CODEWITHSUNDEEP
c++cvisual-c++sizeofcompile-time-constant
xmllmx
xmllmx

Reputation: 42215

Why is the sizeof expression not a compile-time constant like 2, 4, 8, etc.?

My compiler is the latest VC++ 2013 preview.

int main()
{
    __declspec(align(4))           int n1 = 0; // OK.
    __declspec(align(sizeof(int))) int n2 = 0; // error C2059: syntax error : 'sizeof'
}

Why is the sizeof expression not a compile-time constant like 2, 4, 8, etc.?

Upvotes: 3

Views: 2310

Answers (2)

Jesse Chisholm
Jesse Chisholm

Reputation: 4005

Rather than asking: Why is the sizeof expression not a compile-time constant like 2, 4, 8, etc.?

(because, actually, it is a compile time constant just like those examples. (: barring Variable Length Arrays from the newer C standards, for which it must be a run time expression :))

It would be better to ask: Why is it that align(...) does not accept a compile time constant like a sizeof expression?

Microsoft defined __declspec(align(#)) to only accept a small set of values: See: https://msdn.microsoft.com/en-us/library/83ythb65.aspx

"# is the alignment value. Valid entries are integer powers of two from 1 to 8192 (bytes), such as 2, 4, 8, 16, 32, or 64."

So even with simple constants, not just any value is allowed. __declspec(align(7)) would not be allowed, as it isn't a power of 2. Even simple expressions like __declspec(align(4+4)) are not allowed.

Upvotes: 4

user1781626
user1781626

Reputation:

In the C Standard

The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member. The _Alignof operator shall not be applied to a function type or an incomplete type.

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

Upvotes: 0

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