When do compilers inline C++ code?

Question

In C++, do methods only get inlined if they are explicitly declared inline (or defined in a header file), or are compilers allowed to inline methods as they see

Answer 1

Compilers may ignore your inline declaration. It is basically used by the compiler as a hint in order decide whether or not to do so. Compilers are not obligated to inline something that is marked inline, or to not inline something that isn't. Basically you're at the mercy of your compiler and the optimization level you choose.