如何对STL容器的元素强制执行主要键约束，例如行为

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.

Answer 2

Based on your description, it sounds like you are attempting to make pre-existing STL classes handle higher-level application logic for you and that's just not going to happen (at least not cleanly or elegantly).

My only suggestion would be something similar to what Jerry Coffin suggested.

Create your own class that represents your data set. Have that class encapsulate a vector (or map, list...) of your columns but provide a very clean public interface to the rest of the world. Nothing outside of your class should ever have to know that inside there's a vector. Other code only talks to public functions like AddColumn, DeleteColumn, SetKey. So essentially you are creating your own custom container which uses STL under the hood but extends it to provide the functionality you need.

As far as enforcement of the key, leave it up to your class to handle the enforcement internally. And just like Jerry said, instead of storing 49 false's and one true, it might be easier to just have a separate member variable sitting next to the vector that would simply indicate which of the entries happens to be the key.