如何确保initializer_list不为零

Question

I have a constructor that accepts a initializer_list as argument:

A::A(std::initializer_list<uint32_t> arg)
    : vec(arg)
{
}

The issue is that it allows initializer_list zero or empty values for arg:

A {}

How can I force a non-zero initializer_list ?

Answer 1

When the initializer is {} , a default constructor takes precedence over a std::initializer_list constructor, but the latter will be used when the former is not present. Therefore, in order to force a compile error, you need to explicitly delete the default constructor, so that the default constructor is still selected but cannot be used:

struct A {
    A() = delete;
    A(std::initializer_list<uint32_t>) { /* ... */ }
};

However, at compile time, you can't stop the user from manually constructing an empty std::initializer_list object and passing it in. You would need to throw an exception to signal construction failure in that case.

Answer 2

You can't do this with initializer lists -- you'd have to do run-time (not compile-time) validation.

But, just for fun: if you are willing to forego initializer lists, and accept an array as an input into your constructor, you can get what you want.

class A {
 private:
  std::vector<int> v_;
 public:
  template<typename T, std::size_t N>
  A(const T(&v)[N]) : v_(std::begin(v), std::end(v)) {
    static_assert(N > 0, "Requires nonempty array.");
  }
};

int main() {
  int empty_arr[] = {};
  int arr[] = {1};
  // The following no longer works as there is no constructor that takes in as
  // input an initializer list.
  // A a{1};
  // A b{};
  A c({2});
  // So does this:
  A d(arr);
  // And this would be a compilation error, even if there was no static_assert.
  // C++ standard says you can't have an array of size zero. The static_assert in 
  // the class is just to make it easier for others to see what your intention is.
  // A e(empty_arr);
}