static_assert on initializer_list::size()

Question

Why is std::initializer_list<_E>::size not allowable in a static_assert, even though it\'s declared as a constexpr in my libstdc++ (v. 4.6)?

For example, the foll

Answer 1

I haven't really figured out what's going on here.

If I say

const std::initializer_list<double> li = { 1, 2.5, 3.7, 4.3 };
static_assert(li.size() == 4, "fail");

I get a complain that 'li' was not declared 'constexper'.

But if I say

constexpr std::size_t dSize(std::initializer_list<double> di)
{
    return di.size();
}

then

static_assert(dSize({1, 2.5, 3.7, 4.3}) == 4, "failed");   // works

but

static_assert(dSize(li) == 4, "failed");

fails with "error: the value of 'li' is not usable in a constant expression"

This is all with -std=c++11

So, somehow, an initializer list passed into an arg list can be part of a constant expression, but an initializer list just declared as a const variable can't.

Answer 2

Use following syntax:

LIVE DEMO

#include <initializer_list>

template<class T, int Length>
class Point
{
    std::initializer_list<T> data;
public:
    constexpr Point(std::initializer_list<T> init)
        : data
        (
            init.size() == Length ?
            init : throw 0
        )
    {}
};

int main()
{
    constexpr Point<int, 3> a{{1,2,3}};
    constexpr Point<int, 2> b{{1,2,3}}; // compile time error
}

Refer following SO.

---

EDIT: Interesting that works on GCC 4.8.1, but does not work on Clang 3.4. Maybe this is related to constexpr of .size() (afaik, in C++14 it is constexpr).

Answer 3

From my discussion with @Evgeny, I realized that this just works (with gcc 4.8 c++11) and may as well do the size check by only accepting a compatible size in the initializer list (in main).

(code link: http://coliru.stacked-crooked.com/a/746e0ae99c518cd6)

#include<array>
template<class T, int Length>
class Point
{
  public:
    Point(std::array<T, Length> init)
    {
//not needed//      static_assert(init.size() == Length, "Wrong number of dimensions");
    }
};

int main()
{
  Point<int, 3> q({1,2,3}); //ok
//  Point<int, 3> q2({1,2,3,4}); //compile error (good!)
  Point<int, 3> q2({1,2}); // ok, compiles, same as {1,2,0}, feature?
  return 0;
}

Answer 4

"Initializer lists" are just horrible kludges.

Don't:

#include <initializer_list>

template<typename T>
void Dont(std::initializer_list<T> list) { // Bad!
    static_assert(list.size() == 3, "Exactly three elements are required.");
}

void Test() { Dont({1,2,3}); }

Do:

template<typename T, std::size_t N>
void Do(const T(&list)[N]) { // Good!
    static_assert(N == 3, "Exactly three elements are required.");
}

void Test() { Do({1,2,3}); }

Answer 5

The compiler says that init is the problem, not init.size().

I guess that the constructor could be called from different places with different length initializers.

(To elaborate: You're trying to write a static_assert that depends on the run-time value of the variable init, namely how many elements it has. static_asserts have to be evaluable at the time the function is compiled. Your code is analogous to this trivially invalid example:)

void foo(int i) { static_assert(i == 42, ""); }
int main() { foo(42); }  // but what if there's a caller in another translation unit?