c++ - c++将equal_to设置为模板类的默认模板参数

Question

您究竟应该如何将泛型类设置为默认模板参数？我设法使非泛型类可以编译，但没有泛型类。

using namespace std;
template <typename T, typename Cmp = std::equal_to<T> >
class CSearch
{
   public:
    CSearch(){
     MComp = equal_to<T>();
    }
    CSearch(Cmp compare){
     MComp = compare;

    }
    //more functions
   private:
    Cmp MComp;
    //more 
};

T是任何带有迭代器的数据类型

Cmp应该是Binary Predicate函数/ functor / lambda函数。
Cmp的调用方式如下:

MComp(*it2,*it3);

现在有了这个主要功能，代码就可以工作了。请注意，CharComparator实际上与equal_to没什么不同，除了非泛型。

class CharComparator
{
  public:
               CharComparator ( bool caseSensitive = true )
      : m_CaseSensitive ( caseSensitive )
    {
    }
    bool       operator () ( const char & a, const char & b ) const
    {
      return m_CaseSensitive ? a == b : toupper (a) == toupper (b);
    }
  private:
    bool       m_CaseSensitive;
};
int main(){
CSearch <string, CharComparator> test3 ( CharComparator(false) );
//functions,calling Cmp
}

但是在调用Cmp时不会编译该代码:

int main(){
CSearch <string> test1;
//functions,calling Cmp
}

这是编译器错误。

test.cpp: In instantiation of 'bool CSearch<T, Cmp>::SearchFragment(const T&, const T&) const [with T = std::__cxx11::basic_string<char>; Cmp = std::equal_to<std::__cxx11::basic_string<char> >]':
test.cpp:88:6:   required from 'std::set<int> CSearch<T, Cmp>::Search(const T&) const [with T = std::__cxx11::basic_string<char>; Cmp = std::equal_to<std::__cxx11::basic_string<char> >]'
test.cpp:130:3:   required from here
test.cpp:60:3: error: no match for call to '(const std::equal_to<std::__cxx11::basic_string<char> >) (const char&, const char&)'
   60 |   if(MComp(*it2,*it3)){
      |   ^~
In file included from /usr/include/c++/9.3/string:48,
                 from /usr/include/c++/9.3/bits/locale_classes.h:40,
                 from /usr/include/c++/9.3/bits/ios_base.h:41,
                 from /usr/include/c++/9.3/ios:42,
                 from /usr/include/c++/9.3/ostream:38,
                 from /usr/include/c++/9.3/iostream:39,
                 from test.cpp:5:
/usr/include/c++/9.3/bits/stl_function.h:355:7: note: candidate: 'constexpr bool std::equal_to<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = std::__cxx11::basic_string<char>]'
  355 |       operator()(const _Tp& __x, const _Tp& __y) const
      |       ^~~~~~~~
/usr/include/c++/9.3/bits/stl_function.h:355:29: note:   no known conversion for argument 1 from 'const char' to 'const std::__cxx11::basic_string<char>&'
  355 |       operator()(const _Tp& __x, const _Tp& __y) const
      |                  ~~~~~~~~~~~^~~

Answer 1

错误的这一行说出了什么问题:

error: no match for call to '(const std::equal_to<std::__cxx11::basic_string<char> >) (const char&, const char&)'

您尝试调用 std::equal_to<std::string>的实例来比较两个字符，但是 std::equal_to<std::string>比较字符串。

您希望 std::equal_to< typename T::value_type>而不是 std::equal_to<T>，因为您要比较的是字符而不是字符串(或 T是容器类型时的一般元素)。

另外，您不应在默认构造函数中分配 MComp = equal_to<T>();。如果 Cmp不同于 equal_to<T>，则默认构造函数将无法工作。您应该改为使用 MComp类型的默认实例初始化 Cmp。