c++ - Inline keyword vs header definition

Question

What's the difference between using the inline keyword before a function and just declaring the whole function in the header?

so...

int whatever() { return 4; }

vs

.h:

inline int whatever();

.cpp:

inline int myClass::whatever()
{
    return 4;
}

for that matter, what does this do:

inline int whatever() { return 4; }

Answer 1

There are several facets:

Language

• When a function is marked with the inline keyword, then its definition should be available in the TU or the program is ill-formed.
• Any function defined right in the class definition is implicitly marked inline.
• A function marked inline (implicitly or explicitly) may be defined in several TUs (respecting the ODR), whereas it is not the case for regular functions.
• Template functions (not fully specialized) get the same treatment as inline ones.

Compiler behavior

• A function marked inline will be emitted as a weak symbol in each object file where it is necessary, this may increase their size (look up template bloat).
• Whereas the compiler actually inlines the call (ie, copy/paste the code at the point of use instead of performing a regular function call) is entirely at the compiler's discretion. The presence of the keyword may, or not, influence the decision but it is, at best, a hint.

Linker behavior

• Weak symbols are merged together to have a single occurrence in the final library. A good linker could check that the multiple definitions concur but this is not required.