How to find out template parameter Type that STL container holds

Question

For example vector<int> coll , holds int type objects. What if I don't know that coll is holds int (but I know it is vector), how would I go about finding type information ?

Answer 1

You can grab the information from the object's value_type :

using value_type = decltype(coll)::value_type;

static_assert(std::is_same<value_type, int>::value, "Type is not an int");

_{Use of the using alias and static_assert is available in C++11+}

Though this can always be determined by using templates (which is more common):

template <class T>
void f(std::vector<T>& v); // use T as the type