Inline Functions (C++)

A function defined in the body of a class declaration is an inline function.

Example

In the following class declaration, the Account constructor is an inline function. The member functions GetBalance, Deposit, and Withdraw aren't specified as inline but can be implemented as inline functions.

// Inline_Member_Functions.cpp
class Account
{
public:
    Account(double initial_balance) { balance = initial_balance; }
    double GetBalance();
    double Deposit( double Amount );
    double Withdraw( double Amount );
private:
    double balance;
};

inline double Account::GetBalance()
{
    return balance;
}

inline double Account::Deposit( double Amount )
{
    return ( balance += Amount );
}

inline double Account::Withdraw( double Amount )
{
    return ( balance -= Amount );
}
int main()
{
}

Note

In the class declaration, the functions were declared without the inline keyword. The inline keyword can be specified in the class declaration; the result is the same.

A given inline member function must be declared the same way in every compilation unit. This constraint causes inline functions to behave as if they were instantiated functions. Additionally, there must be exactly one definition of an inline function.

A class member function defaults to external linkage unless a definition for that function contains the inline specifier. The preceding example shows that you don't have to declare these functions explicitly with the inline specifier. Using inline in the function definition causes it to be an inline function. However, it's not allowed to redeclare a function as inline after a call to that function.

inline, __inline, and __forceinline

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

The insertion, called inline expansion or inlining, occurs only if the compiler's cost-benefit analysis shows it's worthwhile. Inline expansion minimizes 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 (because of the 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's no guarantee that functions will be inlined. You can't force the compiler to inline a particular function, even with the __forceinline keyword. When compiling with /clr, the compiler won't inline a function if there are security attributes applied to the function.

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 and _forceinline are synonyms for __inline, and __forceinline unless compiler option /Za (Disable language extensions) is specified.

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 behavior 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 shouldn't depend on the inline specifier to cause a function to be inlined.

As with normal functions, there's no defined order for argument evaluation in an inline function. In fact, it could be different from the argument evaluation order when passed using the normal function-call protocol.

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

/LTCG does cross-module inlining whether it's requested in source code or not.

Example 1

// inline_keyword1.cpp
// compile with: /c
inline int max( int a , int b ) {
   if( a > b )
      return a;
   return b;
}

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 /c
#include <iostream>
using namespace std;

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

Microsoft-specific

The __inline keyword is equivalent to inline.

Even with __forceinline, the compiler can't inline code in all circumstances. The compiler can't 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 /Ox, /O1, or /O2.

  • The function is recursive and doesn't have #pragma inline_recursion(on) set. With the pragma, recursive functions are inlined to a default depth of 16 calls. To reduce 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 can't inline a function declared with __forceinline, it generates a level 1 warning, except when:

  • The function is compiled by using /Od or /Ob0. No inlining is expected in these cases.

  • The function is defined externally, in an included library or another translation unit, or is a virtual call target or indirect call target. The compiler can't identify non-inlined code that it can't find in the current translation unit.

Recursive functions can be replaced with inline code to a depth specified by the inline_depth pragma, up to a maximum of 16 calls. After that depth, recursive function calls are treated as calls to an instance of the function. The depth to which recursive functions are examined by the inline heuristic can't exceed 16. 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:

When to use inline functions

Inline functions are best used for small functions such as accessing private data members. The main purpose of these one- or two-line "accessor" functions is to return state information about objects. Short functions are sensitive to the overhead of function calls. Longer functions spend proportionately less time in the calling and returning sequence and benefit less from inlining.

A Point class can be defined as follows:

// when_to_use_inline_functions.cpp
class Point
{
public:
    // Define "accessor" functions as
    //  reference types.
    unsigned& x();
    unsigned& y();
private:
    unsigned _x;
    unsigned _y;
};

inline unsigned& Point::x()
{
    return _x;
}
inline unsigned& Point::y()
{
    return _y;
}
int main()
{
}

Assuming coordinate manipulation is a relatively common operation in a client of such a class, specifying the two accessor functions (x and y in the preceding example) as inline typically saves the overhead on:

  • Function calls (including parameter passing and placing the object's address on the stack)

  • Preservation of caller's stack frame

  • New stack frame setup

  • Return-value communication

  • Restoring the old stack frame

  • Return

Inline functions vs. macros

Inline functions are similar to macros, because the function code is expanded at the point of the call at compile time. However, inline functions are parsed by the compiler, and macros are expanded by the preprocessor. As a result, there are several important differences:

  • Inline functions follow all the protocols of type safety enforced on normal functions.

  • Inline functions are specified using the same syntax as any other function except that they include the inline keyword in the function declaration.

  • Expressions passed as arguments to inline functions are evaluated once. In some cases, expressions passed as arguments to macros can be evaluated more than once.

The following example shows a macro that converts lowercase letters to uppercase:

// inline_functions_macro.c
#include <stdio.h>
#include <conio.h>

#define toupper(a) ((a) >= 'a' && ((a) <= 'z') ? ((a)-('a'-'A')):(a))

int main() {
   char ch;
   printf_s("Enter a character: ");
   ch = toupper( getc(stdin) );
   printf_s( "%c", ch );
}
//  Sample Input:  xyz
// Sample Output:  Z

The intent of the expression toupper(getc(stdin)) is that a character should be read from the console device (stdin) and, if necessary, converted to uppercase.

Because of the implementation of the macro, getc is executed once to determine whether the character is greater than or equal to "a," and once to determine whether it's less than or equal to "z." If it is in that range, getc is executed again to convert the character to uppercase. It means the program waits for two or three characters when, ideally, it should wait for only one.

Inline functions remedy the problem previously described:

// inline_functions_inline.cpp
#include <stdio.h>
#include <conio.h>

inline char toupper( char a ) {
   return ((a >= 'a' && a <= 'z') ? a-('a'-'A') : a );
}

int main() {
   printf_s("Enter a character: ");
   char ch = toupper( getc(stdin) );
   printf_s( "%c", ch );
}
Sample Input: a
Sample Output: A

See also

noinline
auto_inline