C++ Inline Functions

---

---

What is an Inline Function in C++

C++ inline function is powerful concept that is commonly used with classes. If a function is inline, the compiler places a copy of the code of that function at each point where the function is called at compile time.

Any change to an inline function could require all clients of the function to be recompiled because compiler would need to replace all the code once again otherwise it will continue with old functionality.

Defining an Inline Function

To define an inline function, place the keyword inline before the function name and define the function before any calls are made to the function. The compiler can ignore the inline qualifier in case defined function is more than a line.

A function definition in a class definition is an inline function definition, even without the use of the inline specifier.

Example

Following is an example, which makes use of inline function to return max of two numbers −

#include <iostream>
 
using namespace std;

inline int Max(int x, int y) {
   return (x > y)? x : y;
}

// Main function for the program
int main() {
   cout << "Max (20,10): " << Max(20,10) << endl;
   cout << "Max (0,200): " << Max(0,200) << endl;
   cout << "Max (100,1010): " << Max(100,1010) << endl;
   
   return 0;
}

When the above code is compiled and executed, it produces the following result −

Max (20,10): 20
Max (0,200): 200
Max (100,1010): 1010

Inline Function with Classes

By default, all the functions that are defined inside a class are implicitly inline. If you want to explicitly define a function as inline, then you have to declare a function inside the class and write its definition outside the class. The inline keyword is used with the function definition that is outside of the class declaration.

Example

In the following example, we are defining an inline function with the class −

#include <iostream>
using namespace std;

class Number {
private:
    int num1;
    int num2;

public:
    // Function to set the values
    void setValues(int a, int b);

    // Function to print the values
    void printValues();

    // Inline function to add the two numbers
    inline int addNumbers();
};

// Member function definitions
void Number::setValues(int a, int b) {
    num1 = a;
    num2 = b;
}

void Number::printValues() {
    cout << "Number 1: " << num1 << ", Number 2: " << num2 << endl;
}

// Inline function definition
inline int Number::addNumbers() {
    return num1 + num2;
}

int main() {
    // Create an object
    Number n;

    // Set the values
    n.setValues(10, 20);

    // Print the values
    n.printValues();

    // Add the numbers and print the result
    int sum = n.addNumbers();
    cout << "Sum of the numbers: " << sum << endl;

    return 0;
}

Advantages of Inline Function

The following are the advantages of using inline function −

• In the case of an inline function, the function call overhead does not occur.
• The inline function saves the overhead of the push and pop variables on the stack, when the function is called.
• An inline function saves the overhead of a return call from a function.
• When an inline function is created, the compiler may perform context specific optimization on the body of the function. This kind of optimization is not performed for normal functions.
• Using small inline functions may be useful for embedded systems, because inline can yield less code than the function call preamble and return.

Disadvantages of Inline Function

Some of the disadvantages of inline functions are as follows –

• When we use an inline function, the size of the code is increased because the compiler replaces each function call with the inline function code.
• Large sized code takes more memory and time to compiler the code.
• The compilation process slows down because the compiler evaluates and replaces the function code at the place function calls.
• In some of the cases, the program's performance may slow down.
• Based on the function code complexity, the compiler may ignore the inline keyword. Thus, the application of the inline function is limited.

Some of the disadvantages of inline function are as follows −