Inline function: significance and limits

Significance of inline function

An inline function is a function whose body is substituted at the place of each call by the compiler, instead of doing a normal function call. A normal call costs time for pushing arguments, jumping to the function and returning. Inline substitution removes this overhead, so small functions run faster. It is requested with the inline keyword.

#include <iostream>
using namespace std;

inline int square(int x) {       // request for inline expansion
    return x * x;
}

int main() {
    cout << square(5) << endl;   // compiler treats it like: 5 * 5
    cout << square(8) << endl;
    return 0;
}

Output

25
64

Unlike a macro, an inline function is type checked and evaluates its arguments only once.

Advantages

• Saves function call overhead, so execution is faster.
• Type safe, unlike #define macros.
• Keeps the readability of functions while giving the speed of inline code.
• Member functions defined inside a class are inline by default.

Disadvantages

• Code size grows, because the body is copied at every call.
• Large inline code may reduce cache efficiency and make the executable big.
• Changing an inline function requires recompiling all files that use it.

When inline may not work

inline is only a request; the compiler may ignore it. It usually does not work when the function:

• contains loops (for, while, do-while) or switch or goto,
• contains static variables,
• is recursive,
• is large or complex,
• has its address taken (a pointer to the function is used).