Question

在某些C ++程序中，我有两个类 - Abc 和 Xyz 。 Abc 类包含一个整数成员 Foo ，如下面的清单所示：

class Abc
{
 public:
 int Foo;
 Abc(int f)
 {
  Foo = f;
 }
};
class Xyz{};

我正在使用地图＆lt; Abc ， Xyz ＆gt;但表现极差。这就是为什么我想使用hash_map＆lt; Abc ， Xyz ＆gt; （Dev-Cpp扩展库）和一些棘手的哈希。让我们假设以下 AbcHash 一元函数对象将作为哈希生成器运行良好：

class AbcHash
{
 public:
 size_t operator() (const Abc &a)
 {
  return a.Foo % 123;
 } 
};

为了与STL保持一致，我还实现了一个 AbcCmp 比较器：

class AbcCmp
{
 public:
 bool operator() (const Abc &a1, const Abc &a2)
 {
  return (a1.Foo == a2.Foo);
 }
};

当我想最后使用hash_map时，我测试了 Abc 和 Xyz 类。那是我的主要：

int main(int argc, char *argv[])
{
 hash_map myMap;

 Abc a(10);
 a.Foo = 23;

 Xyz x();

 myMap[a] = x; // this line fails the compilation
}

最后一行会导致一些奇怪的编译错误：

Compiler: Default compiler
Building Makefile: "C:\user\hdr\hashfunc\Makefile.win"
Executing  make...
make.exe -f "C:\user\hdr\hashfunc\Makefile.win" all
g++.exe -c main.cpp -o main.o -I"C:/Dev-Cpp/lib/gcc/mingw32/3.4.2/include"  -I"C:/Dev-Cpp/include/c++/3.4.2/backward"  -I"C:/Dev-Cpp/include/c++/3.4.2/mingw32"  -I"C:/Dev-Cpp/include/c++/3.4.2"  -I"C:/Dev-Cpp/include"   

In file included from C:/Dev-Cpp/include/c++/3.4.2/backward/hash_map.h:59,
                 from main.cpp:2:
C:/Dev-Cpp/include/c++/3.4.2/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please consider using one of the 32 headers found in section 17.4.1.2 of the C++ standard. Examples include substituting the  header for the  header for C++ includes, or  instead of the deprecated header . To disable this warning use -Wno-deprecated.
main.cpp: In function `int main(int, char**)':
main.cpp:45: error: no match for 'operator=' in '(&myMap)->__gnu_cxx::hash_map::operator[] [with _Key = Abc, _Tp = Xyz, _HashFcn = AbcHash, _EqualKey = AbcCmp, _Alloc = std::allocator](((const Abc&)((const Abc*)(&a)))) = x'
main.cpp:14: note: candidates are: Xyz& Xyz::operator=(const Xyz&)

C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h: In member function `size_t __gnu_cxx::hashtable::_M_bkt_num_key(const _Key&, size_t) const [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]':
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:523:   instantiated from `size_t __gnu_cxx::hashtable::_M_bkt_num(const _Val&, size_t) const [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:887:   instantiated from `void __gnu_cxx::hashtable::resize(size_t) [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:701:   instantiated from `typename __gnu_cxx::hashtable::reference __gnu_cxx::hashtable::find_or_insert(const _Val&) [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hash_map:181:   instantiated from `_Tp& __gnu_cxx::hash_map::operator[](const typename __gnu_cxx::hashtable, _Key, _HashFcn, std::_Select1st >, _EqualKey, _Alloc>::key_type&) [with _Key = Abc, _Tp = Xyz, _HashFcn = AbcHash, _EqualKey = AbcCmp, _Alloc = std::allocator]'
main.cpp:45:   instantiated from here
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:518: error: passing `const AbcHash' as `this' argument of `size_t AbcHash::operator()(const Abc&)' discards qualifiers

make.exe: *** [main.o] Error 1

Execution terminated

我完全不知道这里有什么不妥。如何解决？

Answer 1

我的第一个方法是确定为什么地图的表现很差。地图实际上是非常好的全能表演者 - 在某些情况下，他们可能会表现优于哈希表。在不知道问题的情况下更换容器不是一个好主意。

然后我会破坏DevC ++。这个古董非常多，并且不再处于积极的发展状态 - 我对使用它附带的任何库特别怀疑。我将切换到Code::Blocks的夜间版本，GCC的Twilight Dragon版本，以及（如果我仍然需要哈希）TR1或Boost哈希表实现。

Answer 2

要解释第一个错误，我需要看看你如何键入deff的hash_map。（你确实键入了它，不是吗？）。

你得到第二个错误的原因是因为你的AbcHash和AbcCmp声明应该将它们的operator（）函数声明为const：

size_t operator() (const Abc &a) const

bool operator() (const Abc &a1, const Abc &a2) const

但是，我必须同意Neil的观点，即使用积极开发的库会更加明智。

C ++中的哈希对象问题

2 个答案: