Please go through the codes one by one.
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1. Let us say we have created a code to calculate square of an input number:

   	#include<iostream>
   	int square(int a)
   	{
   	return a*a;
   	}
   	int main()
   	{
   	std::cout<<square(5)<<std::endl;
   	std::cout<<square(5.5)<<std::endl;
   	std::cout<<square(5.5f)<<std::endl;
   	}
   	/*
   	It will generate the output as :
   	25
   	25 
   	25
   	*/
   

2. 	#include<iostream>
   	int square(int a)
   	{
   	return a*a;
   		}
   	int square(double a)
   	{
   	return a*a;
   		}
   	int square(float a)
   	{
   	return a*a;
   	}
   	int main()
   	{
   	std::cout<<square(5)<<std::endl;
   	std::cout<<square(5.5)<<std::endl;
   	std::cout<<square(5.5f)<<std::endl;
   	}
   	/*
   	It will generate the output as :
   	25
   	30 
   	30
   	*/
   

3. 	#include<iostream>
   	int square(int a)
   	{
   	return a*a;
   		}
   	double square(double a)
   	{
   	return a*a;
   		}
   	float square(float a)
   	{
   	return a*a;
   	}
   	int main()
   	{
   	std::cout<<square(5)<<std::endl;
   	std::cout<<square(5.5)<<std::endl;
   	std::cout<<square(5.5f)<<std::endl;
   	}
   	/*
   	It will generate the output as :
   	25
   	30.25 
   	30.25
   	*/
   

So you see, we can make mistakes if we are dealing with long codes, to avid such mistakes, instead we use idea of templates in c++.

Use of Templates

	#include<iostream>
	template <typename T>
	T square(T a)
	{
	return a*a;
	}
	int main()
	{
	std::cout<<square(5)<<std::endl;
	std::cout<<square(5.5)<<std::endl;
	std::cout<<square(5.5f)<<std::endl;
	}
	/*
	It will generate the output as :
	25
	30.25 
	30.25
	*/

OR

	#include<iostream>
	template <typename T>
	T square(T a)
	{
	return a*a;
	}
	int main()
	{
	std::cout<<square<int>(5)<<std::endl;
	std::cout<<square<double>(5.5)<<std::endl;
	std::cout<<square<float>(5.5f)<<std::endl;
	}
	/*
	It will generate the output as :
	25
	30.25 
	30.25
	*/

• User of template allows user to avoid copy paste the code, it serves as blueprint for compiler to generate the code itself. To see what is happening visit the website . Basically compiler generates the code for us.
  • compiler is generating the below code:

    	#include<iostream>
    	template<typename T>
    	T square(T a)
    	{
    	  return a * a;
    	}
    
    	
    	/* First instantiated from: insights.cpp:10 */
    	#ifdef INSIGHTS_USE_TEMPLATE
    	template<>
    	int square<int>(int a)
    	{
    	  return a * a;
    	}
    	#endif
    	
    	
    	/* First instantiated from: insights.cpp:11 */
    	#ifdef INSIGHTS_USE_TEMPLATE
    	template<>
    	double square<double>(double a)
    	{
    	  return a * a;
    	}
    	#endif
    	
    	
    	/* First instantiated from: insights.cpp:12 */
    	#ifdef INSIGHTS_USE_TEMPLATE
    	template<>
    	float square<float>(float a)
    	{
    	  return a * a;
    	}
    	#endif
    	
    	int main()
    	 {
    	   std::cout.operator<<(square(5)).operator<<(std::endl);
    	   std::cout.operator<<(square(5.5)).operator<<(std::endl);
    	   std::cout.operator<<(square(5.5F)).operator<<(std::endl);
    	   return 0;
    	 }
     
    	/*
    	It will generate the output as :
    	25
    	30.25 
    	30.25
    	*/