C++11: "narrowing conversion inside { }" with modulus

Question

I try to compile the following code with `gcc` and `C++11` enabled: ``` unsigned int id = 100; unsigned char array[] = { id % 3, id % 5 }; ``` I get these warnings: narrowing conversion of ‘(id % 3u)’ from ‘unsigned int’ to ‘unsigned char’ inside { } [-Wnarrowing] see demo online Is there a way to help the compiler find out that the result of id % 3 fits into an unsigned char?

Accepted Answer

In this specific case making `id` const or constexpr will fix the problem: ``` constexpr unsigned int id = 100; ``` since there is an exception for the case where you have a constant expression whose result after conversion will fit into the target type. In the more general case you may also use static_cast to cast the result to unsigned char: ``` { static_cast( id % 3), static_cast( id % 5) } ^^^^^^^^^^^ ^^^^^^^^^^^ ``` We can find he exception for constant expressions and narrowing conversions in the draft C++ standard section `8.5.4` List-initialization which says: A narrowing conversion is an implicit conversion and include the following bullet (emphasis mine): - from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression whose value after integral promotions will fit into the target type. Note, the wording changed from the original draft C++11 standard to what I quote above due to defect report 1449.

C++11: "narrowing conversion inside { }" with modulus

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