Is it possible to place function implementations inside header files in C++?

Yes, in C++ it is possible to place function implementations inside header files, but with some caveats:

• Functions implemented in header files should usually be declared as inline or be template functions to avoid multiple definition errors during linking.
• This is often used for template functions and classes, as their implementation needs to be available in all translation units.

Example:

// header.h
inline int add(int a, int b) {
    return a + b;
}

If a function is not declared as inline and implemented in a header file, including this file in multiple cpp files will cause a multiple definition error.

Therefore, implementing functions in header files is a common practice for templates and inline functions, but for regular functions, it is preferable to place the implementation in cpp files.