Where to put inline function during the frequent calling inside the program from multiple source files?

Inline functions are typically defined in header files. Here are the steps to do it:

1. First, you need to define your function in a header file (.h or .hpp). The function should be prefixed with the inline keyword. This suggests to the compiler that the function is an inline function.

// In your header file
inline int add(int a, int b) {
    return a + b;
}

2. Next, you include this header file in any source file (.cpp) where you want to use the function.

// In your source file
#include "your_header_file.h"

int main() {
    int result = add(5, 3);
    // ...
}

3. Repeat step 2 for any other source files where you want to use the function.

Remember, the decision to inline a function is ultimately made by the compiler. Using the inline keyword is merely a suggestion that the compiler can choose to ignore. The purpose of inlining is to eliminate the function-call overhead during runtime. However, if a function is large and complex, the compiler might choose not to inline it to avoid increasing the binary size.