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Difference between Preprocessor Directives and Function Templates in C++
  • Difficulty Level : Hard
  • Last Updated : 05 Jan, 2021

Preprocessor Directives are programs that process our source code before compilation. There are a number of steps involved between writing a program and executing a program in C / C++.

Below is the program to illustrate the functionality of Function Templates:

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// C++ program to illustrate
// preprocessor directives
#include <bits/stdc++.h>
  
#define min(a, b) ((a < b) ? a : b)
  
using namespace std;
  
// Driver code
int main()
{
    int a = 2, b = 4;
  
    // Find the minimum of a and b
    cout << "Minimum of both is: "
         << min(a, b);
  
    return 0;
}

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Output:

Minimum of both is: 2

Function Templates are the generic function that can handle different data types without the need for any separate code.

Below is the program to illustrate the functionality of Function Templates:



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// C++ program to illustrate the use
// of Function Templates
#include <iostream>
#include <stdio.h>
using namespace std;
  
// Function Template
template <class T>
T Min(T x, T y)
{
    return (x < y) ? x : y;
}
  
// Driver Code
int main()
{
    int a = 4, b = 8;
  
    // Find the minimum of a and b
    cout << "Minimum of both is: " << min(a, b);
  
    return 0;
}

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Output:

Minimum of both is: 4

Function templates are used to make generic functions that can work with any data type. For example, the function template used for calculating the minimum of 2 values of any type can be defined as:

template <class T>
T minimum(T a, T b)
{
   return (a < b) ? a : b;
}

But, this task can also be performed using Pre-processor directives created using preprocessor directive #define. So, the minimum of the two numbers can be defined as as:

#define minimum(a, b) ((a < b) ? a : b)

The Pre-processor directives can also be achieved by using the below statements:

minimum(30, 35);
minimum(30.5, 40.5);

In C++, most of us prefer using templates over Pre-processor directives because:

  • In the case of Pre-processor directives, there is no type checking. But in the case of templates, full type checking is done by the compiler.
  • Pre-processor directives can call an unexpected result. Consider a macro that calculates the square of any number as:
#define sqr(x) (x*x)
  • Now if this macro is called using the following statement, x = sqr(4 + 4); then expected output is 64 but it generates 24 because any x in macro body replaced by 4 + 4 which leads to x = 4 + 4 * 4 + 4 = 24 but in the case of templates, such unexpected results are not obtained.

Below is the tabular difference between the two:

S. No. Pre-processor directives Function Templates
1 There is no type checking There is full type checking
2 They are preprocessed They are compiled
3 They can work with any data type They are used with #define preprocessor directives
4 They can cause an unexpected result No such unexpected results are obtained.
5 They don’t ensure type safety in instance They do ensure type safety
6 They have explicit full specialization They don’t have explicit full specialization

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