阵列内存分配不工作 [英] Array memory Allocation doesn't work

问题描述

我有下面的课程：

  A类{
} 
 
 class B：public A {
 int num; 
}; 
  

在我的主要有：

  int main（）{
 A * vec; // A是一个带有纯虚函数的类
 vec = new B [2]; //想创建一个向量B 
} 
  

vec [0] ，但vec [1]为NULL。为什么没有给我分配合适的记忆？

我不想改变主线。

（我知道我可以把main改成：B * vec = new B [2]但我不想要）

解决方案

不能以多态方式处理数组，C ++语言不支持它。表达式 vec [x] 使用指针算术来确定元素的位置。如果你通过一个基类指针访问它，如果对象的大小以任何方式变化将不会工作。

例如，你的基类是4字节大小，子类大小为8字节。

  base * a = new child [4] 
  

当您访问 a [1] 编译器使用基类的大小计算偏移量。在这种情况下，偏移量为4个字节，最后指向第一个元素的中间。

我建议使用 std :: vector 或 std :: array 的指针。

  //对于需要调整大小的数组（需要为每个新的删除）
 std :: vector< A *> vec（5，NULL）; 
 for（int i = 0; i< vec.size（）; i ++）
 {
 vec [i] = new B（）; 
} 
 
 //对于大小固定的数组（每个新需要删除）
 std :: array< A *，5> vec; 
 for（int i = 0; i< vec.size（）; i ++）
 {
 vec [i] = new B（）; 
} 
 
 //对于用智能指针固定大小的数组
 //不需要删除
 std :: array< std :: unique_ptr< A> ，5> vec; 
 for（int i = 0; i< vec.size（）; i ++）
 {
 vec [i] .reset（new B（））; 
} 
  

I have the next classes:

class A {
};

class B : public A {
  int num;
};

in my main I have:

int main() {
    A* vec; // A is a class with pure virtual functions
    vec = new B[2]; // want to create a vector of B
}

vec[0] is defined correctly, but vec[1] is NULL. why didn't it allocate me a fit memory?

I don't want to change the lines of the main. just make it working.

(I know I can change the main into: B* vec = new B[2] but I don't want)

any help appreciated!

解决方案

You cannot treat arrays polymorphically, the C++ language does not support it. The expression vec[x] uses pointer arithmetic to determine the location of the element. If you are accessing it through a base class pointer it will not work if the size of the objects vary in any way.

For example, you have base class that is 4 bytes in size and the subclass is 8 bytes in size.

base *a = new child[4];

When you access a[1] the compiler calculates the offset using the size of the base class. In this case the offset is 4 bytes which ends up pointing to the middle of the first element.

I recommend using a std::vector or std::array of pointers with an appropriate smart pointer.

// For arrays that needs to be resized (requires delete for each new)
std::vector<A*> vec(5, NULL);
for(int i = 0; i < vec.size(); i++)
{
    vec[i] = new B();
}

// for arrays that are fixed in size (requires delete for each new)
std::array<A*, 5> vec;
for(int i = 0; i < vec.size(); i++)
{
    vec[i] = new B();
}

// for arrays that are fixed in size with smart pointers
// no delete needed 
std::array<std::unique_ptr<A>, 5> vec;
for(int i = 0; i < vec.size(); i++)
{
    vec[i].reset(new B());
}

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