Is there a NULL equivalent for pairs in C++?

Question

What would I want to use instead of NULL if I have an unassigned pair in C++?

As an example, suppose I have (pseudo)code like the following:

pair<int,int> bestPair; //Global variable

updateBestPair(vector<int> a, vector<int> b) {

    bestPair = NULL;

    for (/* loop through a and b */) {
        if (/* pair(a,b) is better than bestPair and better than some baseline */)
            bestPair = make_pair(a,b);
    }

    if (bestPair != NULL) //Found an acceptable best pair
        function(bestPair);
    else
        cout<<"No acceptable pairs found"<<endl;
}

Answer 1

Is there a NULL equivalent for pairs in C++?

No.

What would I want to use instead of NULL if I have an unassigned pair in C++?

Here are a few options:

• you can use a pointer to a pair, which can be set to NULL; This is probably not the best solution (since you are clearly not requiring a pointer)

• you can use a boost::optional<std::pair<int,int>>;

• you can (and probably should) rewrite your code not to use a global variable.

• you can restructure your control flow to avoid checking for a valid pair as a separate step:

  pair<int,int> bestPair; //Global variable
  
  updateBestPair(vector<int> a, vector<int> b) {
  
      // not needed
      // bestPair = NULL;
  
      //loop through a and b
      if (/* pair(a,b) is better than bestPair and ... */)
      {
          bestPair = make_pair(a,b);
          function(bestPair);
      }
      else
          cout<<"No acceptable pairs found"<<endl;
  }
  

• you can choose an artificial value to represent "invalid pair value":

  // use as constant, wherever you used NULL before
  const auto invalid_pair = std::make_pair(
      std::numeric_limits<int>::max(),
      std::numeric_limits<int>::max());
  

• you can use a boolean flag:

  pair<int,int> bestPair; //Global variable
  
  updateBestPair(vector<int> a, vector<int> b) {
  
      bool initialized = false;
  
      //loop through a and b
      if (/* pair(a,b) is better than bestPair and ... */)
      {
          bestPair = make_pair(a,b);
          initialized = true;
      }
  
      if(initialized)
          function(bestPair);
      else
          cout<<"No acceptable pairs found"<<endl;
  }
  

• you can use a custom solution (similar to boost::optional wrapper or not)

Answer 2

No. C++ objects cannot be "NULLed".

(Even pointers, which are objects, cannot be "NULLed"! This is confusing because their value may be set to a null pointer value, which we sometimes in the past obtained with a macro named NULL; however, this has never been the same as "NULLing" the pointer itself. Er, anyway…)

I recommend either boost::optional, or rethink the idea of having a global variable that can be "has a useful value" or "does not have a useful value". What's the point in it existing if it has no useful value?

Answer 3

No, that is not possible. You could use an additional variable to indicate the validity of the pair (that you have a pair).