c++ - 模板类中的比较器

Question

我正在研究堆实现。它必须是模板类，我需要它有自己的比较器，传入构造函数。我该怎么做？

我已经试过了:

template <typename T>
class Heap{
public:
    Heap(int size, bool (*comparator) (const T & a, const T & b) = [] (const T & a, const T & b){
        return a < b;
    })

    // other unimportant methods
}

适用于:

Heap<int> heap(4);

还有:

Heap<int> heap(4, [](const int & a, const int & b){ return false; })

但是当我尝试将它与这样的指针一起使用时(其中 offer 是某种结构):

Heap<Offer*> heap2(3, [](Offer const * a, Offer const * b){
        return false;
});

我遇到了这个编译错误:

test.cpp: In function ‘int main()’:
test.cpp:126:3: error: invalid user-defined conversion from ‘main()::<lambda(const Offer*, const Offer*)>’ to ‘bool (*)(Offer* const&, Offer* const&)’ [-fpermissive]
  });
   ^
test.cpp:124:59: note: candidate is: main()::<lambda(const Offer*, const Offer*)>::operator bool (*)(const Offer*, const Offer*)() const <near match>
  Heap<Offer*> heap2(3, [](Offer const * a, Offer const * b){
                                                           ^
test.cpp:124:59: note:   no known conversion from ‘bool (*)(const Offer*, const Offer*)’ to ‘bool (*)(Offer* const&, Offer* const&)’
test.cpp:13:5: note:   initializing argument 2 of ‘Heap<T>::Heap(int, bool (*)(const T&, const T&)) [with T = Offer*]’
     Heap(int size, bool (*comparator) (const T & a, const T & b) = [] (const T & a, const T & b){

如何让它适用于这两种情况？

Answer 1

对于指针版本:'T' 是一个指针，然后需要作为 const ref 传递 - 它不是它指向的数据，在这里要被 const 限定，而是指针 ref 本身。

template <typename T>
class Heap{
public:
    Heap(int size, bool (*comparator) (const T & a, const T & b)
                                 = [] (const T & a, const T & b){
        return a < b;
    }) {}; 

    // other unimportant methods
};


int main()
{
    Heap<int*> heap2(3, [](int* const& a, int* const& b){
        return false;
    });

    // analogous to  
    Heap<int> heap(4, [](const int & a, const int & b){ 
         return false; 
    });
    // but it's the pointer to the datum in the first,
    // and the datum itself in the latter case
}

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