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inline, __inline, __forceinline

The inline and __inline specifiers instruct the compiler to insert a copy of the function body into each place the function is called.

inline function_declarator;   
__inline function_declarator;   // Microsoft Specific
__forceinline function_declarator;   // Microsoft Specific

The insertion (called inline expansion or inlining) occurs only if the compiler's cost/benefit analysis show it to be profitable. Inline expansion alleviates the function-call overhead at the potential cost of larger code size.

The __forceinline keyword overrides the cost/benefit analysis and relies on the judgment of the programmer instead. Exercise caution when using __forceinline. Indiscriminate use of __forceinline can result in larger code with only marginal performance gains or, in some cases, even performance losses (due to increased paging of a larger executable, for example).

Using inline functions can make your program faster because they eliminate the overhead associated with function calls. Functions expanded inline are subject to code optimizations not available to normal functions.

The compiler treats the inline expansion options and keywords as suggestions. There is no guarantee that functions will be inlined. You cannot force the compiler to inline a particular function, even with the __forceinline keyword.

The inline keyword is available only in C++. The __inline and __forceinline keywords are available in both C and C++. For compatibility with previous versions, _inline is a synonym for __inline.

The inline keyword tells the compiler that inline expansion is preferred. However, the compiler can create a separate instance of the function (instantiate) and create standard calling linkages instead of inserting the code inline. Two cases where this can happen are:

  • Recursive functions.
  • Functions that are referred to through a pointer elsewhere in the translation unit.

These reasons may interfere with inlining, as may others, at the discretion of the compiler; you should not depend on the inline specifier to cause a function to be inlined.

Note that for a function to be considered as a candidate for inlining, it must use the new-style function definition. Functions that are declared as inline, and that are not class member functions, have internal linkage unless otherwise specified.

As with normal functions, there is no defined order of evaluation of the arguments to an inline function. In fact, it could be different from the order in which the arguments are evaluated when passed using normal function call protocol.

The /Ob compiler optimization option helps to determine whether inline function expansion actually occurs.

Example 1

// inline_keyword1.cpp
inline int max( int a , int b )
{
   if( a > b ) return a;
   return b;
}
int main()
{
}

A class's member functions can be declared inline either by using the inline keyword or by placing the function definition within the class definition.

Example 2

// inline_keyword2.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;

class MyClass
{
public:
   void print() { cout << i << ' '; }   // Implicitly inline
private:
   int i;
};

int main()
{
}

Microsoft Specific

The __inline keyword is equivalent to inline.

Even with __forceinline, the compiler cannot inline code in all circumstances. The compiler cannot inline a function if:

  • The function or its caller is compiled with /Ob0 (the default option for debug builds).
  • The function and the caller use different types of exception handling (C++ exception handling in one, structured exception handling in the other).
  • The function has a variable argument list.
  • The function uses inline assembly, unless compiled with /Og, /Ox, /O1, or /O2.
  • The function returns an unwindable object by value, when compiled with /GX, /EHs, or /EHa.
  • The function receives an unwindable copy-constructed object passed by value, when compiled with /GX, /EHs,, or /EHa.
  • The function is recursive and not accompanied by #pragma inline_recursion(on). With the pragma, recursive functions can be inlined to a default depth of eight calls. To change the inlining depth, use inline_depth pragma.
  • The function is virtual and is called virtually. Direct calls to virtual functions can be inlined.
  • The program takes the address of the function and the call is made via the pointer to the function. Direct calls to functions that have had their address taken can be inlined.
  • The function is also marked with the naked __declspec modifier.

If the compiler cannot inline a function declared with __forceinline, it generates a level 1 warning (4714).

Recursive functions can be substituted inline to a depth specified by the inline_depth pragma. After that depth, recursive function calls are treated as calls to an instance of the function. The inline_recursion pragma controls the inline expansion of a function currently under expansion. See the Inline-Function Expansion (/Ob) compiler option for related information.

END Microsoft Specific

For more information on using the inline specifier, see:

See Also

C++ Keywords | noinline | auto_inline pragma

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