Inline, inline, inline

Ben says: This stuff is tricky. gcc info docs have this to say: Note that in C and Objective C, unlike C++, the `inline' keyword does not affect the linkage of the function. The upcoming C standard http://www.dkuug.dk/JTC1/SC22/ introduces the `inline' keyword into the C language References (Grab 'em with `wget -r' before they become standards and disappear forever) http://osiris.dkuug.dk/JTC1/SC22/WG14/www/docs/ http://www.maths.warwick.ac.uk/cpp/pub/wp/html/cd2/ 6.5.4 Function specifiers Syntax [#1] function-specifier: inline Constraints [#2] Function specifiers shall be used only in the declaration of an identifier for a function. [#3] An inline definition (see below) of a function with external linkage shall not contain a definition of an object with static storage duration that can be modified, and shall not contain a reference to an identifier with internal linkage. [#4] The inline function specifier shall not appear in a declaration of main. Semantics [#5] A function declared with an inline function specifier is an inline function. Making a function an inline function suggests that calls to the function be as fast as possible by using, for example, an alternative to the usual function call mechanism known as ``inline substitution''.101 The __________ 101. Inline substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro used within the body of the Language 135 Working Draft, 1997-11-21, WG14/N794 J11/97-158 extent to which such suggestions are effective is implementation-defined.102 [#6] Any function with internal linkage can be an inline function. For a function with external linkage, the following restrictions apply. If a function is declared with an inline function specifier, then it shall also be defined in the same translation unit. If all of the file scope declarations for a function in a translation unit include the inline function specifier without extern, then the definition in that translation unit is an inline definition. An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit. An inline definition provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition. Examples [#7] The declaration of an inline function with external linkage can result in either an external definition, or a definition available for use only within the translation unit. A file scope declaration with extern creates an external definition. The following example shows an entire translation unit. ____________________________________________________________ function uses the definition it had at the point the function body appears, and not where the function is called; and identifiers refer to the declarations in scope where the body occurs. Similarly, the address of the function is not affected by the function's being inlined. 102. For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration. 136 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 inline double fahr(double t) { return (9.0 * t) / 5.0 + 32.0; } inline double cels(double t) { return (5.0 * (t - 32.0)) / 9.0; } /* Creates an external definition. */ extern double fahr(double); double convert(int is_fahr, double temp) { /* A translator may perform inline substitutions. */ return is_fahr ? cels(temp) : fahr(temp); } [#8] Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels is an inline definition. Because there is a call to cels, an external definition of cels in another translation unit is still required by 6.7. This proposal adds the inline keyword to Standard C in such a way that it can be implemented with existing linker tech- nology and also is compatible with C++. This is achieved by requiring that one and only one of the translation units 25 containing the definition of an inline function be specified as the one that provides an external definition for the function. Because that specification consists simply of a declaration that lacks the inline keyword, it will also be accepted by a C++ translator. 30 This proposal does extend the C++ specification in two ways. First, if a function is declared inline in one translation unit, it need not be declared inline in every other transla- tion unit. This allows, for example, a library function that is to be inlined within the library but to be available 35 only through an external definition elsewhere. (The alter- native of using a wrapper function for the external function both requires an additional name and may adversely impact performance, if a translator does not actually do inline substitution.) 40 Second, the requirement that all definitions of an inline function be ``exactly the same'' is replaced by the require- ment that the behavior of the program should not depend on - 1 - 30 May 1997 Inlining Proposal, ReviWG14/N709 (J11/97-072) whether a call is implemented with a visible inline defini- tion or the external definition of a function. This allows an inline definition to be specialized for its use within a particular translation unit. For example, the external 5 definition of a library function might include some argument validation that is not needed for calls made from other functions in the same library. These extensions do offer some advantages, and programmers who are concerned about compatibility can simply abide by 10 the stricter C++ rules. Nevertheless, the extensions are not central to the proposal and the C++ rules could easily be substituted. Still reading??? For the masochists, here are excerpts from the C++ standard: 2 A function declaration (_dcl.fct_, _class.mfct_, _class.friend_) with an inline specifier declares an inline function. The inline specifier indicates to the implementation that inline substitution of the func- tion body at the point of call is to be preferred to the usual func- tion call mechanism. An implementation is not required to perform this inline substitution at the point of call; however, even if this inline substitution is omitted, the other rules for inline functions defined by this subclause shall still be respected. 3 A function defined within a class definition is an inline function. The inline specifier shall not appear on a block scope function declaration.2) 4 An inline function shall be defined in every translation unit in which it is used and shall have exactly the same definition in every case (_basic.def.odr_). [Note: a call to the inline function may be _________________________ 2) The inline keyword has no effect on the linkage of a function. encountered before its definition appears in the translation unit. ] If a function with external linkage is declared inline in one transla- tion unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required. [Note: a static local variable in an extern inline function always refers to the same object. ]