加密模板模板参数错误 [英] Cryptic template template parameter error
问题描述
我正在尝试创建一个从std::map
或std::unordered_map
获取键的函数.我可以使用简单的重载,但首先,我想知道这段代码有什么问题.
I'm trying to create a function that gets the keys from a std::map
or an std::unordered_map
. I could use a simple overload, but first I'd love to know what's wrong with this code.
template<typename K, typename V, template<typename, typename> class TContainer>
std::vector<K> getKeys(const TContainer<K, V>& mMap)
{
std::vector<K> result;
for(const auto& itr(std::begin(mMap)); itr != std::end(mMap); ++itr) result.push_back(itr->first);
return result;
}
使用std::unordered_map
调用它时,即使手动指定 all 模板类型名称,clang ++ 3.4也会说:
When calling it with an std::unordered_map
, even specifying all template typenames manually, clang++ 3.4 says:
模板模板参数与相应的模板模板参数具有不同的模板参数.
template template argument has different template parameters than its corresponding template template parameter.
推荐答案
问题是std::map
和std::unordered_map
实际上不是带有两个参数的模板.他们是:
The problem is that std::map
and std::unordered_map
are not in fact templates with two parameters. They are:
namespace std {
template <class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T>>>
class map;
template <class Key, class T, class Hash = hash<Key>,
class Pred = equal_to<Key>,
class Allocator = allocator<pair<const Key, T>>>
class unordered_map;
}
这里有一些类似的方法可以工作:
Here's something similar that does work:
template <typename K, typename... TArgs, template<typename...> class TContainer>
std::vector<K> getKeys(const TContainer<K, TArgs...>& mMap)
{
std::vector<K> result;
for (auto&& p : mMap)
result.push_back(p.first);
return result;
}
我想要的版本:
template <typename Container>
auto getKeys2(const Container& mMap) -> std::vector<typename Container::key_type>
{
std::vector<typename Container::key_type> result;
for (auto&& p : mMap)
result.push_back(p.first);
return result;
}
同时使用这两个功能的演示程序: http://ideone.com/PCkcu6
A demo program using both functions: http://ideone.com/PCkcu6
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