The sizeof() function followed by square brackets

Question

I've seen an example showing as

int n = sizeof(0)["abcdefghij"];
cout<<n;

What does that thing in square brackets mean? I've read somewhere that (0)["abc"] is equivalent to ("abc")[0]. Meaning the above expression is simply

n = sizeof("abcdefghij")[0];

i.e. the first element.

`"abcdefghij"` is just `const char [N]`, i.e. an array, so it's equivalent to `sizeof("abcdefghij"[0])` because [`sizeof` has higher precedence than `[]`](https://en.cppreference.com/w/cpp/language/operator_precedence) — phuclv, Feb 07 '21 at 06:34
@phuclv - You had correct initially, pre-edit. It's not `sizeof("..."[0])`. — StoryTeller - Unslander Monica, Feb 07 '21 at 06:40
Naval, this example has a pretty poor practical value, if any at all. It may be a cool puzzle, but is not indicative of any real world construct. — StoryTeller - Unslander Monica, Feb 07 '21 at 06:43
@StoryTeller-UnslanderMonica the precedence is wrong but the result is actually correct. I've checked it — phuclv, Feb 07 '21 at 06:58
@phuclv - I'm surprised. Though not by a lot. It's those darn parentheses. They only carry special meaning if they contain a typename. — StoryTeller - Unslander Monica, Feb 07 '21 at 07:02
another duplicate: [Why does `sizeof(my_arr)[0]` compile and equal `sizeof(my_arr[0])`?](https://stackoverflow.com/q/46653406/995714) — phuclv, Feb 09 '21 at 10:19

phuclv · Accepted Answer · 2021-03-14T05:33:37.200

First, sizeof is not a function but an operator

sizeof(0)["abcdefghij"] can be parsed as either

sizeof( (0)["abcdefghij"] ), or
( sizeof(0) )["abcdefghij"]

Since sizeof has lower precedence than [], the former will take place

(0)["abcdefghij"] is equivalent to "abcdefghij"[0] which is just 'a', so the whole thing is the same as sizeof('a') which is 1 in C++

Demo on GodBolt, ideone

If you replace sizeof(0) with sizeof(int) then the same thing happens, but now (int)["abcdefghij"] is invalid so it should result in a compilation fail. Most compilers report an error as expected that except ICC so it looks like that's an ICC bug which chooses (sizeof(int))["abcdefghij"] over sizeof((int)["abcdefghij"]) just because the latter is invalid

Related: Why does sizeof(my_arr)[0] compile and equal sizeof(my_arr[0])?

It would be an interesting exercise to `s/0/int` and discuss the difference. — StoryTeller - Unslander Monica, Feb 07 '21 at 07:03

The sizeof() function followed by square brackets

1 Answers1