Why is 'new int (*)[10]' wrong?

Question

I tried this code:

auto p = new int (*)[10];

but I got error messeage:

test.cc:8:21: error: expected expression
        auto p = new int (*)[10];
                           ^
1 error generated.

I changed my code:

typedef int array[10];
auto p = new array *;

And then everything goes well. Why is this?

If that was really the only error message you got it kinda sucks, because it doesn't do much to identify the issue for you. What toolchain was that from anyway? — WhozCraig, Mar 07 '22 at 12:31
@WhozCraig Both GCC and Clang give similar such error messages. MSVC is even worse and just says `syntax error: ')'`. But the error is exactly correct (but maybe not easy to understand), there should be an expression after the unary `*` operator. — user17732522, Mar 07 '22 at 12:34
@user17732522 interesting. my clang (apple 1300) reports: "this operator is not allowed at this point; parenthesize the preceding new expression" . must have gotten ahold of some special sauce. — WhozCraig, Mar 07 '22 at 12:43
@WhozCraig That's what nvcc says: https://godbolt.org/z/Yo1jbjoPM and ICC similarly: https://godbolt.org/z/bbqcfanYs — user17732522, Mar 07 '22 at 12:50
In modern C++ explicit use of `new` is a bad practice. In this case you should use `std::vector` or `std::array`. — Marek R, Mar 07 '22 at 12:58
new has its uses, particularly in polymorphism so I wouldn't call it bad practice. in this case those container classes would indeed be more appropriate and clearer. — Abel, Mar 07 '22 at 13:31

score 15 · Answer 1 · answered Mar 07 '22 at 12:34

Syntax for new without initializer is either

new (type)

or

new type

In the second case type may not contain parenthesis. This is also demonstrated in the above linked page:

new int(*[10])();    // error: parsed as (new int) (*[10]) ()
new (int (*[10])()); // okay: allocates an array of 10 pointers to functions

For your case that means:

auto p = new int (*)[10];     // error: parsed as (new int) ((*)[10])
auto p = new ( int (*)[10] ); // ok

When you use the alias, you can write

auto p = new array *;

because here type does not contain parenthesis.

1 Answers1