As for the request about "why ... need a wrapper struct", it is because the default "Compare" of the priority_queue does not support the type "DistVal" and you need to provide your own "Compare" version. The following is the priority_queue definition obtained from http://en.cppreference.com/w/cpp/container/priority_queue.

        class T,
        class Container = std::vector<T>,
        class Compare = std::less<typename Container::value_type>
    > class priority_queue;

    The "T", "Container" and "Compare" stand for type rather than object. That is why we "need a wrapper struct" and why we use the 'decltype' to obtain the type and pass this type in (instead of directly passing the object "lessDistVal ").

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