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  1.     #include <iostream>
  2.     using namespace std;
  3.  
  4.     class Base
  5.     {
  6.     public:
  7.     	Base() {};
  8.     	virtual ~Base() {};
  9.  
  10.     	void* operator new[](size_t size)
  11.     	{
  12.     		size_t* ptr = static_cast<size_t*>(malloc((size + 1) * sizeof(size_t)));
  13.     		if (ptr == nullptr)
  14.     		{
  15.     			throw std::bad_alloc();
  16.     		}
  17.     		*ptr = size;
  18.     		return (ptr + sizeof(size_t));
  19.     	}
  20.  
  21.     	void operator delete[](void* ptr, size_t size)
  22.     	{
  23.     		if (ptr != nullptr)
  24.     		{
  25.     			free(static_cast<size_t*>(ptr) - sizeof(size_t));
  26.     		}
  27.     	}
  28.     };
  29.  
  30.     class Derived : public Base
  31.     {
  32.     public:
  33.     	Derived(int value) :
  34.     		m_value(value)
  35.     		{
  36.     		}
  37.  
  38.     private:
  39.     	int m_value;
  40.     };
  41.  
  42.     int main() {
  43.     	Derived* i = new Derived[18] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 42, 100 };
  44.     	void* i2 = i;
  45.     	size_t* size = (static_cast<size_t*>(i2) - sizeof(size_t));
  46.     	printf("Size: '%d'\n", (int)*size);
  47.     	return 0;
  48.     }
  49.  
Success #stdin #stdout 0s 3468KB
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Size: '0'
https://ideone.com/sfjBMo
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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