When to write the inline keyword for functions and methods in C++?

Hello everyone! I'm a beginner C++ developer, and I'm constantly troubled by the question about the inline keyword. I kind of understand that it "substitutes" the function code at the call site to avoid call overhead. But in practice, I see that modern compilers (GCC, Clang, MSVC) often decide for themselves whether to inline a function or not, even if I didn't write inline. And conversely, if I write inline, the compiler may ignore it and not inline.\n\nAs a result, I'm confused. Here's an example:\n\n```cpp\n// File utils.h\nint square(int x) {\n return x * x;\n}\n\n// File main.cpp\n#include "utils.h"\nint main() {\n int a = square(5);\n return 0;\n}\n```\n\nIf I include this header file in multiple .cpp files, the linker will give a multiple definition error. I was told that I need to add