The below fails to compile:
typedef int arr[10]; int main(void) { return sizeof arr; } sizeof.c:3: error: expected expression before ‘arr’
but if I change it to
sizeof(arr);
everything is fine. Why?
In C language, sizeof( ) is an operator. Though it looks like a function, it is an unary operator.
Sizeof is a much used operator in the C or C++. It is a compile time unary operator which can be used to compute the size of its operand. The result of sizeof is of unsigned integral type which is usually denoted by size_t.
According to 6.5.3, there are two forms for sizeof
as the following:
sizeof unary-expression sizeof ( type-name )
Since arr
in your code is a type-name
, it has to be parenthesized.
That's the way the language is specified, type names must be parenthesized here.
Suppose the grammar looked like this:
sizeof unary-expression
sizeof type-name
Now, e.g. the following expression would be ambiguous:
sizeof int * + 0
It could be either sizeof(int *) + 0
or sizeof(int) * +0
. This ambiguity doesn't arise for unary expressions, as an asterisk appended to an expression isn't an expression (but for some type names, appending one, is again a type name).
Something had to be specified here and requiring type-names to be parenthesized is a way to solve the ambiguity.
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