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 might inline any function or might not inline it. They are allowed to use the inline decoration as a hint for this decision, but they're also allowed to ignore it.

Also note that class member functions have an implicit inline decoration if they are defined right in the class definition.