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,如以下 list 所示:
class Abc { public: int Foo; Abc(int f) { Foo = f; } }; class Xyz{};
我正在使用 map <Abc, Xyz> 但性能极差。这就是为什么我想使用 hash_map<Abc、Xyz>(Dev-Cpp 扩展库)和一些棘手的散列。让我们假设以下 AbcHash 一元函数对象将充当哈希生成器:
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); } };
当我最后想使用 hash_map 时,我已经测试了 Abc 和 Xyz 类。这是我的主要:
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问题。如何修复?
最佳答案
我的第一个方法是找出 map 性能不佳的原因。 map 实际上在各方面都表现出色——在某些情况下它们可能会胜过哈希表。在不知道问题的情况下更换容器不是一个好主意。
然后我会把 DevC++ 当作垃圾。这个古董有很多问题,不再积极开发——我特别怀疑是否使用它附带的任何库。我会切换到 Code::Blocks 的夜间构建, Twilight Dragon GCC 的构建,以及(如果我仍然想要哈希)到 TR1 或 Boost 哈希表实现。
关于c++ - C++中的哈希对象问题,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/1932959/