c++ - 创建在 C++ 结构体中声明的数组 a[1] 的多个实例

Question

假设我有如下两个结构:

struct address{
  int x;
  int y;
} addr;

struct details{
  int count;
  int size;
  addr addre[1];// Instances of count number of addresses
} detail;                 

如何创建一个变量(例如 det)，该变量具有由计数定义的多个 addre 实例？

Answer 1

这是通过在对象末尾(具有固定大小)分配动态大小的容器来减少内存分配数量并提高引用局部性的常见技巧。

但在 C++ 中，使用额外的 member[1] 会导致一些麻烦 - 该成员会自动初始化，而其余元素则不会。最好完全避免声明该成员，而是为元素提供访问器/迭代器。然后手动初始化和销毁​​所有成员。例如:

struct address {
    int x;
    int y;
};

struct details {
    int count;
    int size;

    address* addr_begin() { return reinterpret_cast<address*>(this + 1); }
    address* addr_end() { return addr_begin() + count; }

    static void* operator new(size_t sizeof_details, int count) {
        return ::operator new(sizeof_details + count * sizeof(address));
    }

    static void operator delete(void* p) {
        ::operator delete(p);
    }

    static std::unique_ptr<details> create(int count, int size) {
        return std::unique_ptr<details>(new(count) details(count, size));
    }

    ~details() {
        std::for_each(addr_begin(), addr_end(), [](address& a) { a.~address(); });
    }

private:
    details(int count, int size)
        : count(count)
        , size(size)
    {
        std::uninitialized_fill(addr_begin(), addr_end(), address{});
    }
};

int main() {
    auto det = details::create(10, 10);
}

---

如果您无法更改结构，则:

#include <new>
#include <algorithm>

struct address {
    int x;
    int y;
};

struct details {
    int count;
    int size;
    address addre[1];
};

details* create_details(int count, int size) {
    void* mem = ::operator new(sizeof(details) + (count - 1) * sizeof(address));
    auto* p = new (mem) details{count, size};
    std::uninitialized_fill(p->addre + 1, p->addre + count, address{});
    return p;
}

void destroy_details(details* p) {
    std::for_each(p->addre + 1, p->addre + p->count, [](address& a) { a.~address(); });
    p->~details();
    ::operator delete(p);
}

int main() {
    auto* p = create_details(10, 10);
    destroy_details(p);
}