Question

Let’s say I have the class Foo. The following works fine:

class Foo
{
    public:
        const int* bar;

        Foo()
        {
            bar = new int[2] {1, 2};
        }
};

However, I tried to change this very slightly to use a template:

template<int A, int B>
class Foo
{
    public:
        const int* bar;

        Foo()
        {
            bar = new int[2] {A, B};
        }
};

My understanding of the way templates work is that A and B are essentially compile time constants, so it should still work the same.

The error message I get when compiling with g++ (4.5 in the link, same error with 4.6.3) is:

error: ISO C++ forbids initialization in array new [-fpermissive]

With 4.7 a similar error occurs, though slightly different:

error: parenthesized initializer in array new [-fpermissive]

The problem also occurs in template functions, and not just when template parameters are used within the braces for initialization, code and output. (thanks Philipp)

Answer 1

Looks like this is a GCC bug. Clang accepts it, and the standard allows it:

new-initializer:

( expression-list_opt)
braced-init-list

And the rules for this initialization are not special:

A new-expression that creates an object of type T initializes that
object as follows:

— If the new-initializer is omitted, the object is
default-initialized (8.5); if no initialization is performed, the
object has indeterminate value.

— Otherwise, the new-initializer is
interpreted according to the initialization rules of 8.5 for direct-initialization.