Preserving strict aliasing Union example, C++ (vc++)

Preserving strict aliasing Union example

//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

#include <iostream>
#include <windows.h>

int main()
{
    // Example of using Union to create binary compability type 
    // It preserves strict aliasing rule
    
    union FILE_NAME_INFO_UNION {
        FILE_NAME_INFO info;
        struct MY_FILENAME_INFO {
            DWORD FileNameLength;
            WCHAR FileName[MAX_PATH];
        } myInfo;
    };
    
    FILE_NAME_INFO_UNION u = FILE_NAME_INFO_UNION();
    
    std::wcout << L"sizeof: " << sizeof(u) << std::endl;
    std::wcout << L"FileNameLength offset 1: " << offsetof(FILE_NAME_INFO_UNION,info.FileNameLength) << std::endl;
    std::wcout << L"FileNameLength offset 2: " << offsetof(FILE_NAME_INFO_UNION,myInfo.FileNameLength) << std::endl;
    
    std::wcout <<  L"FileName offset 1: " << offsetof(FILE_NAME_INFO_UNION,info.FileName) << std::endl;
    std::wcout <<  L"FileName offset 2: " << offsetof(FILE_NAME_INFO_UNION,myInfo.FileName) << std::endl;
    
    std::wstring hello = L"Hello World!";
    
    // Copy to system struct:
    u.info.FileNameLength = hello.size();
    wcscpy(u.info.FileName,hello.c_str());
    
    // Read from myInfo struct:
    std::wcout << L"Output: " << u.myInfo.FileNameLength << L" " << u.info.FileName << std::endl;
}


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