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    <p>The <code>*_PTR</code> types were added to the Windows API in order to support Win64's 64bit addressing.</p> <p>Because 32bit APIs typically passed pointers using data types like <code>DWORD</code>, it was necessary to create new types for 64 bit compatibility that could substitute for <code>DWORD</code> in 32bit applications, but were extended to 64bits when used in a 64bit applications.</p> <p>So, for example, application developers who want to write code that works as 32bit OR 64bit the windows 32bit API <code>SetWindowLong(HWND,int,LONG)</code> was changed to <code>SetWindowLongPtr(HWND,int,LONG_PTR)</code></p> <p>In a 32bit build, <code>SetWindowLongPtr</code> is simply a macro that resolves to <code>SetWindowLong</code>, and <code>LONG_PTR</code> is likewise a macro that resolves to <code>LONG</code>. In a 64bit build on the other hand, <code>SetWindowLongPtr</code> is an API that accepts a 64bit long as its 3rd parameter, and <code>ULONG_PTR</code> is typedef for <code>unsigned __int64</code>.</p> <p>By using these <code>_PTR</code> types, one codebase can compile for both Win32 and Win64 targets.</p> <hr> <p>When performing pointer arithmetic, these types should also be used in 32bit code that needs to be compatible with 64bit.</p> <p>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</p> <pre><code> 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. </code></pre>
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    1. COI would add that, to use them, you do an #include <windows.h> with a new'ish compiler system.
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