To make a function inline should I add the keyword inline in function prototype or in function definition or should I add the inline keyword both in function declaration and function definition.
If you already have a declaration and definition, you don't need to add inline anywhere. The inline keyword is not a magical optimization incantation, but more of a linkage specifier.
An inline function must be defined in every translation unit it's used in. How can it be defined there? Simple, it's put in header:
// foo.h
inline void foo() {}
But wait, I hear you say, won't putting it in header cause a multiple definitions error? Not when it's marked inline, that's why I called it more of a linkage specifier. The inline specifier tells the compiler that all those multiple definition it sees are the same. So it may in fact use any one of them. Or it may inspect the function body and choose to do code inlining, but it doesn't have to.
The primary purpose of the keyword is to allow those multiple definitions to appear inline, not to cause the function's code to be inlined. So again, if you have a definition in a cpp file and a declaration in a header, you don't need to do anything. If you have a utility you want to appear in a header, put it there - definition and all - and add the inline keyword.
How to make function inline:
To make any function as inline, start its definitions with the keyword “inline”.
From the article.
Related
I defined a class in header file and implemented its function in same header file. I didn't put inline keyword with function definition because I think compiler will regard it as a inline function by default -- but inline is only a hint to compiler, right? What if compiler doesn't regard it as inline function because of its length? I never get error message 'multiple definitions' in reality.
struct tmp {
void print() {
...(very long)
}
};
I didn't put inline keyword with function definition because I think compiler will regard it as a inline function by default
Yes, member functions defined in the body of a class are implicitly inline. The keyword is not necessary.
inline is only a hint to compiler, right? What if compiler doesn't regard it as inline function because of its length?
Yes, sort of. Actually, the inline keyword has two meanings.
The first one is the one you are thinking of, the one that hints to the optimizer to inline the code in the function body at the call site. As you said, this is just a hint—the optimizer is free to ignore this request if it determines that it would be a performance pessimization to do so (or if it is unable to inline for some other technical reason). This meaning of the inline keyword is arguably obsolete. All optimizing compilers nowadays ignore the inline keyword because their authors consider their heuristics to be smarter than the programmer. This is almost always the case, making it rather pointless to try and second-guess the optimizer by marking your functions inline.
The second meaning of the inline keyword is to relax the one-definition rule (ODR), making it legal for there to be multiple definitions of the same function visible to the linker. (Although the behavior of the linker under such circumstances is an implementation detail, most of them will just arbitrarily pick one of the definitions. Which of course only works out well if they are all the same.) This meaning of the inline keyword is still very important, and explains why it is still used today in code.
This is the meaning that your code is benefitting from. Since member functions defined in the body of a class are implicitly marked inline, you do not get multiply-defined symbol errors from the linker.
If you had defined the function in the header file but not within the class definition—in other words, if you had done this:
struct tmp {
void print();
};
void tmp::print()
{ ... }
then you would start getting the multiply-defined symbol errors as soon as that header file was included in two or more compilands (i.e., translation units). This is where you would need to add the inline keyword on the function's definition, not because you want the compiler to "inline" it, but because you want to exempt yourself from the ODR.
EDIT #Leon (below) stated that my answer (reproduced below) was INCORRECT. The correct answer is described here - in short, if the compiler decides to not make a function inline, it still puts it in the object module. But the linker will then pick one of the (potentially many) copies in the different modules and discard all the others.
You are right: you won't get the "multiple definition" error because every time the compiler decides to not put a function inline, it makes the function static within the current module. That means that you could have a large number of copies of your large function littered through your code.
I know that the inline keyword is only a hint for the compiler, and not forced (unless using __forceinline e.g. in MSVC).
Is it also the case when declaring the inlined function in header? In which compilation unit will the compiler put the code?
inline is not just a hint to the compiler.
An inline function may be defined in multiple translation units, and all of these definitions will have the same type, address, and definition.
If a function is defined in a header, then it must be declared inline, or it will violate the One Definition Rule when it is included in multiple translation units.
An inline function is either:
A function at global scope can be declared inline using the keyword inline.
A function defined entirely inside a class/struct/union definition, whether it's a member function or a non-member friend function, is always inline.
A function declared constexpr is always inline.
(source)
Is it also the case [that inline is a hint] when declaring the inlined function in header?
Yes. The inline keyword is always a hint to the compiler to perform "inlining".
However, please note that this is only a hint. The compiler is free to ignore it (and many do).
The real reason compilers are able to perform inlining on inline functions is that the whole definition is available. You will notice the same inlining with static functions and instantiated function templates.
In which compilation unit will the compiler put the code?
Before linkage, the inline function will be fully defined in any compilation unit that defines it. It will be compiled in its entirety into each object file.
During linkage, the linker will determine which definition to use, and discard all the others.
See also this question and its answers.
The code will be present in all compillation units that include this header. The main point of inline is saying to the linker that this function can be found in multiple object files and any of these copies can be chosen by linker.
Inline is not forced, ever. If you define a method inside the class definition, it is implicitly inlined. It's like defining it outside the class definition except with inline implied. This has nothing to do with what file the definition is in.
When a function you requested to inline is not actually inlined, it's up to the compiler to decide where to put it. In early days, you could get a non-exported copy in each file that header file was included in. Now, some strategy is applied like putting it in the same place as the first constructor, the first method, or where the virtual function table is. It's compiler-dependent.
I need to define inline functions to improve performance of my code. At the moment declaration of functions are in .h file and definitions are in .cpp file. I added inline keyword at the front of each declaration of functions but I am getting link error. Is possible to separate declaration and definition of inline functions ?
You can separate the declaration and definition fine, but that definition must be available in every translation unit that uses the function, e.g.:
#include <iostream>
inline void foo();
int main() {
foo();
}
inline void foo() {
std::cout << "Hi\n";
}
is perfectly legal and correct.
The exact quote from n3290 § 7.1.2.4 is:
An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly
the same definition in every case (3.2). [ Note: A call to the inline function may be encountered before its
definition appears in the translation unit. —end note ]
Where § 3.2 basically says that it has to be identical everywhere, even overload resolutions etc.
Are you absolutely sure that making your functions 'inline' would improve your performance? I am pretty sure it will not.
The compiler is able to inline some function calls if and only if it can see the body of the inlined functions. So you need to include the body of the function as well, but if do it, you do not need to annotate your function with 'inline' because the compiler only needs the body of the function -- not your 'inline' keyword. Compilers nowadays are smart and know without your hints whether and when to inline functions. And inlining does not necessarily increase your program's performance, and it is likely to increase your executable's size.
See this article by Herb Sutter. He argues that keyword "inline" has no meaning in C++. But I disagree with him. Keyword "inline" makes one difference: you can specify the body of the inline function more than once in the program (provided that it is exactly the same definition each time) -- this is useful when putting function bodies in headers (if you need this for any reason).
Yes, but you have to put the implementation in the header file. That is because in order to be inlined, the definition has to be known, when including the header.
If you do so, modern compilers will automatically inline the function even without the inline keyword.
Use a separate "implementation header" that you will still include everywhere?
You are need specify only once prototype or realization. Both is eligible.
By definition, inline functions must be known at compile time.
If you want to define them in a separate .h file, you can use a
#pragma once
trick to only insert their definition once.
I have found that in some embedded toolchains, the #inline keyword is not allowed in the function declaration. You can specify it in the definition, but (as stated above) many compilers ignore the keyword anyway.
What's the difference between using the inline keyword before a function and just declaring the whole function in the header?
so...
int whatever() { return 4; }
vs
.h:
inline int whatever();
.cpp:
inline int myClass::whatever()
{
return 4;
}
for that matter, what does this do:
inline int whatever() { return 4; }
There are several facets:
Language
When a function is marked with the inline keyword, then its definition should be available in the TU or the program is ill-formed.
Any function defined right in the class definition is implicitly marked inline.
A function marked inline (implicitly or explicitly) may be defined in several TUs (respecting the ODR), whereas it is not the case for regular functions.
Template functions (not fully specialized) get the same treatment as inline ones.
Compiler behavior
A function marked inline will be emitted as a weak symbol in each object file where it is necessary, this may increase their size (look up template bloat).
Whereas the compiler actually inlines the call (ie, copy/paste the code at the point of use instead of performing a regular function call) is entirely at the compiler's discretion. The presence of the keyword may, or not, influence the decision but it is, at best, a hint.
Linker behavior
Weak symbols are merged together to have a single occurrence in the final library. A good linker could check that the multiple definitions concur but this is not required.
without inline, you will likely end up with multiple exported symbols, if the function is declared at the namespace or global scope (results in linker errors).
however, for a class (as seen in your example), most compilers implicitly declare the method as inline (-fno-default-inline will disable that default on GCC).
if you declare a function as inline, the compiler may expect to see its definition in the translation. therefore, you should reserve it for the times the definition is visible.
at a higher level: a definition in the class declaration is frequently visible to more translations. this can result in better optimization, and it can result in increased compile times.
unless hand optimization and fast compiles are both important, it's unusual to use the keyword in a class declaration these days.
The purpose of inline is to allow a function to be defined in more than one translation unit, which is necessary for some compilers to be able to inline it wherever it's used. It should be used whenever you define a function in a header file, although you can omit it when defining a template, or a function inside a class definition.
Defining it in a header without inline is a very bad idea; if you include the header from more than one translation unit, then you break the One Definition Rule; your code probably won't link, and may exhibit undefined behaviour if it does.
Declaring it in a header with inline but defining it in a source file is also a very bad idea; the definition must be available in any translation unit that uses it, but by defining it in a source file it is only available in one translation unit. If another source file includes the header and tries to call the function, then your program is invalid.
This question explains a lot about inline functions What does __inline__ mean ? (even though it was about inline keyword.)
Basically, it has nothing to do with the header. Declaring the whole function in the header just changes which source file has that the source of the function is in. Inline keyword modifies where the resulting compiled function will be put - in it's own place, so that every call will go there, or in place of every call (better for performance). However compilers sometimes choose which functions or methods to make inline for themselves, and keywords are simply suggestions for the compiler. Even functions which were not specified inline can be chosen by the compiler to become inline, if that gives better performance.
If you are linking multiple objects into an executable, there should normally only be one object that contains the definition of the function. For int whatever() { return 4; } - any translation unit that is used to produce an object will contain a definition (i.e. executable code) for the whatever function. The linker won't know which one to direct callers to. If inline is provided, then the executable code may or may not be inlined at the call sites, but if it's not the linker is allowed to assume that all the definitions are the same, and pick one arbitrarily to direct callers to. If somehow the definitions were not the same, then it's considered YOUR fault and you get undefined behaviour. To use inline, the definition must be known when compiler the call, so your idea of putting an inline declaration in a header and the inline definition in a .cpp file will only work if all the callers happen to be later in that same .cpp file - in general it's broken, and you'd expect the (nominally) inline function's definition to appear in the header that declares it (or for there to be a single definition without prior declaration).
Non inline function defined in header file with guards
#if !defined(HEADER_RANDOM_H)
#define HEADER_RANDOM_H
void foo()
{
//something
}
#endif
Results in linker error : Already defined in someother.obj file
Making the function inline works fine but I am not able to understand why the function is already erroring out in first case.
If the header is included in more than one source file and the function is not marked as "inline" you will have more than one definition. The include guards only prevent multiple inclusions in the same source file.
You're violating the one definition rule. If you want to define a function directly in the header, you must mark it as inline -- that will allow the function to be defined multiple times. Also note that inline has no other meaning, particularly it doesn't force the compiler to inline calls (contrary to popular belief).
Since it is not inline, each translation unit will have its own copy of the function resulting in the function being defined multiple times.