[pjc50]

If you put a bare function or variable in a header, then use it in multiple compilation units, the linker will reject your multiply-defined symbols.

(Templates get de-duplicated).

[vonmoltke]

Unless you mark them 'static', which is common practice for constants and small inlined functions.

[gpderetta]

Inline is the keyword here (litterally). Inline, in C++, is defined as disabling the requirement of having exactly one definition of a function (it is also a weak hint for the compiler to perform inlining).

Template functions are implicitly inline.

Inline in C has a similar but subtly different definition.