在 C++ 中释放分配器

Question

我定义了一个 class 模板 Vec，它包含一个 T 序列，例如向量：

template <typename T> class Vec {
    public:
        size_t size() const { return first_free - elements; }
        size_t capacity() const { return cap - elements; }
        T *begin() const { return elements; }
        T *end() const { return first_free; }
        void resize(const size_t); // something wrong in this function
        ... 
    private:
        allocator<T> alloc; //allocates the elements
        T *elements; // pointer to the first element in the array
        T *first_free; // pointer to the first free element in the array
        T *cap; // pointer to one past the end of the array
        ...
};

当我的代码调用这个函数时，程序会崩溃，

template <typename T> void Vec<T>::resize(const size_t newcap)
{
    // this part goes well
    if(newcap > capacity()) {
        auto newdata = alloc.allocate(newcap);
        auto dest = newdata;
        auto orig = elements;
        for(size_t i = 0; i != size(); ++i) 
            alloc.construct(dest++, std::move(*orig++));
        elements = newdata;
        first_free = dest;
        cap = elements + newcap;
    }
    // crash happens in the following two parts
    else if(size() <= newcap && newcap < capacity()) { // deallocate the redundant memory where no element exists
       alloc.deallocate(elements + newcap, capacity() - newcap); // this line causes crash 
       cap = elements + newcap; 
    }   
    else if(size() < newcap) { // destroy the elements and deallocate the memory
        for(auto p = first_free; p != elements + newcap; /*empty*/)
            alloc.destroy(--p);
        alloc.deallocate(elements + newcap, capacity() - newcap); // this line causes crash 
        first_free = cap = elements + newcap;
    }
}

调用代码：

Vec<string> v{"aaa", "bbb", "ccc"}; // size() == 3, capacity == 3
v.resize(2); // to resize v to size() == 2, capacity == 2

我想我已经进行了正确的解除分配调用和正确的指针算法。谢谢。

Answer 1

alloc.deallocate(elements + newcap, capacity() - newcap);

你不能那样做。 Allocator are all or nothing：你要么取消分配整个分配的块，要么不理会它。

要缩小分配的内存，执行与新大小大于当前容量时相同的分配和复制操作，但分配新的较小大小。

顺便说一句，标记为//this part goes well的部分有一个主要问题：它从不释放旧内存块。

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