用大括号从double初始化float

Question

Why does the compiler (clang,gcc) not warn about narrowing conversions when doing this

float a{3.1231231241234123512354123512341235123541235};
float a = {double(3.1231231241234123512354123512341235123541235)}

I expected a warning because I do explicit value-initialization with braces. Following this answer Link it should spit out an error.

Compilation here

Answer 1

[dcl.init.list]/§7 (standard draft)

A narrowing conversion is an implicit conversion

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• from long double to double or float, or from double to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly) , or

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Both expressions 3.14159 and double(3.141) are constant expressions and the value is within the range of values representable by float . Therefore the conversion isn't narrowing as defined by the standard and there is no requirement to warn about the conversion.

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but it does not give a warning also for longer inputs

Sure it does , as long as the value is outside of the range of values representable by float .

Answer 2

Because the source is a constant expression and overflow does not occur for these cases, then narrowing conversion error won't be triggered.

(emphasis mine)

conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression and overflow does not occur

If you use it with a double variable(ie a non-constant-expression) or a constant with big value which causes overlow, the diagnostic message will be generated. eg

double d = 3.14159;
float a {d}; // non-constant-expression cannot be narrowed from type 'double' to 'float' in initializer list

EDIT (for longer input)

Because even if the value cannot be represented exactly by float , overflow still doesn't occur, then it's allowed.

$8.6.4/7.2 List-initialization (emphasie mine)

from long double to double or float, or from double to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented ( even if it cannot be represented exactly ), or