Question

I’m curious if the default constructor and destructor that the compiler generates are inline or not, because I can justify it either way. On the one hand, you want the default constructor/destructor to not be inline so that adding them later doesn’t break ABI (because object files compiled when only the defaults were there will have inlined the generated definitions instead of what you define). On the other hand, for a C++ compiler to compile C code that performs as well as when compiled with a C compiler, it can’t be adding constructor/destructor calls for every allocated struct, and in C++ the only functional difference between a class and a struct is supposed to be the default access protection. Maybe the linker addresses this somehow? Maybe the answer varies across compilers?

A consequence of this question: if I have a POD struct in C++, can I theoretically benefit under some compilers by defining empty inline constructor/destructors myself in place of the defaults?

Answer 1

The C++ standard says, in 12.1[class.ctor]/5

An implicitly-declared default constructor is an inline public member of its class

and in 12.4[class.dtor]/3

An implicitly-declared
destructor is an inline public member of its class.