Question

bool HuffmanNode::Compare::operator()(const HuffmanNode &n1, const HuffmanNode &n2) const
{
    if (n1.frequency == n2.frequency) {
        return lessThan ? n1.character < n2.character : n1.character >= n2.character;
    } else {
        return lessThan ? n1.frequency < n2.frequency : n1.frequency >= n2.frequency;
    }
}

bool HuffmanNode::Compare::operator()(const HuffmanNode *n1, const HuffmanNode *n2) const
{
    return operator()(*n1, *n2);
}

在这里，我具有另一个对象（HuffmanNode）中的比较对象的比较函数。我想通过模板将compare函数传递给另一个对象：

HeapQueue<HuffmanNode, HuffmanNode::Compare> queue;

这似乎不起作用，我什至尝试使用运算符功能。

什么是正确的格式？谢谢。

这是我的编译器错误

/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1826:31: error: 
      no matching constructor for initialization of 'HuffmanNode'
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1718:18: note: 
      in instantiation of function template specialization
      'std::__1::allocator<HuffmanNode>::construct<HuffmanNode>' requested here
            {__a.construct(__p, _VSTD::forward<_Args>(__args)...);}
                 ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1561:14: note: 
      in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<HuffmanNode>
      >::__construct<HuffmanNode>' requested here
            {__construct(__has_construct<allocator_type, _Tp*, _Args...>(),
             ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/vector:1030:25: note: 
      in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<HuffmanNode>
      >::construct<HuffmanNode>' requested here
        __alloc_traits::construct(__a, _VSTD::__to_raw_pointer(this->__end_));
                        ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/vector:1121:9: note: 
      in instantiation of member function 'std::__1::vector<HuffmanNode,
      std::__1::allocator<HuffmanNode> >::__construct_at_end' requested here
        __construct_at_end(__n);
        ^
./HeapQueue.hpp:25:26: note: in instantiation of member function
      'std::__1::vector<HuffmanNode, std::__1::allocator<HuffmanNode> >::vector'
      requested here
  VectorCompleteTree() : V(1) {}
                         ^
./HeapQueue.hpp:46:7: note: in instantiation of member function
      'VectorCompleteTree<HuffmanNode>::VectorCompleteTree' requested here
class HeapQueue
      ^
./HuffmanBase.hpp:8:7: note: candidate constructor (the implicit copy
      constructor) not viable: requires 1 argument, but 0 were provided
class HuffmanNode {
      ^
./HuffmanBase.hpp:8:7: note: candidate constructor (the implicit move
      constructor) not viable: requires 1 argument, but 0 were provided
./HuffmanBase.hpp:11:3: note: candidate constructor not viable: requires 2
      arguments, but 0 were provided
  HuffmanNode(char c, size_t f) : HuffmanNode(c, f, nullptr, nullptr, nu...
  ^
./HuffmanBase.hpp:10:3: note: candidate constructor not viable: requires 5
      arguments, but 0 were provided
  HuffmanNode(char c, size_t f, HuffmanNode *p, HuffmanNode *l, HuffmanN...
  ^
1 error generated.

Answer 1

从错误消息的底部开始，编译器正在尝试执行HuffmanNode{}，但是由于HuffmanNode没有默认的构造函数而失败。此处的密钥为but 0 were provided。

./HuffmanBase.hpp:8:7: note: candidate constructor (the implicit copy
      constructor) not viable: requires 1 argument, but 0 were provided
class HuffmanNode {
      ^
./HuffmanBase.hpp:8:7: note: candidate constructor (the implicit move
      constructor) not viable: requires 1 argument, but 0 were provided
./HuffmanBase.hpp:11:3: note: candidate constructor not viable: requires 2
      arguments, but 0 were provided
  HuffmanNode(char c, size_t f) : HuffmanNode(c, f, nullptr, nullptr, nu...
  ^
./HuffmanBase.hpp:10:3: note: candidate constructor not viable: requires 5
      arguments, but 0 were provided
  HuffmanNode(char c, size_t f, HuffmanNode *p, HuffmanNode *l, HuffmanN...
  ^

HeapQueue的默认构造函数正在调用VectorCompleteTree<HuffmanNode>的默认构造函数。

./HeapQueue.hpp:46:7: note: in instantiation of member function
      'VectorCompleteTree<HuffmanNode>::VectorCompleteTree' requested here
class HeapQueue

VectorCompleteTree<HuffmanNode>的默认构造函数是使用一个元素构造一个std::vector<HuffmanNode>。此元素需要默认构造。

./HeapQueue.hpp:25:26: note: in instantiation of member function
      'std::__1::vector<HuffmanNode, std::__1::allocator<HuffmanNode> >::vector'
      requested here
  VectorCompleteTree() : V(1) {}

std::vector<HuffmanNode>试图默认构造HuffmanNode，但不能。

/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1826:31: error: 
      no matching constructor for initialization of 'HuffmanNode'
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1718:18: note: 
      in instantiation of function template specialization
      'std::__1::allocator<HuffmanNode>::construct<HuffmanNode>' requested here
            {__a.construct(__p, _VSTD::forward<_Args>(__args)...);}
                 ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1561:14: note: 
      in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<HuffmanNode>
      >::__construct<HuffmanNode>' requested here
            {__construct(__has_construct<allocator_type, _Tp*, _Args...>(),
             ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/vector:1030:25: note: 
      in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<HuffmanNode>
      >::construct<HuffmanNode>' requested here
        __alloc_traits::construct(__a, _VSTD::__to_raw_pointer(this->__end_));
                        ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/vector:1121:9: note: 
      in instantiation of member function 'std::__1::vector<HuffmanNode,
      std::__1::allocator<HuffmanNode> >::__construct_at_end' requested here
        __construct_at_end(__n);

所有错误消息都在那里！简单的解决方案是为HuffmanNode提供默认的构造函数。

如何将对象函数作为模板参数传递

1 个答案: