Non-uniform byte initialization

February 4, 2024

Is there a reason, simple enough to explain to non language lawyers, why the initializations commented out fail to compile while the first one succeeds?

#include <cstddef>

void test() {
  std::byte a{42};
  //std::byte b = {42};
  //std::byte c[]{48, 49};
}

Code on https://godbolt.org/z/nGfzjnh4f

The error is:

error: cannot initialize a variable of type 'std::byte' with an rvalue of type 'int'

>Solution :

Because std::byte is an enum class and std::byte b = {42} is not direct-list-initialization which uses C++17’s relaxed enum class initialization rules and only allow for direct list initialization.

This can be seen from relaxed enum class initialization rules:

An enumeration can be initialized from an integer without a cast, using list initialization, if all of the following are true:

The initialization is direct-list-initialization.

The initializer list has only a single element.

The enumeration is either scoped or unscoped with underlying type fixed.

The conversion is non-narrowing.

(emphasis mine)

And since std::byte b = {42} doesn’t satisfy the first bullet requirement, we get the mentioned error.