Question

Only one element in the container can have the key property similar to how rdbms will not allow you to declare
more than one primary key on a table.
Example follows using a vector ( please consider using any other container ( std or boost ) that can accomplish the task
elegantly.

struct Element
{
  std::wstring val_;
  bool key_;
};
std::vector<Element> v;
Element e1;
e1.val_ = L"Jupiter";
e1.key_ = false;
v.push_back(e1);

Element e2;
e2.val_ = L"Mars";
e2.key_ = true;
v.push_back(e2);

Element e3;
e3.val_ = L"Venus";
e3.key_ = false;
v.push_back(e3);

Element e4;
e4.val_ = L"Venus";
e4.key_ = false;
v.push_back(e4);

Key requirement is that if for example an attempt is made to make e3.key_ = true an exception should be thrown because
e2 (“Mars”) already plays that role

Note that duplicates are allowed in this container.

Answer 1

If you only want to allow one of them to be a key, I would store the indication of the key separately from the individual items:

struct column_set {
    std::vector<std::wstring> elements;
    size_t key;
};

column_set v;
v.elements.push_back(L"Jupiter");
v.elements.push_back(L"Mars");
v.elements.push_back(L"Venus");
v.elements.push_back(L"Venus");
v.key = 1;

If you want to, it should be fairly easy to write a little front-end code that acts like an array of bool that will return true for an index equal to the value you’ve give for key, and false for any other value.