Static array size determined from template values

Question

I'm trying to use static array which size needs to be determined by given template values. However size will be constant across program runtime - thats why I decided not to use std::vector.

template<uint32_t BAR_WIDTH>
class Bar 
{
//do_stuff...
Foo mapper[ [&]()->int{ uint32_t tmp = BAR_WIDTH / Foo:FOO_EDGE; return (BAR_WIDTH % 10 == 0) ? tmp : tmp + 1; }; ];
};

FOO_EGDE is const static value. IDE gives me a hint that

Array size expression must have an integer type instead of int(*)()

I wonder if I can make it work this way without using std::vector. Any advice is welcomed and appreciated.

The error message is telling you the problem - your array size isn't an integer, it's a lambda taking no arguments and returning `int`. — Chowlett, Aug 16 '17 at 08:10

score 3 · Accepted Answer · answered Aug 16 '17 at 08:15

3

The problem is, that you are using a lambda for determining the size of the array. If you leave it off and just use the ternary operator, it works:

int main() {
    const bool z = true;
    const int x = 5, y = 3;
    int arr[z ? x : y];
    return 0;
}

Ideone

As opposed to:

int main() {
    const bool z = true;
    const int x = 5, y = 3;
    int arr[[&]() -> int { return z ? x : y; }];
    return 0;
}

Ideone

answered Aug 16 '17 at 08:15

Post Self

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Indeed it works, thank you :) it seems I tried to be too clever. – 412131 Aug 16 '17 at 08:27

score 2 · Answer 2 · answered Aug 16 '17 at 08:29

As described here, lambda expressions can't be constexpr yet, and you can only declare the size of an array with a constexpr value (even then, you are not trying to invoke declared lambda (to invoke it - () is required at the end of declaration).

To work around such problem, you could use a private static constexpr method, and use the return value of it, for the array size declaration:

static constexpr uint32_t GetArraySize ()
    {
    uint32_t tmp = BAR_WIDTH / Foo::FOO_EDGE;
    return (BAR_WIDTH % 10 == 0) ? tmp : tmp + 1;
    }

Foo mapper[GetArraySize ()];

Static array size determined from template values

2 Answers2