QVector 要求默认构造函数的原因是什么?

  
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问题描述

我可以看到这些类被视为调用默认构造函数所需的复杂对象:

I can see that that classes are treated as complex objects which are required for calling default constructor:

void QVector<T>::defaultConstruct(T *from, T *to)
{
    if (QTypeInfo<T>::isComplex) {
        while (from != to) {
            new (from++) T();
        }
    ...
}

但不清楚为什么需要在 QVector 的隐藏"区域中构造对象.我的意思是这些对象根本无法访问，那么为什么不只是保留内存而不是真正的对象创建呢?

But it's not clear why is it needed to construct objects in the 'hidden' area of QVector. I mean these objects are not accessible at all, so why not just to reserve the memory instead of the real object creation?

作为一个额外的问题，我想问一下，如果我想要一个非默认可收缩对象数组，我可以安全地将 QVector 替换为 QVector<;包装器<T>?其中 Wrapper 是这样的:

And as a bonus question, I would like to ask, if I want to have an array of non-default-constractible objects, can I safely replace QVector<T> with QVector<Wrapper<T>? where Wrapper is something like that:

class Wrapper {
public:
    union {
        T object;
        bool hack;
    };
    Wrapper() {}
    Wrapper(const T &t) : object { t }  {}
    Wrapper(const Wrapper &t) : object { t.object } {}

    Wrapper &operator=(const Wrapper &value) {
        object = value.object;
        return *this;
    }

    ~Wrapper() {}
};

推荐答案

很容易让 QVector 为非默认可构造类型 T 工作:

It's easy enough to make the QVector work for a non-default-constructible type T:

#define QVECTOR_NON_DEFAULT_CONSTRUCTIBLE(Type) 
template <> QVector<Type>::QVector(int) = delete; 
template <> void QVector<Type>::resize(int newSize) { 
   Q_ASSERT(newSize <= size()); 
   detach(); 
} 
template <> void QVector<Type>::defaultConstruct(Type*, Type*) { Q_ASSERT(false); }

宏需要出现在 MyType 声明之后 - 在头文件(如果有)中，并且必须在命名空间或全局范围内:

The macro needs to be present right after MyType declaration - in the header file (if any), and it must be in namespace or global scope:

struct MyType { ... };
QVECTOR_NON_DEFAULT_CONSTRUCTIBLE(MyType)

struct A {
  struct MyType2 { ... };
};
QVECTOR_NON_DEFAULT_CONSTRUCTIBLE(A::MyType2);

不，包装器不正确.它不会破坏 object 成员.它也不提供移动语义，不保护默认构造等. hack 联合成员不是必需的.联合中的任何内容都不会为您默认构建.

No, the wrapper is not correct. It doesn't destruct the object member. It also doesn't offer move semantics, doesn't protect from being default-constructed, etc. The hack union member is not necessary. Nothing in a union will be default-constructed for you.

这是一个更正确的包装器 - 它非常类似于 std::optional.查看 这里 了解 optional 的细微差别代码>需要:)

Here's a more correct wrapper - it pretty much resembles std::optional. See here to see how much nuance an optional needs :)

// https://github.com/KubaO/stackoverflown/tree/master/questions/vector-nodefault-33380402

template <typename T> class Wrapper final {
   union {
      T object;
   };
   bool no_object = false;
   void cond_destruct() {
      if (!no_object)
         object.~T();
      no_object = true;
   }
public:
   Wrapper() : no_object(true) {}
   Wrapper(const Wrapper &o) : no_object(o.no_object) {
      if (!no_object)
         new (&object) T(o.object);
   }
   Wrapper(Wrapper &&o) : no_object(o.no_object) {
      if (!no_object)
         new (&object) T(std::move(o.object));
   }
   Wrapper(const T &o) : object(o) {}
   Wrapper(T &&o) : object(std::move(o)) {}
   template <class...Args> Wrapper(Args...args) : object(std::forward<Args>(args)...) {}
   template <class U, class...Args> Wrapper(std::initializer_list<U> init, Args...args) :
      object(init, std::forward<Args>(args)...) {}
   operator T&      () &      { assert(!no_object); return object; }
   operator T&&     () &&     { assert(!no_object); return std::move(object); }
   operator T const&() const& { assert(!no_object); return object; }
   Wrapper &operator=(const Wrapper &o) & {
      if (no_object)
         ::new (&object) T(o);
      else
         object = o.object;
      no_object = false;
      return *this;
   }
   Wrapper &operator=(Wrapper &&o) & {
      if (no_object)
         ::new (&object) T(std::move(o.object));
      else
         object = std::move(o.object);
      no_object = false;
      return *this;
   }
   template<class... Args> T &emplace(Args&&... args) {
      cond_destruct();
      ::new (&object) T(std::forward<Args>(args)...);
      no_object = false;
      return object;
   }
   ~Wrapper() {
      cond_destruct();
   }
};

由于赋值运算符是 ref-qualified，它不允许分配给右值，因此它具有以下不会编译的 IMHO 正属性:

Since the assignment operators are ref-qualified, it disallows assigning to rvalues, so it has the IMHO positive property that the following won't compile:

Wrapper<int>() = 1   // likely Wrapper<int>() == 1 was intended

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