Implicit instantiations

When a function is called, the definition of that function needs to exist. If this function has not been explicitly instantiated, an implicit instantiation approach is reached, in which the list of template arguments need to be either explicitly supplied or deduced from the context. Part A of the following program provides some examples of the implicit instantiation of app_max() in this catalog:

//ch4_2_func_template_implicit_inst.cpp
#include <iostream>
template <class T>
T app_max (T a, T b) { return (a>b?a:b); }
using namespace std;
int main(){
 //Part A: implicit instantiation in an explicit way 
 cout << app_max<int>(5, 8) << endl;       //line A 
 cout << app_max<float>(5.0, 8.0) << endl; //line B
 cout << app_max<int>(5.0, 8) << endl;     //Line C
 cout << app_max<double>(5.0, 8) << endl;  //Line D
 
 //Part B: implicit instantiation in an argument deduction way
 cout << app_max(5, 8) << endl;           //line E 
 cout << app_max(5.0f, 8.0f) << endl;     //line F 

 //Part C: implicit instantiation in a confuse way
 //cout<<app_max(5, 8.0)<<endl;          //line G  
 return 0;
}

The implicit instantiations of lines A, B, C, and D are int app_max<int>(int,int), float app_max<float>(float, float>), int app_max<int>(int,int), and double app_max<double>(double, double), respectively.