Are inline virtual functions really a non-sense?

Question

I got this question when I received a code review comment saying virtual functions need not be inline.

I thought inline virtual functions could come in handy in scenarios where functions are called on objects directly. But the counter-argument came to my mind is -- why would one want to define virtual and then use objects to call methods?

Is it best not to use inline virtual functions, since they're almost never expanded anyway?

Code snippet I used for analysis:

class Temp
{
public:

    virtual ~Temp()
    {
    }
    virtual void myVirtualFunction() const
    {
        cout<<"Temp::myVirtualFunction"<<endl;
    }

};

class TempDerived : public Temp
{
public:

    void myVirtualFunction() const
    {
        cout<<"TempDerived::myVirtualFunction"<<endl;
    }

};

int main(void) 
{
    TempDerived aDerivedObj;
    //Compiler thinks it's safe to expand the virtual functions
    aDerivedObj.myVirtualFunction();

    //type of object Temp points to is always known;
    //does compiler still expand virtual functions?
    //I doubt compiler would be this much intelligent!
    Temp* pTemp = &aDerivedObj;
    pTemp->myVirtualFunction();

    return 0;
}

Answer 1

Actually in some cases adding "inline" to a virtual final override can make your code not compile so there is sometimes a difference (at least under VS2017s compiler)!

Actually I was doing a virtual inline final override function in VS2017 adding c++17 standard to compile and link and for some reason it failed when I am using two projects.

I had a test project and an implementation DLL that I am unit testing. In the test project I am having a "linker_includes.cpp" file that #include the *.cpp files from the other project that are needed. I know... I know I can set up msbuild to use the object files from the DLL, but please bear in mind that it is a microsoft specific solution while including the cpp files is unrelated to build-system and much more easier to version a cpp file than xml files and project settings and such...

What was interesting is that I was constantly getting linker error from the test project. Even if I added the definition of the missing functions by copy paste and not through include! So weird. The other project have built and there are no connection between the two other than marking a project reference so there is a build order to ensure both is always built...

I think it is some kind of bug in the compiler. I have no idea if it exists in the compiler shipped with VS2020, because I am using an older version because some SDK only works with that properly :-(

I just wanted to add that not only marking them as inline can mean something, but might even make your code not build in some rare circumstances! This is weird, yet good to know.

PS.: The code I am working on is computer graphics related so I prefer inlining and that is why I used both final and inline. I kept the final specifier to hope the release build is smart enough to build the DLL by inlining it even without me directly hinting so...

PS (Linux).: I expect the same does not happen in gcc or clang as I routinely used to do these kind of things. I am not sure where this issue comes from... I prefer doing c++ on Linux or at least with some gcc, but sometimes project is different in needs.

Answer 2

Well, actually virtual functions can always be inlined, as long they're statically linked together: suppose we have an abstract class Base with a virtual function F and derived classes Derived1 and Derived2:

class Base {
  virtual void F() = 0;
};

class Derived1 : public Base {
  virtual void F();
};

class Derived2 : public Base {
  virtual void F();
};

An hypotetical call b->F(); (with b of type Base*) is obviously virtual. But you (or the compiler...) could rewrite it like so (suppose typeof is a typeid-like function that returns a value that can be used in a switch)

switch (typeof(b)) {
  case Derived1: b->Derived1::F(); break; // static, inlineable call
  case Derived2: b->Derived2::F(); break; // static, inlineable call
  case Base:     assert(!"pure virtual function call!");
  default:       b->F(); break; // virtual call (dyn-loaded code)
}

while we still need RTTI for the typeof, the call can effectively be inlined by, basically, embedding the vtable inside the instruction stream and specializing the call for all the involved classes. This could be also generalized by specializing only a few classes (say, just Derived1):

switch (typeof(b)) {
  case Derived1: b->Derived1::F(); break; // hot path
  default:       b->F(); break; // default virtual call, cold path
}

Answer 3

Virtual functions can be inlined sometimes. An excerpt from the excellent C++ faq:

"The only time an inline virtual call can be inlined is when the compiler knows the "exact class" of the object which is the target of the virtual function call. This can happen only when the compiler has an actual object rather than a pointer or reference to an object. I.e., either with a local object, a global/static object, or a fully contained object inside a composite."

Answer 4

C++11 has added final. This changes the accepted answer: it's no longer necessary to know the exact class of the object, it's sufficient to know the object has at least the class type in which the function was declared final:

class A { 
  virtual void foo();
};
class B : public A {
  inline virtual void foo() final { } 
};
class C : public B
{
};

void bar(B const& b) {
  A const& a = b; // Allowed, every B is an A.
  a.foo(); // Call to B::foo() can be inlined, even if b is actually a class C.
}

Answer 5

Marking a virtual method inline, helps in further optimizing virtual functions in following two cases:

• Curiously recurring template pattern (http://www.codeproject.com/Tips/537606/Cplusplus-Prefer-Curiously-Recurring-Template-Patt)

• Replacing virtual methods with templates (http://www.di.unipi.it/~nids/docs/templates_vs_inheritance.html)

Answer 6

It does make sense to make virtual functions and then call them on objects rather than references or pointers. Scott Meyer recommends, in his book "effective c++", to never redefine an inherited non-virtual function. That makes sense, because when you make a class with a non-virtual function and redefine the function in a derived class, you may be sure to use it correctly yourself, but you can't be sure others will use it correctly. Also, you may at a later date use it incorrectly yoruself. So, if you make a function in a base class and you want it to be redifinable, you should make it virtual. If it makes sense to make virtual functions and call them on objects, it also makes sense to inline them.

Answer 7

Inlined declared Virtual functions are inlined when called through objects and ignored when called via pointer or references.

Answer 8

I've seen compilers that don't emit any v-table if no non-inline function at all exists (and defined in one implementation file instead of a header then). They would throw errors like missing vtable-for-class-A or something similar, and you would be confused as hell, as i was.

Indeed, that's not conformant with the Standard, but it happens so consider putting at least one virtual function not in the header (if only the virtual destructor), so that the compiler could emit a vtable for the class at that place. I know it happens with some versions of gcc.

As someone mentioned, inline virtual functions can be a benefit sometimes, but of course most often you will use it when you do not know the dynamic type of the object, because that was the whole reason for virtual in the first place.

The compiler however can't completely ignore inline. It has other semantics apart from speeding up a function-call. The implicit inline for in-class definitions is the mechanism which allows you to put the definition into the header: Only inline functions can be defined multiple times throughout the whole program without a violation any rules. In the end, it behaves as you would have defined it only once in the whole program, even though you included the header multiple times into different files linked together.

Answer 9

In the cases where the function call is unambiguous and the function a suitable candidate for inlining, the compiler is smart enough to inline the code anyway.

The rest of the time "inline virtual" is a nonsense, and indeed some compilers won't compile that code.

Answer 10

There is one category of virtual functions where it still makes sense to have them inline. Consider the following case:

class Base {
public:
  inline virtual ~Base () { }
};

class Derived1 : public Base {
  inline virtual ~Derived1 () { } // Implicitly calls Base::~Base ();
};

class Derived2 : public Derived1 {
  inline virtual ~Derived2 () { } // Implicitly calls Derived1::~Derived1 ();
};

void foo (Base * base) {
  delete base;             // Virtual call
}

The call to delete 'base', will perform a virtual call to call correct derived class destructor, this call is not inlined. However because each destructor calls it's parent destructor (which in these cases are empty), the compiler can inline those calls, since they do not call the base class functions virtually.

The same principle exists for base class constructors or for any set of functions where the derived implementation also calls the base classes implementation.

Answer 11

A compiler can only inline a function when the call can be resolved unambiguously at compile time.

Virtual functions, however are resolved at runtime, and so the compiler cannot inline the call, since at compile type the dynamic type (and therefore the function implementation to be called) cannot be determined.

Answer 12

With modern compilers, it won't do any harm to inlibe them. Some ancient compiler/linker combos might have created multiple vtables, but I don't believe that is an issue anymore.

Answer 13

inline really doesn't do anything - it's a hint. The compiler might ignore it or it might inline a call event without inline if it sees the implementation and likes this idea. If code clarity is at stake the inline should be removed.