Question

I’d like to use boost::array as a class member, but I do not know the size at compile time.
I thought of something like this, but it doesn’t work:

int main() {
    boost::array<int, 4> array = {{1,2,3,4}};
    MyClass obj(array);
}

class MyClass {
    private:
        boost::array<int, std::size_t> array;
    public:
        template<std::size_t N> MyClass(boost::array<int, N> array)
        : array(array) {};
};

The compiler, gcc, says:

error: type/value mismatch at argument 2 in template parameter list for
  ‘template<class _Tp, long unsigned int _Nm> struct boost::array’
error:   expected a constant of type ‘long unsigned int’, got ‘size_t’

Which obviously means that one cannot use variable-sized arrays as class members. If so, this would negate all the advantages of boost::array over vectors or standard arrays.

Can you show me what I did wrong?

Answer 1

Boost’s array is fixed-size based on the second template parameter, and boost::array<int,4> is a different type from boost::array<int,2>. You cannot have instances of the same class (MyClass in your example) which have different types for their members.

However, std::vectors can have different sizes without being different types:

struct MyClass {
  template<std::size_t N>
  explicit
  MyClass(boost::array<int, N> const& array)
  : data(array.begin(), array.end())
  {}

private:
  std::vector<int> data;
};

int main() {
  boost::array<int, 4> a = {{1,2,3,4}};
  MyClass obj(a);

  boost::array<int, 2> a2 = {{42,3}};
  MyClass obj2(a2);

  // notice obj.data.size() != obj2.data.size()

  return 0;
}

That said, boost::array is still useful (it’s even useful in this example code), just not in the exact way you want to use it.