Question

I would like to templatize “first” type of std::pair using the following construction

template <typename T>
struct TPair
{
typedef std::pair <T, short> Type;
};

and create a vector of such pairs.

template <typename T>
struct TPairs
{
typedef std::vector <TPair <T> > Type;
};

But this code seems to be screwed for common usage and it is uncomfortable:

TPair <double> ::Type my_pair (1.0, 0 ); //Create pairs
TPair <double> my_pair2 (1.0, 0 ); //Create object, needs a constructor

TPairs <double> ::Type pairs; //Create vector
TPairs <double> pairs2; //Create object

pairs.push_back(my_pair); //Need a constructor
pairs2.push_back(my_pair); //No push_back method for the structure...
....

Is there any more simple and comfortable solution?

Answer 1

template <typename T>
struct TPairs
{
  typedef std::vector <TPair <T> > Type;
};

There’s a problem here: you’re creating a type that is a vector of TPair<T>, which is in fact not what you want. You want a vector of TPair<T>::Type.

template <typename T>
struct TPairs
{
  typedef std::vector <typename TPair <T>::Type > Type;
};

As for your use cases, remember that those two structs you created are there just to simulate a template typedef, you should never instantiate them at all, just use their Type member typedef. So:

TPair <double> ::Type my_pair (1.0, 0 ); // Good, creates a std::pair
TPair <double> my_pair2 (1.0, 0 ); // Not good, does not create an std::pair


TPairs <double> ::Type pairs; //Good, creates a vector
TPairs <double> pairs2;       //Not good, doesn't create a vector

pairs.push_back(my_pair);   // Ok, does what you mean
pairs2.push_back(my_pair);  // Can't compile, the `TPairs` struct ins't a vector