The *_PTR types
were added to the Windows API in order to support Win64's 64bit addressing.
Because 32bit APIs are used to store pointers to things using data types like DWORDS,
it was necessary to create new types for 64 bit compatibility that acted like a DWORD in
32bits, but were extended to 64bits when used in a 64bit app.
So, for example, application developers who want to write code that works as 32bit OR 64bit the windows 32bit API SetWindowLong(HWND,int,LONG) was
changed to SetWindowLongPtr(HWND,int,LONG_PTR)`
In a 32bit build, SetWindowLongPtr is
simply a macro that resolves to SetWindowLong,
and LONG_PTR is
likewise a macro that resolves to LONG.
In a 64bit build on the other hand, SetWindowLongPtr is
an API that accepts a 64bit long as its 3rd parameter, and ULONG_PTR is
typedef for unsigned
__int64.
By using these _PTR types,
one codebase can compile for both Win32 and Win64 targets.
When performing pointer arithmetic, these types should also be used in 32bit code that needs to be compatible with 64bit.
so, if you need to access an array with more than 4billion elements, you would need to use an INT_PTR rather than an INT
CHAR* pHuge = new CHAR[0x200000000]; // allocate 8 billion bytes
INT idx;
INT_PTR idx2;
pHuge[idx]; // can only access the 1st 4 billion elements.
pHuge[idx2]; // can access all 64bits of potential array space.目的是爲了在32位系統和64位系統保持兼容。舉例來說， INT_PTR在32位系統下表示32bit的值，在64位系統下可表示64位的值。就是這樣。