Why are C inline functions in the header

Question

NB This is not a question about how to use inline functions or how they work  more why they are done the way they are   The declaration of a class member function does not need to define a function as inline  it is only the actual implementation of the function   For example  in the header file   struct foo      void bar       no need to define this as inline     So why does the inline implementation of a classes function have to be in the header file  Why can t I put the inline function the  cpp file  If I were to try to put the inline definition in the  cpp file I would get an error along the lines of   error LNK2019  unresolved external symbol   public  void   thiscall foo  bar void     bar foo  QAEXXZ  referenced in function  main  1 gt C  Users Me Documents Visual Studio 2012 Projects inline Debug inline exe    fatal error LNK1120  1 unresolved externals

User · Answer

The reason is that the compiler has to actually see the definition in order to be able to drop it in in place of the call.

Remember that C and C++ use a very simplistic compilation model, where the compiler always only sees one translation unit at a time. (This fails for export, which is the main reason only one vendor actually implemented it.)

User · Answer

There are two ways to look at it    Inline functions are defined in the header because  in order to inline a function call  the compiler must be able to see the function body  For a naive compiler to do that  the function body must be in the same translation unit as the call   A modern compiler can optimize across translation units  and so a function call may be inlined even though the function definition is in a separate translation unit  but these optimizations are expensive  aren t always enabled  and weren t always supported by the compiler  functions defined in the header must be marked inline because otherwise  every translation unit which includes the header will contain a definition of the function  and the linker will complain about multiple definitions  a violation of the One Definition Rule   The inline keyword suppresses this  allowing multiple translation units to contain  identical  definitions    The two explanations really boil down to the fact that the inline keyword doesn t exactly do what you d expect   A C   compiler is free to apply the inlining optimization  replace a function call with the body of the called function  saving the call overhead  any time it likes  as long as it doesn t alter the observable behavior of the program   The inline keyword makes it easier for the compiler to apply this optimization  by allowing the function definition to be visible in multiple translation units  but using the keyword doesn t mean the compiler has to inline the function  and not using the keyword doesn t forbid the compiler from inlining the function

User · Answer

This is a limit of the C   compiler  If you put the function in the header  all the cpp files where it can be inlined can see the  source  of your function and the inlining can be done by the compiler  Otherwhise the inlining would have to be done by the linker  each cpp file is compiled in an obj file separately   The problem is that it would be much more difficult to do it in the linker  A similar problem exists with  template  classes functions  They need to be instantiated by the compiler  because the linker would have problem instantiating  creating a specialized version of  them  Some newer compiler linker can do a  two pass  compilation linking where the compiler does a first pass  then the linker does its work and call the compiler to resolve unresolved things  inline templates

User · Answer

Because the compiler needs to see them in order to inline them  And headers files are the  components  which are commonly included in other translation units    include  file h     Ok  now me  the compiler  can see the definition of that inline function      So I m able to replace calls for the actual implementation

User · Answer

Inline Functions  In C   a macro is nothing but inline function  SO now macros are under control of compiler    Important   If we define a function inside class it will become Inline automatically   Code of Inline function is replaced at the place it is called  so it reduce the overhead of calling function   In some cases Inlining of function can not work  Such as    If static variable used inside inline function  If function is complicated  If recursive call of function  If address of function taken implicitely or explicitely   Function defined outside class as below may become inline   inline int AddTwoVar int x int y     This may not become inline   inline int AddTwoVar int x int y    return x   y       This becomes inline   Function defined inside class also become inline      Inline SpeedMeter functions class SpeedMeter       int speed      public      int getSpeed   const   return speed        void setSpeed int varSpeed    speed   varSpeed       int main         SpeedMeter objSM      objSM setSpeed 80       int speedValue   A getSpeed         Here both getSpeed and setSpeed functions will become inline

User · Answer

I know this is an old thread but thought I should mention that the extern keyword  I ve recently ran into this issue and solved as follows  Helper h  namespace DX       extern inline void ThrowIfFailed HRESULT hr       Helper cpp  namespace DX       inline void ThrowIfFailed HRESULT hr                if  FAILED hr                         std  stringstream ss              ss  lt  lt       lt  lt  hr              throw std  exception ss str   c str

User · Answer

The c   inline keyword is misleading  it doesn t mean  inline this function   If a function is defined as inline  it simply means that it can be defined multiple times as long as all definitions are equal  It s perfectly legal for a function marked inline to be a real function that is called instead of getting code inlined at the point where it s called   Defining a function in a header file is needed for templates  since e g  a templated class isn t really a class  it s a template for a class which you can make multiple variations of  In order for the compiler to be able to e g  make a Foo lt int gt   bar   function when you use the Foo template to create a Foo class  the actual definition of Foo lt T gt   bar   must be visible

User · Answer

The definition of an inline function doesn t have to be in a header file but  because of the one definition rule  ODR  for inline functions  an identical definition for the function must exist in every translation unit that uses it  The easiest way to achieve this is by putting the definition in a header file  If you want to put the definition of a function in a single source file then you shouldn t declare it inline  A function not declared inline does not mean that the compiler cannot inline the function  Whether you should declare a function inline or not is usually a choice that you should make based on which version of the one definition rules it makes most sense for you to follow  adding inline and then being restricted by the subsequent constraints makes little sense

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