c++ – Why are template parameters needed?

Question:

Please give real examples of using these parameters, when they are indispensable or they greatly simplify the code.

Answer:

Let's look at the code below:

#include <vector>
#include <list>

using namespace std;

template <template <typename T> typename Cont>
class Foo
{
private:
    Cont<int> values;
};

template <typename Cont>
class Bar
{
private:
    Cont values;
};

int main()
{
    Foo<vector> fooVec;
    Bar<vector<int>> barVec;
    
    Foo<list> fooList;
    Bar<list<int>> barList;
    
    return 0;
}

As you can see, we can pass any container to both classes. So what's the difference? The difference is that we have the opportunity to pass a container of a different type to the Bar class. Nobody forbids us to write

Bar<list<double>> barList;

In turn, the Foo class allows the user of the class to choose the appropriate container for storing values, but does not allow the user to choose the type for these values.

---

Here is another use case. For example, you need to store two containers in a class object. And it is important for you that these are the same containers.

#include <iostream>
#include <vector>
#include <list>

template <typename Type1, typename Type2, template<typename T> typename Cont>
class Foo
{
private:
    Cont<Type1> first;
    Cont<Type2> second;
};

int main()
{
    Foo<int, double, std::vector> obj1;
    Foo<char, bool, std::list> obj2;
    
    return 0;
}

By passing a pattern as a parameter, you can ensure that first and second are the same containers.

---

This language feature is more commonly used in metaprogramming. Try to search on github for such libraries. But you can start with Boost . It contains MPL (Metaprogramming Library) , which has a quote* class template that uses template template parameter .
Synopsis :

template<
      template< typename P1 > class F
    , typename Tag = unspecified
    >
struct quote1
{
    // unspecified
    // ...
};

...

template<
      template< typename P1,... typename Pn > class F
    , typename Tag = unspecified
    >
struct quoten
{
    // unspecified
    // ...
};