c – 是否可以在已分配的内存上初始化std :: vector？

T1 *buffer = new T1[some_size];
std::vector<T2> v(buffer,some_sizE); // <- ofc doesn't work

std::vector<T2> v(some_sizE);
T1 *buffer = &v[0];

像这样..标准中的容器通常采用分配器.使用c 11的分配器特性,创建分配器非常容易,因为您不必拥有分配器中的所有成员.但是,如果使用旧版本的C,则需要实现每个成员并进行重新绑定！

对于Pre-C 11,您可以使用以下内容：

#include <iterator>
#include <vector>
#include <iostream>

template<typename T>
class PreAllocator
{
    private:
        T* memory_ptr;
        std::size_t memory_size;

    public:
        typedef std::size_t size_type;
        typedef ptrdiff_t difference_type;
        typedef T* pointer;
        typedef const T* const_pointer;
        typedef T& reference;
        typedef const T& const_reference;
        typedef T value_type;


        PreAllocator(T* memory_ptr,std::size_t memory_sizE) throw() : 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 address (reference value) const {return &value;}
        const_pointer address (const_reference value) const {return &value;}

        pointer allocate (size_type n,const void* hint = 0) {return memory_ptr;}
        void deallocate (T* ptr,size_type n) {}

        void construct (pointer ptr,const T& val) {new (ptr) T (val);}

        template<typename U>
        void destroy (U* ptr) {ptr->~U();}
        void destroy (pointer ptr) {ptr->~T();}

        size_type max_size() const {return memory_size;}

        template<typename U>
        struct rebind
        {
            typedef PreAllocator<U> other;
        };
};

int main()
{
    int my_arr[100] = {0};
    std::vector<int,PreAllocator<int> > my_vec(PreAllocator<int>(&my_arr[0],100));
    my_vec.push_BACk(1024);
    std::cout<<"My_Vec[0]: "<<my_vec[0]<<"\n";
    std::cout<<"My_Arr[0]: "<<my_arr[0]<<"\n";

    int* my_heap_ptr = new int[100]();
    std::vector<int,PreAllocator<int> > my_heap_vec(PreAllocator<int>(&my_heap_ptr[0],100));
    my_heap_vec.push_BACk(1024);
    std::cout<<"My_Heap_Vec[0]: "<<my_heap_vec[0]<<"\n";
    std::cout<<"My_Heap_Ptr[0]: "<<my_heap_ptr[0]<<"\n";

    delete[] my_heap_ptr;
    my_heap_ptr = NULL;
}

对于C 11,您可以使用以下内容：

#include <cstdint>
#include <iterator>
#include <vector>
#include <iostream>

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],PreAllocator<int>> my_heap_vec(0,PreAllocator<int>(&my_heap_ptr[0],100));
    my_heap_vec.push_BACk(1024);
    std::cout<<"My_Heap_Vec[0]: "<<my_heap_vec[0]<<"\n";
    std::cout<<"My_Heap_Ptr[0]: "<<my_heap_ptr[0]<<"\n";

    delete[] my_heap_ptr;
    my_heap_ptr = nullptr;
}

注意两个分配器之间的区别！这将适用于堆缓冲区/数组和堆栈缓冲区/数组.它也适用于大多数容器.使用Pre-C 11版本更安全,因为它将向后兼容并使用更多容器(即：std :: List).

您可以将分配器放在标题中,并在任何项目中尽可能多地使用它.如果要使用SharedMemory或已分配的任何缓冲区,这是很好的.

警告：
不要同时为多个容器使用相同的缓冲区！缓冲区可以重复使用,但只要确保没有两个容器同时使用它.

例：

int my_arr[100] = {0};
std::vector<int,100));
std::vector<int,PreAllocator<int> > my_vec2(PreAllocator<int>(&my_arr[0],100));

my_vec.push_BACk(1024);
my_vec2.push_BACk(2048);

std::cout<<"My_Vec[0]: "<<my_vec[0]<<"\n";
std::cout<<"My_Arr[0]: "<<my_arr[0]<<"\n";

以上的输出是2048！为什么？因为最后一个向量覆盖了第一个向量的值,因为它们共享相同的缓冲区.