Viewing double bit pattern in Visual Studio C++ Debugger

Question

I'm working with IEEE-754 doubles, and I'd like to verify that the bit patterns match between different platforms. For this reason I would like to see the bit pattern of a double in the Visual Studio C++ Debugger.

I've tried format specifiers, but they don't seem to allow me to format a double as anything which would allow me to see the bit pattern.

One way I finally found was to use Memory View and enter the address of the variable (&x) in the address field. This allows me to set for instance 8-bit integer hex display, which gives me what I need. But is there any other more convenient way of formatting a double this way in the debugger?

user_995714 · Answer

To view the exact binary floating-point value you should print the it as hexadecimal with %a/%A or std::hexfloat instead of examining its bit pattern

printf("Hexadecimal: %a %A
", 1.5, 1.5);
std::out << std::hexfloat << 1.5 << '
';

However if you really need to view the actual bit pattern then you just need to reinterpret the type of the underlying memory region like auto bits = reinterpret_cast(doubleValue). You don't need to open the Memory View to achieve this, a simple cast would work in the Watch window. So to get the bit pattern of double and float use *(__int64*)&doubleValue,x and *(int*)&floatValue,x respectively. Strict aliasing does occur but you don't actually need to care about it in MSVC debugger
_{Note that __int64 is a built-in type of MSVC so you might want to use long long instead. Typedefs and macros like uint64_t won't work while watching}

Alternatively you can access the bytes separately by casting to char* and print as an array with (char*)&doubleValue, 8, (char*)&floatValue, 4 or (char*)&floatingPoint, [sizeof floatingPoint]. This time strict aliasing doesn't occur but the output may be less readable

Viewing double bit pattern in Visual Studio C++ Debugger

1 Answers