为什么我需要在以下示例中在 block_cache.h 之前包含 block_cache_key.h 和 block.h

通常我们会使用标准类型作为std::unordered_map<key, value>的键和值。但是现在我需要自定义自己的键和值类。

键类在block_cache_key.h中定义如下：

#ifndef BLOCK_CACHE_KEY_H_
#define BLOCK_CACHE_KEY_H_
#include <functional>
namespace wcg{
class BlockCacheKey{
public:
BlockCacheKey(const std::string &name, int64_t offset) : name_(name), offset_(offset) {}
bool operator==(const BlockCacheKey &other) const{
return offset_ == other.offset_ && name_ == other.name_;
}
const std::string &name() const{
return name_;
}
const int64_t offset() const{
return offset_;
}
std::string to_string() const{
return name_ + "_" + std::to_string(offset_);
}
private:
std::string name_;
int64_t offset_;
};
}
namespace std{
template <>
class hash<wcg::BlockCacheKey>{
public:
size_t operator()(const wcg::BlockCacheKey &key) const{
return std::hash<std::string>()(key.name()) ^ (std::hash<int64_t>()(key.offset()));
}
};
}
#endif

值类在block.h中定义如下：

#ifndef BLOCK_H_
#define BLOCK_H_
namespace wcg{
class Block{
public:
Block() {}
};
}
#endif

使用std::unorded_map的类在block_cache.h中定义如下：

#ifndef BLOCK_CACHE_H_
#define BLOCK_CACHE_H_
#include <iostream>
#include <memory>
#include <unordered_map>
namespace wcg{
class BlockCacheKey;
class Block;
class BlockCache{
public:
BlockCache() : count_(0) {
std::cout << "initial stats: " << to_string() << std::endl;
}
~BlockCache(){
std::cout << "final stats: " << to_string() << std::endl;
}
void CacheBlock(const BlockCacheKey &key, std::shared_ptr<Block> block){
std::cout << "cache put: " << key.to_string() << std::endl;
map_[key] = block;
count_++;
}
std::shared_ptr<Block> GetBlock(const BlockCacheKey &key){
auto pos = map_.find(key);
if(pos != map_.end()){
std::cout << "cache get: " << key.to_string() << std::endl;
return pos->second;
}
return nullptr;
}
std::string to_string() const{
return "block count: " + std::to_string(count_);
}
private:
std::unordered_map<BlockCacheKey, std::shared_ptr<Block>> map_;
int count_;
};
}
#endif

main 函数在main.cpp中定义如下：

#include <memory>
#include "block_cache_key.h"
#include "block.h"
#include "block_cache.h"
// g++ -std=c++11 -g -Wall main.cpp
int main(int argc, char* argv[]){
wcg::BlockCache bc;
return 0;
}

当#include "block_cache.h"终于结束时，编译是可以的。但是当#include "block_cache.h"在#include "block_cache_key.h"和#include "block.h"之前时，编译会失败，错误消息非常丑陋，很难理解。

我已经知道的，std::unordered_map是一个模板。 当GCC编译时，它将首先预处理包含部分，即将它们全部展开。 这给我的印象是，包含顺序不会是一个问题。

最后要注意的是，我不想在block_cache.h中包含block_cache_key.h和block.h

部分编译错误消息： (整个错误消息以 https://paste.ubuntu.com/p/VyrWKCTG4q/为单位(

In file included from /usr/include/c++/7/bits/hashtable.h:35:0,
from /usr/include/c++/7/unordered_map:47,
from block_cache.h:13,
from main.cpp:5:
/usr/include/c++/7/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> >':
/usr/include/c++/7/type_traits:143:12:   required from 'struct std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > >'
/usr/include/c++/7/type_traits:154:31:   required from 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
/usr/include/c++/7/bits/unordered_map.h:103:66:   required from 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >'
block_cache.h:50:60:   required from here
/usr/include/c++/7/bits/hashtable_policy.h:87:34: error: no match for call to '(const std::hash<wcg::BlockCacheKey>) (const wcg::BlockCacheKey&)'
noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/bits/move.h:54:0,
from /usr/include/c++/7/bits/stl_pair.h:59,
from /usr/include/c++/7/bits/stl_algobase.h:64,
from /usr/include/c++/7/memory:62,
from main.cpp:3:
/usr/include/c++/7/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >':
/usr/include/c++/7/bits/unordered_map.h:103:66:   required from 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >'
block_cache.h:50:60:   required from here
/usr/include/c++/7/type_traits:154:31: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > >'
: public __bool_constant<!bool(_Pp::value)>
^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/unordered_map:48:0,
from block_cache.h:13,
from main.cpp:5:
/usr/include/c++/7/bits/unordered_map.h: In instantiation of 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >':
block_cache.h:50:60:   required from here
/usr/include/c++/7/bits/unordered_map.h:103:66: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc>  _Hashtable;
^~~~~~~~~~
/usr/include/c++/7/bits/unordered_map.h:110:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
typedef typename _Hashtable::key_type key_type;