C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s, an inline function is a function upon which the compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
In computing, inline expansion, or inlining, is a manual or compiler optimization that replaces a function call site with the body of the callee. This optimization may improve time and space usage at runtime, at the possible cost of increasing the final size of the program In computing, inline...
. In other words, the programmer has requested that the compiler insert the complete body of the function in every place that the function is called, rather than generating code to call the function in the one place it is defined. (However, compilers are not obligated to respect this request.)
Motivation
Inline expansion is used to eliminate the time overhead when a function is called. It is typically used for functions that execute frequently. It also has a space benefit for very small functions, and is an enabling transformation for other optimizations
Optimization (computer science)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
.
Without inline functions, however, the compiler decides which functions to inline. The programmer has little or no control over which functions are inlined and which are not. Giving this degree of control to the programmer allows for the use of application-specific knowledge in choosing which functions to inline.
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, inline expansion was accomplished at the source level using parameterized macro
Parameterized macro
In computer science, a parameterized macro is a type of macro that is able to insert given objects into its expansion. This gives the macro some of the power of a function....
s. Use of true inline functions, as are available in C99
C99
C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...
, provides several benefits over this approach:
Macro invocations do not perform type checking, or even check that arguments are well-formed, whereas function calls usually do.
In C, a macro cannot use the return keyword with the same meaning as a function would do (it would make the function that asked the expansion terminate, rather than the macro). In other words, a macro cannot return anything which is not the result of the last expression invoked inside it.
Since C macros use mere textual substitution, this may result in unintended side-effects and inefficiency due to re-evaluation of arguments and order of operations
Order of operations
In mathematics and computer programming, the order of operations is a rule used to clarify unambiguously which procedures should be performed first in a given mathematical expression....
.
Compiler errors within macros are often difficult to understand, because they refer to the expanded code, rather than the code the programmer typed.
Many constructs are awkward or impossible to express using macros, or use a significantly different syntax. Inline functions use the same syntax as ordinary functions, and can be inlined and un-inlined at will with ease.
Debugging information for inlined code is usually more helpful than that of macro-expanded code.
Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem. The approach can be applied to many types of problems, and is one of the central ideas of computer science....
Bjarne Stroustrup ; born December 30, 1950 in Århus, Denmark) is a Danish computer scientist, most notable for the creation and the development of the widely used C++ programming language...
, the designer of C++, likes to emphasize that macros should be avoided wherever possible, and advocates extensive use of inline functions.
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...
, and GNU C each have support for inline functions. Different compilers vary in how complex a function they can manage to inline. Mainstream C++ compilers like Microsoft Visual C++ and GCC
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
support an option that lets the compilers automatically inline any suitable function, even those not marked as inline functions.
An inline function can be written in C99 (see below for a problem in C99 regarding this definition) or C++ like this:
inline int max(int a, int b)
{
return (a > b) ? a : b;
}
Then, a statement such as the following:
a = max(x, y);
may be transformed into a more direct computation:
a = (x > y) ? x : y;
Analogs in other languages
In the Ada programming language, there exists a pragma
Directive (programming)
In computer programming, the term directive is applied in a variety of ways that are similar to the term command. It is also used to describe some programming language constructs ....
for inline functions. Most other languages, including Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
and functional languages, do not provide language constructs for inline functions, but often do perform aggressive inline expansion
Inline expansion
In computing, inline expansion, or inlining, is a manual or compiler optimization that replaces a function call site with the body of the callee. This optimization may improve time and space usage at runtime, at the possible cost of increasing the final size of the program In computing, inline...
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
Microsoft Visual C++ and few other compilers support non-standard constructs for defining inline functions, such as __inline and __forceinline specifiers.
The __inline keyword is equivalent to inline.
The __forceinline keyword allow the programmer to force the compiler to inline the function, but indiscriminate use of __forceinline can result in larger code (bloated executable file), minimum or none performance gain and in some cases even performance losses. The compiler cannot inline the function in all circumstances, even with the __forceinline keyword applied. If the compiler cannot inline a function declared with __forceinline, a warning of level 1 is generated. A list of cases when __forceinline will not take effect is listed below (based on Microsoft Specifications at MSDN):
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.
Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem. The approach can be applied to many types of problems, and is one of the central ideas of computer science....
and not accompanied by #pragma inline_recursion(on). With the pragma, recursive functions are inlined to a default depth of 16 calls. To reduce the inlining depth, use inline_depth pragma.
In object-oriented programming, a virtual function or virtual method is a function or method whose behaviour can be overridden within an inheriting class by a function with the same signature...
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.
__forceinline is useful if:
inline or __inline is not respected by the compiler (ignored by compiler cost/benefit analyzer)
code portability is not required
inlining results in a necessary performance boost
Example of portable code:
ifdef _MSC_VER
#define INLINE __forceinline /* use __forceinline (VC++ specific) */
In computing, inline expansion, or inlining, is a manual or compiler optimization that replaces a function call site with the body of the callee. This optimization may improve time and space usage at runtime, at the possible cost of increasing the final size of the program In computing, inline...
in general, inline functions as a language feature may not be as valuable as they appear, for a number of reasons:
Often, a compiler is in a better position than a human to decide whether a particular function should be inlined. Sometimes the compiler may not be able to inline as many functions as the programmer indicates.
An important point to note is that the code (of the inline function) gets exposed to its client(the calling function).
As functions evolve, they may become suitable for inlining where they were not before, or no longer suitable for inlining where they were before. While inlining or un-inlining a function is easier than converting to and from macros, it still requires extra maintenance which typically yields relatively little benefit.
Inline functions used in proliferation in native C-based compilation systems can increase compilation time, since the intermediate representation of their bodies is copied into each call site where they are
The specification of inline in C99 requires exactly one additional external definition of a function in another compilation unit, when the corresponding inline definition, that can occur multiple times in different compilation units, if that function is used somewhere. That can easily lead to linker errors because such a definition wasn't provided by the programmer. For this reason, inline in C99 often is used together with static, which gives the function internal linkage.
In C++, it is necessary to define an inline function in every module (compilation unit) that uses it, whereas an ordinary function must be defined in only a single module. Otherwise it would not be possible to compile a single module independently of all other modules.
For problems with the optimization itself, rather than the language feature, see problems with inline expansion.
Quotes
"A function declaration [ . . . ] with an inline specifier declares an inline function. The inline specifier indicates to the implementation that inline substitution of the function body at the point of call is to be preferred to the usual function 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 7.1.2 shall still be respected."
— ISO 14882:1998(E), the current C++ standard, section 7.1.2
"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. The extent to which such suggestions are effective is implementation-defined (footnote: For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration.)
"[ . . . ] An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit
Translation unit (programming)
In C programming language terminology, a translation unit is the ultimate input to a C compiler from which an object file gets generated.-Context:...
. 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."
— ISO 9899:1999(E), the C99 standard, section 6.7.4
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
