There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have "vague linkage". Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.
Local static variables and string constants used in an inline function
are also considered to have vague linkage, since they must be shared
between all inlined and out-of-line instances of the function.
Note: If the chosen key method is later defined as inline, the
vtable will still be emitted in every translation unit which defines it.
Make sure that any inline virtuals are declared inline in the class
body, even if they are not defined there.
typeidand exception handling. For polymorphic classes (classes with virtual functions), the type_info object is written out along with the vtable so that
dynamic_castcan determine the dynamic type of a class object at runtime. For all other types, we write out the type_info object when it is used: when applying
typeidto an expression, throwing an object, or referring to a type in a catch clause or exception specification.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support.
On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable.
For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.
See Declarations and Definitions in One Header, for another way to control placement of these constructs.