Difference between Preprocessor Directives and Function Templates in C++

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:

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:

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:

#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: