Assigning values to std::array of std::optional objects

Question

I am trying to fill a std::array of std::optional objects as below.

class MyClass
{
   private:
    int distance;
    MyClass(int x, int y);

   friend class MasterClass;
};

MyClass::MyClass(int x, int y)
{
  distance = x+y;
}

class MasterClass
{
  public:
  MasterClass(std::array<std::optional<int>,5> xs, std::array<std::optional<int>,5> ys);
  
  private:
  std::array<std::optional<MyClass>, 5> myclassarray{};

};

MasterClass::MasterClass(std::array<std::optional<int>,5> xs, std::array<std::optional<int>,5> ys)
 {
     for(int i=0; i<5;i++)
     {
         myclassarray[i].emplace(new  MyClass(*xs[i], *ys[i])); //---(1)
     }

 }

From the line commented with (1) above, I get the following error,

error: no matching function for call to std::optional<MyClass>::emplace(MyClass&)

I also tried replacing the same line with

 myclassarray[i] = new  MyClass(*xs[i], *ys[i]) ; //---(2)

This will give me

error: no match for ‘operator=’ (operand types are ‘std::array<std::optional<MyClass>,5>::value_type’ {aka ‘std::optional<MyClass>’} and ‘MyClass*’)

How do I solve this issue?

Answer 1

Looks like maybe you are coming from Java, or C#. When you assign a value in c++, it is rare that you will use new. The issue is, that you are basically doing this:

std::optional<MyClass> o = new MyClass();

o is of type, std::optional<MyClass> and new My Class() is of type MyClass *. You can see from here that there is no operation that converts a pointer to an object to an optional of an object. Lets take this back to basics, what we want to do is something like:

std::optional<int> o; // defaults to std::nullopt
o = value; // We set it with some value.

And actually this is all there is to it. So lets expand to an array:

std::array<std::optional<int>, 5> a; // An array with 5 optional, all are std::nullopt
a[2] = value; // Set the optional at position 2 to a value.

And this extends easily to your example:

for(int i=0; i<5;i++) {
     myclassarray[i] = MyClass(*xs[i], *ys[i]));
}

Just be careful with this bit:

*xs[i], *ys[i]

because from here:

The behavior is undefined if *this does not contain a value.

Which will cause you much grief if this is the case.