1. When the template arguments are explicitly specified, there is no need to deduce the template arguments. 
     #include<iostream>
 #include<typeinfo>
	
 template<typename T>
 void foo(T a, T b){
    std::cout << typeid(a).name() << std::endl;
    std::cout << typeid(b).name() << std::endl;
 }
	
 int main(){
    // foo(unsigned int, unsigned int) instantiated
    foo<unsigned int>(2U, 123);
    return 0;
 }
  2. In the explicit specification we need only list the template arguments that cannot be implicitly deduced, with the constraint that, as with default arguments, we can omit only trailing arguments. 
     #include<iostream>
 #include<typeinfo>
	
 template<typename T1, typename T2, typename T3>
 void foo(T1 a, T2 b, T3 c){
    std::cout << typeid(a).name() << std::endl;
    std::cout << typeid(b).name() << std::endl;
    std::cout << typeid(c).name() << std::endl;
 }
	
 int main(){
    // instantiate foo(double,short,float)
    foo<double, short>(2.0d, 123, 1.f);
	
    // instantiate foo(int,int,float)
    foo<int>(2.0d, 123, 1.f);
	
    // instantiate foo(double,int,float)
    foo<double>(2.0d, 123, 1.f);
	
	   
    void (*pf)(double, int, short) = &foo<double>; 
	
    // instantiate foo(double,int,short)
    pf(2,4,5);
	
    return 0;
 }