static_assert negative integer

Question

#include <iostream>

using namespace std;

template <int fact>
constexpr int pow2T()
{
    static_assert(fact < 0, "error");
return fact == 0 ? 1 : pow2T<fact - 1>() * 2;
}

constexpr int e2 = pow2T<2>();

int main(int argc, char *argv[])
{
    cout << e2 << endl;

   return 0;
}

Trying to implement x^2 static calculation with argument static checking. Assertion fails.. why!?

/home/serj/work/untitled/main.cpp:-1: In function 'constexpr int pow2T() [with int fact = 2]':

/home/serj/work/untitled/main.cpp:-1: In function 'constexpr int pow2T() [with int fact = 1]':

...

static_assert does the exact contrary of what you think! It fails if the condition is false. It asserts that the condition is true. — tux3, Jan 07 '15 at 13:35
Note, you can use iteration (loops) instead of recursive function calls if you upgrade to a C++14 compiler. — Potatoswatter, Jan 07 '15 at 13:39
@Potatoswatter: or just `(1u << fact)` (and that works for older compiler too ^_^). — Jarod42, Jan 07 '15 at 14:04

Artem Sobolev · Answer 1 · 2015-01-07T13:44:54.057

7

static_assert fails if condition is false. Obviously, 1 < 0 and 2 < 0 are both false.

BTW, this function computes 2^x, not x^2

edited Jan 07 '15 at 13:44

answered Jan 07 '15 at 13:29

Artem Sobolev

5,891
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score 3 · Answer 2 · answered Jan 07 '15 at 13:29

3

The 2 branches are evaluated, you have to do specialization:

template <int fact>
constexpr int pow2T()
{
    static_assert(fact >= 0, "error");
    return pow2T<fact - 1>() * 2;
}

template <>
constexpr int pow2T<0>()
{
    return 1;
}

answered Jan 07 '15 at 13:29

Jarod42

203,559
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My compiler agrees with you. But could you say why ?: version does not work as expected? Taking into account I'm trying to calculate only powT(2). – noname7619 Jan 07 '15 at 15:39
`pow2T<2>` requires `pow2T<1>` which requires `pow2T<0>`. But `pow2T<0>` also requires `pow2T<-1>` as the 2 branches are instranciated (even if it is not evaluated). – Jarod42 Jan 07 '15 at 16:14

static_assert negative integer

2 Answers2