How many space should be reserved for dynamic class with placement new

[q1670741824]

with placement new, it turned out that sizeof operator can't tell me the real space needed to set an array (buffer) to hold two successive class object.

i set a class, whose size is 28(sizeof told me), so i set a int *buf = new int[14];//for 14*4 = 56 to give just adequate space for 2 object. but while run time, error occurs. and i have to give a lot of additional space.

#include <iostream>
#include <string>
#include <new>

class goodday
{
	std::string a;
public:
	goodday(){ a = "hello"; }
	const std::string & val()const{ return a; }
	~goodday(){}
};

int main()
{
	std::cout << sizeof(goodday);
	int *holder = new int[14];
	goodday *pa = new (holder)goodday;
	goodday *pb = new (holder + sizeof(goodday))goodday;
	std::cout << pa->val() << pb->val();
	pb->~goodday();
	pa->~goodday();
	delete[] holder;
	std::cin.get();
}

but what interesting is that while i give char *holder = new char[56];//56*1=56 it works, no error occurs.

why?

[Aceix]

int *holder = new int[14];

Note that arrays are static pointers so basically holder is a pointer to a "static pointer."

Aceix.

[coder777]

why?

the problem is the pointer arithmetic

holder + sizeof(goodday) is the same as &holder[sizeof(goodday)]

in other words: the pointer is incremented by sizeof(goodday) * sizeof(int)