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Simon Anciaux
1184 posts
Warning to detect initialization of a double with a float literal
Edited by Simon Anciaux on

While I was testing a floating point to string function, I initialized a double variable with a float literal. And the result I got when printing the variable weren't what I expected. Here are the value, bit pattern and print result:

float f =  123400000000000000000000000.0f;
// 01101010'11001100'00100101'11101111
// Prints: 1.234e+26

double d = 123400000000000000000000000.0f;
// 01000101'01011001'10000100'10111101'11100000'00000000'00000000'00000000
// Prints: 1.234000017665421e+26

double d2 = 123400000000000000000000000.0;
// 01000101'01011001'10000100'10111101'11011001'11011110'11111110'11010110
// 1.234e+26

We can see that second value's bit pattern got zero appended, which result in the different print out. Which is expected and OK.

My question is: is there a way to get a warning from MSVC, Clang and GCC when initializing a double variable using a float literal ? I searched a bit for MVSC but couldn't find one.

Mārtiņš Možeiko
2356 posts / 2 projects
Warning to detect initialization of a double with a float literal

I'm aware only of clang's -Wdouble-promotion warning for this. Don't think others have any warnings for this.

Simon Anciaux
1184 posts
Warning to detect initialization of a double with a float literal
Edited by Simon Anciaux on

Thanks.

Do you know of a good place to find Clang flag list with descriptions of what they do ?

Edit: I just found that https://clang.llvm.org/docs/DiagnosticsReference.html