What is the Inline Function in C++

This tutorial will teach us about C++ inline functions and how to use them with the help of examples. The inline function is one of the key features of C++. Before understanding what inline functions are, first let’s try to understand why we need Inline Functions.

Why do we need Inline Function in C++?

When a function call instruction is carried out by the program, the CPU copies the function’s arguments to the stack caches the memory location of the instruction that follows the function call and then passes control to the targeted function. The CPU then runs the function’s logic, saves the return value in a preset memory address and hands control back to the caller function. If the function’s execution time is shorter than the time required to move from the calling function to the called function, this might constitute overhead. Small functions experience this overhead since their execution time is less than their switching time. Inline Functions in C++ reduce the function call overhead.

Inline Function in C++

An Inline Function is a function that is expanded inline when it is called. This speeds up the process and saves time. The compiler decreases the overhead of function calls by replacing the function call with the appropriate function code. When a call is made to an inline function, the compiler during runtime replaces all of the calling statements with the function definition. The compiler creates a duplicate of the function’s code in place each time an inline function is invoked in order to avoid making a function call.

Circumstances Where the Compiler Does Not Consider Inline Functions

The inline Function does not work for the following situations:

  1. When a function has a loop, switch, or goto and returns a value.
  2. If there is a return statement for functions that don’t return values.
  3. The compiler will refuse to allow the function to be made an inline function if it includes one or more static variables.
  4. A recursive function won’t be regarded as an inline function by the compiler.

Declaration of Inline Function in C++

To declare Inline Function in C++, we use the “inline” keyword. For example,

inline return_type function_name(parameters)
{
  //Code
}

Explanation of the Syntax:

  • The keyword "inline" is used at the beginning to inform the compiler that the function is an inline function. However, the choice of whether to inline a function or not is left up to the compiler.
  • When an inline function is declared, it must also be defined. The compiler will simply replace all instances where the function is called in the program with the definition of the function.

Here is an example of an inline function that returns the maximum of two numbers –

#include <bits/stdc++.h>
using namespace std;

inline int max(int num1, int num2)
{
    return (num1 > num2) ? num1 : num2 ;
}

int main() 
{
	cout<<"max(60,15) : "<<max(60,15)<<endl ;
	cout<<"max(11,30) : "<<max(11,30)<<endl ;
	
	return 0;
}

Output

Max(60,15) : 60
Max(11,30) : 30

Here is how the program works –

Here, we created an inline function named max() that takes two integers as parameters. Then, we made 2 inline function calls inside the main() function with different arguments. Each time max() is reached, the compiler copies the code of the function to that call location.

Use of Inline Function in C++

  • The inline function can be used over macros. These functions have an advantage over macros since they are more effective and optimized. Almost all of the capabilities that macros may offer can be obtained by utilizing inline functions.
  • Performance-wise, inline functions consistently outperform other methods. Inline functions are the only option if your software requires performance since they speed up the execution of your code.
  • By utilizing the inline keyword outside of the class, you may build inline functions to hide the implementation details of your functions.

Advantages of Inline Function in C++

These are some of the advantages of inline function in C++

  • Due to the elimination of function call overhead provided by inline functions, the program’s compilation speed is boosted.
  • When a function is invoked, the overhead of pushing and popping variables from the stack is also avoided.
  • Additionally, it avoids the overhead of a function’s return call.
  • They enable the compiler to carry out some content-specific optimizations on the function’s body. Other function calls cannot undergo such optimizations.

Disadvantages of using Inline Function

These are some of the disadvantages of inline function in C++

  • Longer inline functions may reduce the cache hit rate of the instructions, therefore they must be kept to a minimum. The effectiveness of the program may suffer as a result.
  • Due to the repetition of the same code, using too many inline functions will increase the size of the binary executable file.
  • Large inline functions are ineffective for embedded systems since these systems prioritize function size over performance.
  • Large functions may result in compilation overhead, which lowers the code’s efficiency.

Conclusion
In this article, we have learned about What is the inline function in C++. The article begins with a basic overview of inline functions, their uses, advantages of inline function in C++, and how they might improve a program’s overall effectiveness. Moving on, we spoke about how the compiler creates inline functions and a few situations in which it doesn’t. We also learned about when to use the inline functions. In addition, the syntax, arguments, examples, and advantages and disadvantages of inline functions were examined.

Frequently Asked Questions related to Inline Functions:

1. What is the difference between macro and inline functions in C?

S. NO Inline Macro
1. The inline keyword is used to define the inline function. The #define keyword is used to define the macro.
2. In C++, inline can either be declared within or outside of the class. The macro is always defined at the start of the program.
3. The class’s data members are accessible through inline functions. The class’s data members are inaccessible to macro.
4. Program debugging is simple when using inline functions. Program debugging is not simple when using macro.
5. In comparison to macros, inline is less popular. Although the macro is often used.
6. The curly brace at the end signifies the completion of an inline function. The macro is finished with a new line rather than any symbols.
7. Competitive programming avoids using inline. The macro is often used in competitive programming.

2. Can an inline function return value?
Inline functions go beyond simple copy-paste operations. They operate precisely like normal functions would, thus any return value would be sent to the caller in the same way.

3. What is the difference between inline function and normal function?

S. NO Inline Function Normal Function
1. The inline keyword is used to define the inline function. There is no need for a keyword in its declaration.
2. When executed, it is expanded inline. The program’s modularity is provided by this function.
3. Usually, it just has 2 – 3 lines of code. Normal Function contains a lot of code.
4. In general, it is a function that is utilized when other functions are small and often called. In general, it consists of a collection of statements that work together to complete a certain task. When there are large functions, it is used.
5. It is typically used to increase the program’s execution duration. It is frequently used to make code more manageable and reusable.
6. Compared to normal function, it is a little more difficult to understand and test. Compared to the inline function, it is much simpler to understand and test.
7. Too many inline functions increase file size because they duplicate code during compilation. Too many normal functions do not affect the size of the file after compilation.

4. Why inline functions are called macros?
Inline functions and macros are similar in that the function code is expanded at the point of call during compilation, however inline functions are parsed by the compiler while macros are expanded by the preprocessor.

5. Which is faster macro or inline function?
As there is no real function call overhead with macros, they are often quicker than functions.

6. Can inline functions be recursive?
It’s possible for inline functions to be recursive. The overhead of creating a parameter list and calling the function is removed in the calling function since the call to an inline function is replaced with the function itself.

7. In Which case inline function may not work?
The inline Function does not work for the following situations:

  1. When a function has a loop, switch, or goto and returns a value.
  2. If there is a return statement for functions that don’t return values.
  3. The compiler will refuse to allow the function to be made an inline function if it includes one or more static variables.
  4. A recursive function won’t be regarded as an inline function by the compiler.

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