当我尝试从 this post 编译代码时(见下文)在 Visual Studio 2019 中,我在模板化复制 ctor 行收到以下编译器错误:
Error C2439 'PreAllocator<_Newfirst>::memory_ptr': member could not be initialized
Error C2440 'initializing': cannot convert from 'T *const ' to 'T *'
Error C2248 'PreAllocator<int>::memory_size': cannot access private member declared in class 'PreAllocator<int>'
Error C2248 'PreAllocator<int>::memory_ptr': cannot access private member declared in class 'PreAllocator<int>'
这是编译器错误还是我遗漏了什么?template <typename T>
class PreAllocator
{
private:
T* memory_ptr;
std::size_t memory_size;
public:
typedef std::size_t size_type;
typedef T* pointer;
typedef T value_type;
PreAllocator(T* memory_ptr, std::size_t memory_size) : memory_ptr(memory_ptr), memory_size(memory_size) {}
PreAllocator(const PreAllocator& other) throw() : memory_ptr(other.memory_ptr), memory_size(other.memory_size) {};
template<typename U>
PreAllocator(const PreAllocator<U>& other) throw() : memory_ptr(other.memory_ptr), memory_size(other.memory_size) {};
template<typename U>
PreAllocator& operator = (const PreAllocator<U>& other) { return *this; }
PreAllocator<T>& operator = (const PreAllocator& other) { return *this; }
~PreAllocator() {}
pointer allocate(size_type n, const void* hint = 0) { return memory_ptr; }
void deallocate(T* ptr, size_type n) {}
size_type max_size() const { return memory_size; }
};
int main()
{
int my_arr[100] = { 0 };
std::vector<int, PreAllocator<int>> my_vec(0, PreAllocator<int>(&my_arr[0], 100));
}
最佳答案
template<typename U>
PreAllocator(const PreAllocator<U>& other) throw() : memory_ptr(other.memory_ptr), memory_size(other.memory_size) {};
此构造函数尝试初始化 memory_ptr
类型 T*
来自 other.memory_ptr
类型 U*
, 不同于 T*
(因为否则会调用复制构造函数)。 std::vector
实现显然使用内部类型来实例化分配器来分配存储,并且指向该类型的指针不能隐式转换为int*
,你所期望的。您必须显式转换指针,或存储
void*
在内部,然后您将其转换为 T*
在 allocate
成员函数。template <typename T>
class PreAllocator
{
private:
void* memory_ptr;
...
public:
template<typename U>
PreAllocator(const PreAllocator<U>& other) throw() :
memory_ptr(other.memory_ptr), memory_size(other.memory_size) {}
pointer allocate(size_type n, const void* hint = 0)
{
return static_cast< pointer >(memory_ptr);
}
...
};
此外,在同一个构造函数中,您正在访问 memory_ptr
和 memory_size
PreAllocator<U>
的成员,这是与 PreAllocator<T>
不同且不相关的类型(同样,因为 T
和 U
是不同的类型)。这些成员在该类中是私有(private)的,这会给您带来访问错误。您可以通过将这些成员设为公共(public)或为它们添加公共(public)访问器,或通过使
PreAllocator
的所有特化来缓解这种情况。有这样声明的 friend :template <typename T>
class PreAllocator
{
template< typename U >
friend class PreAllocator;
...
};
关于c++ - 模板化 ctor 上的编译器错误(这是编译器错误吗?),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/65807982/