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So I'm using the STL priority_queue<> with pointers... I don't want to use value types because it will be incredibly wasteful to create a bunch of new objects just for use in the priority queue. So... I'm trying to do this:

class Int { public: Int(int val) : m_val(val) {} int getVal() { return m_val; } private: int m_val; } priority_queue<Int*> myQ; myQ.push(new Int(5)); myQ.push(new Int(6)); myQ.push(new Int(3));

Now how can I write a comparison function to get those to be ordered correctly in the Q? Or, can someone suggest an alternate strategy? I really need the priority_queue interface and would like to not use copy constructors (because of massive amounts of data). Thanks

EDIT: Int is just a placeholder/example... I know I can just use int in C/C++ lol...

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3 Answers 3

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10

You can explicitly specify which comparator your queue should use.

#include <iostream> #include <sstream> #include <functional> #include <vector> #include <queue> class Int { public: Int(int val) : m_val(val) {} int getVal() { return m_val; } bool operator<(const Int &other) const { return m_val < other.m_val; } private: int m_val; }; template<typename Type, typename Compare = std::less<Type> > struct pless : public std::binary_function<Type *, Type *, bool> { bool operator()(const Type *x, const Type *y) const { return Compare()(*x, *y); } }; int main(int argc, char *argv[]) { std::priority_queue<Int*, std::vector<Int*>, pless<Int> > myQ; for (int i = 1; i < argc; i++) { std::stringstream ss(argv[i]); int x; ss >> x; myQ.push(new Int(x)); } for (; !myQ.empty(); delete myQ.top(), myQ.pop()) std::cout << myQ.top()->getVal() << std::endl; return 0; }
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3

One option that will surely work is to replace Int* with shared_ptr<Int> and then implement operator< for shared_ptr<Int>

bool operator<(const shared_ptr<Int> a, const shared_ptr<Int> b) { return a->getVal() < b->getVal(); }
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I'm trying this approach out right now... instead of using shared_ptr just using a basic wrapper class.
@Polaris878 Although the shared pointers are irrelevant here. You never said you wanted shared ownership.
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An integer is the same size as a pointer on 32 bit systems. On 64 bit systems, a pointer will be twice as big. Therefore, it is simpler/faster/better to use regular integers.

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Int is just a placeholder for a much bigger class.
in that case, ignore this response.

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