来自另一个 static 常量变量的 static 常量变量给出编译错误？

Question

Why this compiles:

static const short test_value = 0x0100;
static const uint8_t *is_big_endian = ((const uint8_t *) &test_value);

but this doesn't:

static const short test_value = 0x0100;
static const uint8_t is_big_endian = *((const uint8_t *) &test_value);

with the error:

expression must have a constant value

I made up some false sense about it and tried the following:

static uint8_t is_big_endian = *((const uint8_t *) &test_value);

Which also didn't compile and produced the same error!
I also have tried to play with the location of the const but nothing worked.

I'm using Visual Studio 2019.

Edit:
It seems to work in C++ but not C. So now my questions are:

1. Why it isn't compiling in C?
2. What C++ does different that enables it?

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Answer 1

The C standard does not require compilers to figure out the bytes that make up an object while evaluating expressions for initializers. So, while *((const uint8_t *) &test_value) might have a logically determined value that you can figure out from knowledge of test_value , the compiler is not required to figure it out.

C 2018 6.8 7 says a constant expression used in an initializer may be an arithmetic constant expression, a null pointer constant, an address constant, or an address constant plus or minus an integer constant expression. The expression *((const uint8_t *) &test_value) is not any of the latter three forms, since they are pointers of various kinds. Let's consider the first form, an arithmetic constant expression.

C 2018 6.8 8 defines arithmetic constant expression :

An arithmetic constant expression shall have arithmetic type and shall only have operands that are integer constants, floating constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, and _Alignof expressions. Cast operators in an arithmetic constant expression shall only convert arithmetic types to arithmetic types, except as part of an operand to a sizeof or _Alignof operator.

So *((const uint8_t *) &test_value) is not an arithmetic constant expression because it contains a cast of a pointer, not an arithmetic type, and that cast is not part of an operand to sizeof or _Alignof . It also contains test_value as an operand, and that is not one of the list permissible kinds of operand. It also contains &test_value as an operand, and that is also not one of the permissible kinds.