Could a very long Class function member defined in header file? - c++

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.

Related

Inline keyword vs header definition

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).

Are C++ class methods defined in the header always inlined?

Edit: I've restored the original title but really what I should have asked was this: 'How do C++ linkers handle class methods which have been defined in multiple object files'
Say I have a C++ class defined in a header along these lines:
class Klass
{
int Obnoxiously_Large_Method()
{
//many thousands of lines of code here
}
}
If I compile some C++ code which uses 'Obnoxiously_Large_Method' in several locations, will the resulting object file always inline the code for 'Obnoxiously_Large_Method' or will it optimise for size (for example, when using g++ -Os) and create a single instance of 'Obnoxiously_Large_Method' and use it like a normal function?, if so, how do linkers resolve the collisions between other object files which have instantiated the same function?. Is there some arcane C++ namespace Juju which keeps the separate object instances of method from colliding with each other?
7.1.2 Function specifiers
A function declaration (8.3.5, 9.3, 11.4) 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.
So, the compiler is not required to actually 'inline' any function.
However, the standard also says,
An inline function with external linkage shall have the same address in all translation units.
Member functions normally have external linkage (one exception is when the member function belongs to a 'local' class), so inline functions must have a unique address for cases where the address of the function is taken. In this case, the compiler will arrange for the linker to throw away all but one instance of a non-inlined copy of the function and fix-up all address references to the function to be to the one that's kept.
Section [9.3], Member functions, of the C++98 Standard states:
A member function may be defined (8.4) in its class definition, in which case it is an inline member function (7.1.2).
Thus, it has always been the case that marking member functions defined in the class definition explicitly inline is unnecessary.
On the inline function specifier, the Standard states:
A function declaration (8.3.5, 9.3, 11.4) with an inline specifier declares an inline function. The inline specifier indicates to the [C++ compiler] that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism. [However, a C++ compiler] is not required to perform this inline substitution at the point of call;
So, it is up to the compiler whether it will actually inline the definition of the function rather than call it via the usual function call mechanism.
Nothing is always inlined (unless your compiler has an attribute or private keyword to force it to do so...at which point you're writing $(COMPILER)-flavored C++ rather than standard C++). Very long functions, recursive functions, and a few other things generally aren't inlined.
The compiler can choose not to inline stuff if it determines that doing so will degrade performance, unreasonably increase the object file's size, or make things work incorrectly. Or if it's optimizing for size instead of speed. Or if you ask it not to. Or if it doesn't like your shirt. Or if it's feeling lazy today, cause it compiled too much last night. Or for any other reason. Or for no reason at all.
There is no - single answer to this question. Compilers are smart beasts. You can specifically use the inline words if you want, but this doesn't mean that the compiler will actually inline the function.
Inline is there to help the developer with optmization. It hints at the compiler that something should be inlined, but these hints are generally ignored nowadays, since compilers can do better at register assignment and deciding when to inline functions (in fact, a compiler can either inline or not inline a function at different times). Code generation on modern processors is far more complicated than on the more deterministic ones common when Ritchie was inventing C.
What the word means now, in C++, is that it can have multiple identical definitions, and needs to be defined in every translation unit that uses it. (In other words, you need to make sure it can be inlined.) You can have an inline function in a header with no problems, and member functions defined in a class definition are automatically effectively inline.
That said, I used to work with a greenhills compiler, and it actually obeyed my will more than it disobeyed it :).. It's up to the compiler, really.
The inline keyword deals with c++ definition of a function. The compiler may inline object code where ever it wants.
Functions defined inline (eg they use the inline keyword), create object code for the function in every compilation unit. Those functions are marked as special so the linker knows to only use one.
See this answer for more specifics.
It doesn't have to be inlined, no; it's just like if you specified inline explicitly.
When you write inline, you promise that this method won't be called from translation units where it isn't defined, and therefore, that it can have internal linkage (so the linker won't connect one object-file's reference to it to another object-file's definition of it). [This paragraph was wrong. I'm leaving it intact, just struck-out, so that the below comments will still make sense.]

Inline functions in C++

If we define a member function inside the class definition itself, is it necessarily treated inline or is it just a request to the compiler which it can ignore.
Yes, functions that are defined inside a class body are implicitly inline.
(As with other functions declared inline it doesn't mean that the complier has to perform inline expansion in places where the function is called, it just enables the permitted relaxations of the "one definition rule", combined with the requirement that a definition must be included in all translation units where the function is used.)
As stated by others, a method defined within a class is automatically requested inline.
It's useful to understand why.
Suppose it weren't. You'd have to generate code for such a function, and everywhere it is called, a jump to subroutine instruction would have to reference the location, via the linker.
class A {
public:
void f() { ... your code ... }
};
Every time this code is seen, if it's not inline, the compiler can only assume it must be generated, so it would generate a symbol. Suppose it was like this:
A__f_v:
If that symbol were global, then if you happened to include this class code multiple times in different modules, you would have a multiply defined symbol error at link time. So it can't be global. Instead, it's file local.
Imagine you include the above header file in a number of modules. In each one, it's going to generate a local copy of that code. Which is better than not compiling at all, but you're getting multiple copies of the code when you really need only one.
This leads the the following conclusion: if your compiler is not going to inline a function, you are significantly better off declaring it somewhere once, and not requesting it to be inlined.
Unfortunately, what is and is not inline is not portable. It's defined by the compiler writer. A good rule of thumb is to always make every one liner, particularly all functions which themselves just call a function, inline, as you remove overhead. Anything below three lines of linear code is almost certainly ok. But if you have a loop in the code, the question is whether the compiler will allow it inline, and more to the point, how much benefit you would see even if it did what you want.
consider this inline code:
inline int add(int a, int b) { return a + b; }
It's not only almost as small as the prototype would be in source code, but the assembly language generated by the inline code is smaller than the call to a routine would be. So this code is smaller, and faster.
And, if you happen to be passing in constants:
int c= add(5,4);
It's resolved at compile time and there is no code.
In gcc, I recently noticed that even if I don't inline code, if it's local to a file, they will sneakily inline it anyway. It's only if I declare the function in a separate source module that they do not optimize away the call.
On the other end of the spectrum, suppose you request inline on a 1000 line piece of code. Even if your compiler is silly enough to go along with it, the only thing you save is the call itself, and the cost is that every time you call it, the compiler must paste all that code in. If you call that code n times, your code grows by the size of the routine * n. So anything bigger than 10 lines is pretty much not worth inlining, except for the special case where it is only called a very small number of times. An example of that might be in a private method called by only 2 others.
If you request to inline a method containing a loop, it only makes sense if it often executes a small number of times. But consider a loop which iterates one million times. Even if the code is inlined, the percentage of time spent in the call is tiny. So if you have methods with loops in it, which tend to be bigger anyway, those are worth removing from the header file because they a) will tend to be rejected as inline by the compiler and b) even if they were inlined, are generally not going to provide any benefit
It is necessarily treated by the compiler as a request for inline -- which it can ignore. There are some idioms for defining some functions in the header (e.g. empty virtual destructors) and some necessary header definitions (template functions), but other than that see GotW #33 for more information.
Some have noted that the compiler may even inline functions you never asked it to, but I'm not sure whether that would defeat the purpose of requesting to inline a function.
It is indeed inlined - but any inline request can be ignored by the compiler.
It is a request to the compiler that it can ignore.
The 2003 ISO C++ standard says
7.1.2/2 A function declaration (8.3.5, 9.3, 11.4) 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.
7.1.2/3 A function defined within a class definition is an inline
function. The inline specifier shall
not appear on a block scope function
declaration.
7.1.2/4 An inline function shall be defined in every translation unit in
which it is 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 defi-nition appears in the
translation unit. ] If a function
with external linkage is declared
inline in one transla-tion unit, it
shall be declared inline in all
translation units in which it
appears; no diagnostic is required. An
inline function with external
linkage shall have the same address in
all translation units. A static
local variable in an extern inline
function always refers to the same
object. A string literal in an
extern inline function is the same
object in different translation
units.
There are two things that shouldn't be lumped together:
How you mark a function as being inline: define it with inline in front of the signature or define it at declaration point;
What the compiler will treat such inline marking: regardless of how you marked the function as inline it will be treated as a request by the compiler.

Does defining a function inside a header always make the compiler treat it as inline?

I just learned that defining a c++ function inside a class's header file make the function inline. But I know that putting the inline keyword next to a function is only a suggestion and the compiler wont necessarily follow it. Is this the same for header defined c++ functions and is there a difference in behavior between a standalone c++ function and a c++ function that is part of a class?
"defining a c++ function inside a class's header file make the function inline"
That's not true. Defining a function (that is to say, providing the body of the function instead of just a declaration) inside a class definition makes it inline. By "makes it inline", I mean it's the same as giving it the inline keyword. But class definitions don't have to be in headers, and headers can contain other things than class definitions.
So in this example, the function foo is implicitly inline. The function bar is not implicitly inline:
struct Foo {
void foo() {}
void bar();
};
void Foo::bar() {}
"putting the inline keyword next to a function is only a suggestion and the compiler wont necessarily follow it"
inline has two effects. One of them is a hint to the compiler which it can ignore. The other is not optional, and always has its effect. The "hint" is that the compiler is advised to replace calls to that function with a copy of the code for the function itself.
The guaranteed effect is that an inline function can be defined in multiple translation units, and those be linked together, without a multiple definition error, and all but one of the copies is removed by the linker. So, if the example above appears in a header file which is shared between multiple translation units, bar needs to be explicitly marked inline. Otherwise, the linker will discover multiple definitions of bar, which is not allowed.
Despite the name, inline in C++ is mostly about the second, compulsory effect, not the first, optional one. Modern optimising compilers have their own ideas about which calls should be inlined, and don't pay a whole lot of attention to inline when making that decision. For instance I've seen it have an effect in gcc at moderate optimisation levels, but at low levels approximately nothing is inlined, and at high levels approximately everything is (if the definition is available when the call is compiled) unless it makes the function too big.
Whether a function is defined in a header or in a cpp file has absolutely no effect on anything by itself. You can safely imagine that what #include does is copy and paste the header file into the cpp file in the preprocessor, before the compiler ever sees it. If a function is defined in the same translation unit as a call to it, then the function code is available to be inlined by the compiler. If they're in different translation units, then the code is not available and the call can only be inlined by the linker, with whole-program optimisation or similar. A "translation unit" more or less means, "a cpp file, after all the headers have been copy and pasted into it".
C++ compilers are free to choose what will be inline and what won't, no matter what hints you give them. It shouldn't matter if the function is part of a class or not, or whether it is in a header file or source file; the compiler doesn't pay attention to those things while making its decision.
No, not always. The compiler treats it as a hint, just like the inline keyword, but it mostly decides on its own, because it knows better than you what the costs and benefits may be. Inlining the code avoids the function call overhead, but makes the code bigger, which has negative performance impacts on the instruction cache.
These performance hints from the programmer are generally more and more often ignored by the compiler. What it does not ignore (or rather, what the linker does not ignore) is that a function declared inline may appear in several compilation units, and should be treated as multiple copies of the same function without resulting in linker errors.
If you place the definition of a free non-template function in a header file, you will end up with a function definition in each .cpp file that includes the header (directly or indirectly). This can lead to problems when linking.
However, if you declare the function to be inline, the linker will make sure you only use a single definition even if it is included at multiple places.

When do compilers inline C++ code?

In C++, do methods only get inlined if they are explicitly declared inline (or defined in a header file), or are compilers allowed to inline methods as they see fit?
The inline keyword really just tells the linker (or tells the compiler to tell the linker) that multiple identical definitions of the same function are not an error. You'll need it if you want to define a function in a header, or you will get "multiple definition" errors from the linker, if the header is included in more than one compilation unit.
The rationale for choosing inline as the keyword seems to be that the only reason why one would want to define a (non-template) function in a header is so it could be inlined by the compiler. The compiler cannot inline a function call, unless it has the full definition. If the function is not defined in the header, the compiler only has the declaration and cannot inline the function even if it wanted to.
Nowadays, I've heard, it's not only the compiler that optimizes the code, but the linker can do that as well. A linker could (if they don't do it already) inline function calls even if the function wasn't defined in the same compilation unit.
And it's probably not a good idea to define functions larger than perhaps a single line in the header if at all (bad for compile time, and should the large function be inlined, it might lead to bloat and worse performance).
Yes, the compiler can inline code even if it's not explicitly declared as inline.
Basically, as long as the semantics are not changed, the compiler can virtually do anything it wants to the generated code. The standard does not force anything special on the generated code.
Compilers might inline any function or might not inline it. They are allowed to use the inline decoration as a hint for this decision, but they're also allowed to ignore it.
Also note that class member functions have an implicit inline decoration if they are defined right in the class definition.
Compilers may ignore your inline declaration. It is basically used by the compiler as a hint in order decide whether or not to do so. Compilers are not obligated to inline something that is marked inline, or to not inline something that isn't. Basically you're at the mercy of your compiler and the optimization level you choose.
If I'm not mistaken, when optimizations are turned on, the compiler will inline any suitable routine or method.
Text from IBM information Center,
Using the inline specifier is only a
suggestion to the compiler that an
inline expansion can be performed; the
compiler is free to ignore the
suggestion.
C Language Any function, with the exception of main, can be declared or
defined as inline with the inline
function specifier. Static local
variables are not allowed to be
defined within the body of an inline
function.
C++ functions implemented inside of a class declaration are
automatically defined inline. Regular
C++ functions and member functions
declared outside of a class
declaration, with the exception of
main, can be declared or defined as
inline with the inline function
specifier. Static locals and string
literals defined within the body of an
inline function are treated as the
same object across translation units;
Your compiler's documentation should tell you since it is implementation dependent. For example, GCC according to its manual never inlines any code unless optimisation is applied.
If the compiler does not inline the code, the inline keyword will have the same effect as static, and each compilation unit that calls the code will have its own copy. A smart linker may reduce these to a single copy.
The compiler can inline whatever it wants in case inlining doesn't violate the code semantics and it can reach the function code. It can also inline selectively - do inline when it feels it's a good idea and not inline when it doesn't feel it's a good idea or when it would violate the code semantics.
Some compilers can do inlining even if the function is in another translation unit - that's called link-time code generation.
Typical cases of when inlining would violate code semantics are virtual calls and passing a function address into another function or storing it.
Compiler optimize as he wants unless you spec the opposite.
The inline keyword is just a request to the compiler. The compiler reserves the right to make or not make a function inline. One of the major factor that drives the compiler's decision is the simplicity of code(not many loops)
Member functions are declared inline by default.(The compiler decides here also)
These are not hard and fast rules. It varies according to the compiler implementations.
If anybody knows other factors involved, please post.
Some of the situations where inline expansion may NOT work are:
For functions returning values, if a loop, a switch, or a goto exists
For function not returning values, if a return statement exits;
If functions contain static variables
If inline functions are recursive.
Inline expansion makes a program run faster because the overhead of a function call and return statement is eliminated. However, it makes the program to take up more memory because the statements that define the inline functions are reproduced at each point where the function is called. So, a trade-off becomes necessary.
(As given in one of my OOP books)