类中私有结构的构造函数出错

Question

我有这个类：

template <typename C, typename R, typename D>
class Cache {
typedef std::shared_ptr<cc::Distance<C,D>> DistancePtr;
public:
Cache(const DistancePtr distance, const std::function<R(C)> &backEnd, const size_t size = 10000, const float treshold = 0);
...
private:
struct CacheElem{
CacheElem(const C code, const R result, std::list<size_t>::iterator listElem) : code(code), result(result), listElem(listElem) {}
C code;
R result;
std::list<size_t>::iterator listElem; //pointing to corresponding element in lru0
};
...

我用cc::Cache<int,int,int> cache(...)实例化了这个对象(我不知道你是否需要知道所有传递的参数，在这种情况下让我知道)，但我得到这个我根本不明白的错误：

In file included from /usr/include/c++/5/memory:64:0,
from ../main.cpp:9:
/usr/include/c++/5/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = cc::Cache<int, int, int>::CacheElem; _Args = {}]’:
/usr/include/c++/5/bits/stl_uninitialized.h:519:18:   required from ‘static _ForwardIterator std::__uninitialized_default_n_1<_TrivialValueType>::__uninit_default_n(_ForwardIterator, _Size) [with _ForwardIterator = cc::Cache<int, int, int>::CacheElem*; _Size = long unsigned int; bool _TrivialValueType = false]’
/usr/include/c++/5/bits/stl_uninitialized.h:575:20:   required from ‘_ForwardIterator std::__uninitialized_default_n(_ForwardIterator, _Size) [with _ForwardIterator = cc::Cache<int, int, int>::CacheElem*; _Size = long unsigned int]’
/usr/include/c++/5/bits/stl_uninitialized.h:637:44:   required from ‘_ForwardIterator std::__uninitialized_default_n_a(_ForwardIterator, _Size, std::allocator<_Tp>&) [with _ForwardIterator = cc::Cache<int, int, int>::CacheElem*; _Size = long unsigned int; _Tp = cc::Cache<int, int, int>::CacheElem]’
/usr/include/c++/5/bits/stl_vector.h:1311:36:   required from ‘void std::vector<_Tp, _Alloc>::_M_default_initialize(std::vector<_Tp, _Alloc>::size_type) [with _Tp = cc::Cache<int, int, int>::CacheElem; _Alloc = std::allocator<cc::Cache<int, int, int>::CacheElem>; std::vector<_Tp, _Alloc>::size_type = long unsigned int]’
/usr/include/c++/5/bits/stl_vector.h:279:30:   required from ‘std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>::size_type, const allocator_type&) [with _Tp = cc::Cache<int, int, int>::CacheElem; _Alloc = std::allocator<cc::Cache<int, int, int>::CacheElem>; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<cc::Cache<int, int, int>::CacheElem>]’
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:59:85:   required from ‘cc::Cache<C, R, D>::Cache(cc::Cache<C, R, D>::DistancePtr, const std::function<R(C)>&, size_t, float) [with C = int; R = int; D = int; cc::Cache<C, R, D>::DistancePtr = std::shared_ptr<cc::Distance<int, int> >; size_t = long unsigned int]’
../main.cpp:33:53:   required from here
/usr/include/c++/5/bits/stl_construct.h:75:7: error: no matching function for call to ‘cc::Cache<int, int, int>::CacheElem::CacheElem()’
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
^
In file included from ../Core/CCCore.hpp:19:0,
from ../main.cpp:15:
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:36:4: note: candidate: cc::Cache<C, R, D>::CacheElem::CacheElem(C, R, std::__cxx11::list<long unsigned int>::iterator) [with C = int; R = int; D = int; std::__cxx11::list<long unsigned int>::iterator = std::_List_iterator<long unsigned int>]
CacheElem(const C code, const R result, std::list<size_t>::iterator listElem) : code(code), result(result), listElem(listElem) {}
^
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:36:4: note:   candidate expects 3 arguments, 0 provided
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:35:10: note: candidate: constexpr cc::Cache<int, int, int>::CacheElem::CacheElem(const cc::Cache<int, int, int>::CacheElem&)
struct CacheElem{
^
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:35:10: note:   candidate expects 1 argument, 0 provided
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:35:10: note: candidate: constexpr cc::Cache<int, int, int>::CacheElem::CacheElem(cc::Cache<int, int, int>::CacheElem&&)
/home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Core/Cache.hpp:35:10: note:   candidate expects 1 argument, 0 provided
<builtin>: recipe for target 'main.o' failed
make: *** [main.o] Error 1

为什么会这样？

更新到评论一个答案：

好的，我找到了问题所在，但不知道为什么会出现。Cache构造函数实现是这样的：

template <typename C, typename R, typename D>
Cache<C,R,D>::Cache(const DistancePtr distance, const std::function<R(C)> &backEnd, const size_t size, const float treshold)
: distance(distance), backEnd(backEnd), values(size), treshold(treshold), size(size) {}

其中values声明为：

std::vector<CacheElem> values;

但是如果我从上面的构造函数中删除values(size)，一切正常，不会发生编译错误。为什么？并热衷于解决它？

Answer 1

看起来问题是您缺少CacheElem的默认构造函数。 在代码中的某个地方，需要默认构造CacheElem，要么是因为您将其存储在要求元素默认可构造的 STL 容器中，要么是因为您只是尝试在某个地方实例化一个而不调用构造函数。

Answer 2

由于这个答案的提示，我发现了错误。

从这里更改Cache构造函数实现：

template <typename C, typename R, typename D>
Cache<C,R,D>::Cache(const DistancePtr distance, const std::function<R(C)> &backEnd, const size_t size, const float treshold)
: distance(distance), backEnd(backEnd), values(size), treshold(treshold), size(size) {}

对此：

template <typename C, typename R, typename D>
Cache<C,R,D>::Cache(const DistancePtr distance, const std::function<R(C)> &backEnd, const size_t size, const float treshold)
: distance(distance), backEnd(backEnd), treshold(treshold), size(size) {
values.reserve(size);
}

解决了问题。我认为因为调用values(size)是隐式调用resize()这是非法的，而无需提供CacheElem构造函数元素。