类中私有结构的构造函数出错

Question

我有这门课：

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，在这种情况下请告诉我），但我得到了这个我完全不明白的错误：

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？

更新评论一个答案：

好的，我发现问题出在哪里，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) {}

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)，一切正常并且不会发生编译错误。为什么？并且热解决它？

Answer 1

问题似乎在于您缺少CacheElem 的默认构造函数。在代码中的某处，CacheElem 需要默认构造，因为您将其存储在要求元素默认可构造的 STL 容器中，或者因为您只是尝试在不调用构造函数的情况下在某处实例化一个。

Answer 2

感谢this 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) {}

到这里：

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);
}

解决了这个问题。我认为是因为调用values(size) 隐式调用resize()，如果不提供CacheElem 构造函数元素，这是非法的。