MSVC brace initialization with doubles appears to violate the standard?

Question

Check out this simple program:

int main() {
    float f2 = 7.2; // OK, with warning
    float f3 = 7.199999809265137; // OK, no warning
    float f4{ 7.2 };          


        

Answer 1

It looks like a bug, indeed. For a workaround, the following appears to silence both errors and warnings in MSVC 2015.

#pragma float_control(precise, off, push)

float f2 = 7.2; // OK, with warning
//...

#pragma float_control(precise, pop)

The same works globally if using the /fp:fast compiler switch, though that one is incompatible with /Za which disables MS language extensions.

Answer 2

Some floating point numbers can be exactly expressed in a float representation and some can't. If the number can be represented in the form x / 2^y where x is any integer and y is an integer 23 or less, it fits. Most decimal numbers can't be represented in this way, as a binary number they repeat forever. 7.2 is one example.

You can fix this easily by appending f to each number, to indicate to the compiler that this is a float constant rather than a double.

float f4{ 7.2f };