How it works...

Let’s define an example class where automatic template type deduction would be of value:

template <typename T1, typename T2, typename T3>
class my_wrapper {
 T1 t1;
 T2 t2;
 T3 t3;

public:
 explicit my_wrapper(T1 t1_, T2 t2_, T3 t3_) 
 : t1{t1_}, t2{t2_}, t3{t3_}
 {}

 /* … */
};

Okay, this is just another template class. We previously had to write the following in order to instantiate it:

my_wrapper<int, double, const char *> wrapper {123, 1.23, "abc"};

We can now just omit the template specialization part:

my_wrapper wrapper {123, 1.23, "abc"};

Before C++17, this was only possible by implementing a make function helper:

my_wrapper<T1, T2, T3> make_wrapper(T1 t1, T2 t2, T3 t3)
{
 return {t1, t2, t3};
}

Using such helpers, it was possible to have a similar effect:

auto wrapper (make_wrapper(123, 1.23, "abc"));