When a compiler can infer a template parameter?

Question

Sometimes it works sometimes not:

template  
void f(T t) {}

template 
class MyClass {
public:
  MyClass(T t) {}
};

void test          


        

Answer 1

Template parameters can be inferred for function templates when the parameter type can be deduced from the template parameters

So it can be inferred here:

template <typename T>
void f(T t);

template <typename T>
void f(std::vector<T> v);

but not here:

template <typename T>
T f() {
  return T();
}

And not in class templates.

So the usual solution to your problem is to create a wrapper function, similar to the standard library function std::make_pair:

  template <class T>
    class MyClass {
    public:
        MyClass(T t) {}
        void print(){
            std::cout<<"try MyClass"<<std::endl;
        }
    };

    template <typename T>
    MyClass<T> MakeMyClass(T t) { return MyClass<T>(t); }

and then call auto a = MakeMyClass(5); to instantiate the class.

Answer 2

Read up on Template Argument Deduction (and ADL or Koenig lookup).

Answer 3

In C++17, it is possible to infer some types using auto, though the template parameters still need to be specified here:

#include <iostream> 
#include <string>

template <class T1,class T2>   
auto print_stuff(T1 x, T2 y) 
{ 
    std::cout << x << std::endl;
    std::cout << y << std::endl;
}

int main() 
{ 
    print_stuff(3,"Hello!");
    print_stuff("Hello!",4);
    return 0; 
}

Using the -fconcepts flag in gcc, it is possible to infer the parameters, though this is not yet a part of the C++ standard:

#include <iostream> 
#include <string>

auto print_stuff(auto x, auto y) 
{ 
    std::cout << x << std::endl;
    std::cout << y << std::endl;
}

int main() 
{ 
    print_stuff(3,"Hello!");
    print_stuff("Hello!",4);
    return 0; 
}