Inline Function in C++

 

1️⃣ What is an Inline Function?

An inline function is a function that the compiler expands at the point of its call instead of executing a normal function call.

✔ Avoids function call overhead (stack operations).
✔ Increases execution speed by directly inserting the function code.
✔ Best for small, frequently used functions.

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2️⃣ Syntax

inline returnType functionName(parameters) {
    // Function body
}

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3️⃣ Example: Inline Function in Action

#include <iostream>
using namespace std;

inline int square(int x) {  
    return x * x;
}

int main() {
    cout << "Square of 5: " << square(5) << endl;
    cout << "Square of 7: " << square(7) << endl;

    return 0;
}

🔹 Output

Square of 5: 25
Square of 7: 49

✅ Why use inline?
✔ Avoids function call overhead.
✔ square(5) is replaced directly with 5 * 5 in machine code.

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4️⃣ How Inline Functions Work Internally?

Instead of generating a function call, the compiler expands the function at each call site.

🔹 Without Inline (Normal Function Call)

int square(int x) {
    return x * x;
}
int main() {
    int a = square(5);  // CALL square function
}

🔹 With Inline Function (Expanded Code)

int main() {
    int a = 5 * 5;  // No function call, directly replaced!
}

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5️⃣ When to Use Inline Functions?

✅ Use inline when:
✔ The function is small (one-liner or a few lines).
✔ The function is frequently used (e.g., math operations).
✔ The function does not contain loops, recursion, or complex logic.

🚫 Avoid inline when:
❌ The function is large (increases code size).
❌ The function contains loops, recursion, static variables.
❌ The function is virtual (because dynamic binding prevents inlining).

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6️⃣ Example: When Inline is Not Applied

Inline functions won't be expanded if they contain complex operations like loops or recursion.

🔹 Example: Loop inside Inline Function

#include <iostream>
using namespace std;

inline void printNumbers(int n) {
    for (int i = 1; i <= n; i++) {  // Loop inside inline function
        cout << i << " ";
    }
    cout << endl;
}

int main() {
    printNumbers(5);  // Compiler may ignore inline here
    return 0;
}

⚠ Inline may be ignored! The loop makes the function complex, so the compiler may choose not to expand it.

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7️⃣ Summary

✔ Inline functions reduce function call overhead by expanding code at the call site.
✔ Best for small, simple, and frequently used functions.
✔ Avoid for complex functions with loops, recursion, or static variables.

Would you like an example with class and inline functions? 🚀