Do You Have to Know Ascii and Base to Program in C

strange inheritance

Inheritance — What your mother never told you lot

How can I set up my course so it won't be inherited from?

Just declare the class final.

Just likewise inquire yourself why you want to? There are ii common answers:

• For efficiency: to avert your function calls existence virtual.
• For rubber: to ensure that your class is not used as a base class (for example, to be sure that you can re-create objects without fear of slicing).

In today'south usual implementations, calling a virtual part entails fetching the "vptr" (i.e. the pointer to the virtual table) from the object, indexing into information technology via a constant, and calling the function indirectly via the pointer to function institute at that location. A regular phone call is most often a direct call to a literal address. Although a virtual phone call seems to be a lot more work, the right way to judge costs is in comparing to the work actually carried by the function. If that piece of work is pregnant, the cost of the call itself is negligible by comparing and often cannot exist measured. If, however, the function body is simple (i.east. an accessor or a forward), the cost of a virtual call tin can be measurable and sometimes significant.

The virtual part call mechanism is typically used only when calling through a pointer or a reference. When calling a function directly for a named object (e.one thousand. i allocated on the stack of the caller), the compiler inserts lawmaking for a regular call. Note, even so, that frequent such utilize may indicate other problems with the design - virtual functions work only in tandem with polymorphism and indirect use (pointers and references). Such cases may warrant a design review for overuse of virtual.

How can I fix my member function and then it won't be overridden in a derived class?

Merely declare the role final.

But again, inquire yourself why you want to. See the reasons under given for final classes.

Is it okay for a not-virtual role of the base form to call a virtual function?

Aye. Information technology's sometimes (not e'er!) a not bad idea. For example, suppose all Shape objects have a common algorithm for press, but this algorithm depends on their area and they all have a potentially different style to compute their surface area. In this example Shape's area() fellow member function would necessarily have to exist virtual (probably pure virtual) simply Shape::impress() could, if we were guaranteed no derived form wanted a dissimilar algorithm for press, be a non-virtual defined in the base form Shape.

            #include "Shape.h"  void Shape::print() const {     float a = this->surface area();  // area() is pure virtual     // ... }                      

That last FAQ confuses me. Is it a different strategy from the other ways to use virtual functions? What's going on?

Yep, it is a different strategy. Yep, in that location really are two dissimilar basic ways to use virtual functions:

1. Suppose you take the situation described in the previous FAQ: yous have a member function whose overall structure is the same for each derived class, but has little pieces that are different in each derived class. So the algorithm is the same, but the primitives are different. In this case you'd write the overall algorithm in the base of operations course every bit a public member role (that's sometimes not-virtual), and you'd write the piddling pieces in the derived classes. The lilliputian pieces would be alleged in the base form (they're oft protected, they're oft pure virtual, and they're certainly virtual), and they'd ultimately be defined in each derived course. The most critical question in this situation is whether or not the public member function containing the overall algorithm should exist virtual. The answer is to make it virtual if y'all think that some derived grade might demand to override information technology.
2. Suppose you have the verbal contrary situation from the previous FAQ, where you lot take a fellow member office whose overall construction is dissimilar in each derived grade, yet it has fiddling pieces that are the same in virtually (if not all) derived classes. In this case you lot'd put the overall algorithm in a public virtual that's ultimately divers in the derived classes, and the fiddling pieces of common code tin be written once (to avoid code duplication) and stashed somewhere (anywhere!). A common place to stash the niggling pieces is in the protected part of the base course, but that'southward non necessary and information technology might not even be all-time. Just find a place to stash them and you'll be fine. Notation that if yous do stash them in the base form, you should usually make them protected, since unremarkably they do things that public users don't need/want to do. Assuming they're protected, they probably shouldn't be virtual: if the derived class doesn't like the behavior in one of them, information technology doesn't have to call that member part.

For emphasis, the above list is a both/and situation, not an either/or state of affairs. In other words, yous don't have to choose between these two strategies on any given course. It's perfectly normal to have member function f() correspond to strategy #i while fellow member function chiliad() corresponds to strategy #2. In other words, it's perfectly normal to accept both strategies working in the same grade.

Should I use protected virtuals instead of public virtuals?

Sometimes yes, sometimes no.

Outset, stay abroad from always/never rules, and instead utilise whichever arroyo is the best fit for the state of affairs. There are at to the lowest degree 2 proficient reasons to use protected virtuals (see beneath), but simply because you lot are sometimes better off with protected virtuals does not mean you should always use them. Consistency and symmetry are good up to a point, but at the terminate of the day the about important metrics are cost + schedule + risk, and unless an idea materially improves price and/or schedule and/or gamble, it'southward just symmetry for symmetry's sake (or consistency for consistency's sake, etc.).

The cheapest + fastest + lowest risk approach in my experience ends up resulting in most virtuals existence public, with protected virtuals beingness used whenever you lot accept either of these two cases: the situation discussed in the previous FAQ or the situation discussed in relation to the hiding rule.

The latter deserves some additional commentary. Pretend you have a base class with a set of overloaded virtuals. To make the instance easy, pretend there are just 2: virtual void f(int) and virtual void f(double). The idea of the Public Overloaded Non-Virtuals Phone call Protected Non-Overloaded Virtuals idiom is to change the public overloaded fellow member functions to non-virtuals, and make those phone call protected not-overloaded virtuals.

Code using public overloaded virtuals:

            class Base { public:   virtual void f(int x);    // May or may not be pure virtual   virtual void f(double x); // May or may not be pure virtual };                      

Improving this via the Public Overloaded Not-Virtuals Call Protected Non-Overloaded Virtuals idiom:

            class Base of operations { public:   void f(int 10)    { f_int(x); }  // Non-virtual   void f(double x) { f_dbl(10); }  // Non-virtual protected:   virtual void f_int(int);   virtual void f_dbl(double); };                      

Here'south an overview of the original code:

Fellow member function	Public?	Inline?	Virtual?	Overloaded?
f(int) & f(double)	Yes	No	Yeah	Yes

Here'south an overview of the improved code that uses the Public Overloaded Non-Virtuals Telephone call Protected Non-Overloaded Virtuals idiom:

Member function	Public?	Inline?	Virtual?	Overloaded?
f(int) & f(double)	Yep	Yes	No	Yes
f_int(int) & f_dbl(double)	No	No	Yes	No

The reason I and others apply this idiom is to make life easier and less error-decumbent for the developers of the derived classes. Remember the goals stated higher up: schedule + cost + risk? Let's evaluate this Idiom in low-cal of those goals. From a toll/schedule standpoint, the base course (singular) is slightly larger but the derived classes (plural) are slightly smaller, for a internet (small) improvement in schedule and price. The more than signicant improvement is in chance: the idiom packs the complexity of properly managing the hiding rule into the base grade (atypical). This means the derived classes (plural) more than-or-less automatically handle the hiding rule, then the various developers who produce those derived classes can remain almost completely focused on the details of the derived classes themselves — they need not concern themselves with the (subtle and frequently misunderstood) hiding rule. This profoundly reduces the hazard that the writers of the derived classes will spiral up the hiding-rule.

With apologies to Mr. Spock, the needs of the many (the derived classes (plural)) outweigh the needs of the one (the base class (singular)).

(Read upwardly on the Hiding Rule for why you need to be careful about overriding some-but-not-all of a set of overloaded member functions, and therefore why the higher up makes life easier on derived classes.)

When should someone utilise private virtuals?

When you need to brand specific behavior in a base of operations class customizable in derived classes, while protecting the semantics of the interface (and/or the base algorithm therein), which is defined in public fellow member functions that telephone call private virtual fellow member functions.

Ane case where individual virtuals testify up is when implementing the Template Method design pattern. Some experts, e.1000., Herb Sutter's C/C++ Users Periodical article Virtuality, advocate it equally a best exercise to always define virtual functions private, unless at that place is a good reason to brand them protected. Virtual functions, in their view, should never be public, considering they define the grade' interface, which must remain consistent in all derived classes. Protected and individual virtuals define the form' customizable behavior, and in that location is no need to make them public. A public virtual function would define both interface and a customization point, a duality that could reflect weak design.

By the way, it confuses most novice C++ programmers that private virtuals tin can be overridden, allow alone are valid at all. We were all taught that private members in a base class are not accessible in classes derived from it, which is right. However this inaccessibility by the derived class does not have anything to do with the virtual call machinery, which is to the derived course. Since that might confuse novices, the C++ FAQ formerly recommended using protected virtuals rather than private virtuals. However the private virtual approach is at present common enough that defoliation of novices is less of a business organization.

You lot might inquire, What skillful is a function that the derived class tin can't call? Even though the derived class tin't phone call it in the base class, the base of operations class tin can call it which finer calls downwards to the (appropriate) derived grade. And that'due south what the Template Method design is all about.

Recollect of "Back to the Future." Assume the base class is written last year, and yous are about to create a new derived class after today. The base class' member functions, which might have been compiled and stuck into a library months ago, will phone call the private (or protected) virtual, and that will finer "phone call into the futurity" - the lawmaking which was compiled months agone volition call code that doesn't even exist yet - lawmaking you are about to write in the next few minutes. You can't admission individual members of the base class - you can't attain into the by, but the past tin reach into the future and call your member functions which yous haven't even written yet.

Here is what that Template Method pattern looks like:

            class MyBaseClass { public:   void myOp();  private:   virtual void myOp_step1() = 0;   virtual void myOp_step2(); };  void MyBaseClass::myOp() {   // Pre-processing...    myOp_step1();  // telephone call into the future - phone call the derived class   myOp_step2();  // optionally the future - this i isn't pure virtual    // Post-processing... }  void MyBaseClass::myOp_step2() {   // this is "default" code - it tin optionally be customized by a derived class }                      

In this case, public member role MyBaseClass::myOp() implements the interface and basic algorithm to perform some operation. The pre- and post-processing, besides every bit the sequence of stride 1 and step 2, are intentionally fixed and cannot be customized past a derived course. If MyBaseClass::myOp() was virtual, the integrity of that algorithm would be seriously compromised. Instead, customization is restricted to specific "pieces" of the algorithm, implemented in the 2 individual virtual functions. This enforces better compliance of derived classes to the original intent embodied in the base course, and also makes customization easier - the derived class' writer needs to write less code.

If MyBaseClass::myOp_step2() might need to be called by the derived class, for instance, if the derived class might need (or desire) to employ that code to simplify its ain code, so that can exist promoted from a private virtual to a protected virtual. If that is not possible because the base class belongs to a different system, as a rough-and-tumble the code can exist copied.

(At this point I can almost read your thoughts: "What? Re-create code??!? Are yous KIDDING??!? That would increase maintenance price and duplicate bugs!! Are y'all CRAZY??!?" Whether I'chiliad crazy remains to be seen, but I am experienced enough to realize life sometimes paints you into a corner. If the base course tin't be modified, sometimes the "to the lowest degree bad" of the bad alternatives is to re-create some code. Remember, one size does not fit all, and "think" is not a 4-letter word. So hold your olfactory organ and practice whatsoever is the least bad thing. Then shower. Twice. Just if you lot take chances the team'due south success considering yous are waiting for some third party to change their base grade, or if you use #ascertain to change the meaning of private, you might have chosen a worse evil. And oh yea, if yous re-create the lawmaking, mark it with a big fat comment so I won't come up along and think you are crazy!! .)

On the other hand, if y'all are creating the base class and if y'all aren't sure whether derived class's might desire to call MyBaseClass::myOp_step2(), you tin declare it protected merely in example. And in that case, you'd meliorate put a large fat comment next to it so Herb doesn't come along and recollect you're crazy! Either way, somebody is going to retrieve you're crazy.

When my base class's constructor calls a virtual part on its this object, why doesn't my derived class's override of that virtual part become invoked?

Because that would exist very dangerous, and C++ is protecting you from that danger.

The balance of this FAQ gives a rationale for why C++ needs to protect y'all from that danger, only before we starting time that, be advised that y'all can get the effect as if dynamic bounden worked on the this object even during a constructor via The Dynamic Binding During Initialization Idiom.

You lot can phone call a virtual part in a constructor, but exist careful. It may non do what you expect. In a constructor, the virtual call mechanism is disabled because overriding from derived classes hasn't yet happened. Objects are constructed from the base up, "base of operations earlier derived".

Consider:

                          #include<string>     #include<iostream>     using namespace std;      class B {     public:         B(const string& ss) { cout << "B constructor\due north"; f(ss); }         virtual void f(const string&) { cout << "B::f\n";}     };      class D : public B {     public:         D(const cord & ss) :B(ss) { cout << "D constructor\n";}         void f(const string& ss) { cout << "D::f\n"; s = ss; }     individual:         string s;     };      int main()     {         D d("Hullo");     }                      

the plan compiles and produce

                          B constructor     B::f     D constructor                      

Note not D::f. Consider what would happen if the rule were different so that D::f() was called from B::B(): Because the constructor D::D() hadn't yet been run, D::f() would try to assign its argument to an uninitialized cord s. The event would near probable be an firsthand crash. So fortunately the C++ linguistic communication doesn't let this happen: it makes sure any phone call to this->f() that occurs while command is flowing through B'south constructor volition end up invoking B::f(), non the override D::f().

Destruction is done "derived class earlier base of operations class", and then virtual functions behave as in constructors: Only the local definitions are used – and no calls are made to overriding functions to avert touching the (at present destroyed) derived class part of the object.

For more details see D&E xiii.2.iv.2 or TC++PL3 15.4.three.

It has been suggested that this rule is an implementation artifact. Information technology is not so. In fact, it would be noticeably easier to implement the unsafe rule of calling virtual functions from constructors exactly as from other functions. However, that would imply that no virtual part could exist written to rely on invariants established by base classes. That would be a terrible mess.

Okay, but is there a fashion to simulate that behavior as if dynamic bounden worked on the this object within my base of operations grade's constructor?

Aye: the Dynamic Bounden During Initialization idiom (AKA Calling Virtuals During Initialization).

To clarify, we're talking about the state of affairs when Base's constructor calls virtual functions on its this object:

            form Base { public:   Base();   // ...   virtual void foo(int n) const; // frequently pure virtual   virtual double bar() const;    // often pure virtual   // if y'all don't want outsiders calling these, make them protected };  Base of operations::Base() {   // ...   foo(42);  // Warning: does NOT dynamically bind to the derived class   bar();    // (ditto)   // ... }  grade Derived : public Base { public:   // ...   virtual void foo(int due north) const;   virtual double bar() const; };                      

This FAQ shows some ways to simulate dynamic binding equally if the calls made in Base's constructor dynamically bound to the this object's derived class. The ways we'll prove have tradeoffs, then choose the i that all-time fits your needs, or brand up another.

The first approach is a two-phase initialization. In Phase I, someone calls the actual constructor; in Stage Two, someone calls an "init" function on the object. Dynamic binding on the this object works fine during Phase II, and Phase II is conceptually part of construction, then we merely move some lawmaking from the original Base::Base() into Base::init().

            class Base { public:   void init();  // may or may non be virtual   // ...   virtual void foo(int n) const; // often pure virtual   virtual double bar() const;    // often pure virtual };  void Base of operations::init() {   // Almost identical to the body of the original Base::Base()   // ...   foo(42);   bar();   // ... }  course Derived : public Base { public:   // ...   virtual void foo(int n) const;   virtual double bar() const; };                      

The only remaining bug are determining where to call Phase I and where to phone call Phase Two. There are many variations on where these calls tin alive; nosotros will consider two.

The kickoff variation is simplest initially, though the lawmaking that actually wants to create objects requires a tiny fleck of programmer self-discipline, which in practice means you're doomed. Seriously, if there are only one or 2 places that actually create objects of this hierarchy, the programmer self-discipline is quite localized and shouldn't cause bug.

In this variation, the lawmaking that is creating the object explicitly executes both phases. When executing Phase I, the lawmaking creating the object either knows the object's verbal class (e.thousand., new Derived() or peradventure a local Derived object), or doesn't know the object'southward exact class (e.g., the virtual constructor idiom or some other factory). The "doesn't know" case is strongly preferred when you want to go far easy to plug-in new derived classes.

Annotation: Phase I often, simply not always, allocates the object from the heap. When it does, you lot should store the pointer in some sort of managed arrow, such equally a std::unique_ptr, a reference counted pointer, or another object whose destructor deletesouthward the allocation. This is the best way to prevent retentiveness leaks when Phase II might throw exceptions. The following example assumes Stage I allocates the object from the heap.

            #include <retentiveness>  void joe_user() {   std::unique_ptr<Base> p( /*...somehow create a Derived object via new...*/ );   p->init();   // ... }                      

The second variation is to combine the kickoff two lines of the joe_user office into some create role. That'south almost always the right thing to do when there are lots of joe_user-like functions. For example, if y'all're using some kind of manufactory, such every bit a registry and the virtual constructor idiom, y'all could move those ii lines into a static fellow member function called Base of operations::create():

            #include <retentiveness>  form Base { public:   // ...   using Ptr = std::unique_ptr<Base>;  // type aliases simplify the code   static Ptr create();   // ... };  Base::Ptr Base of operations::create() {   Ptr p( /*...use a factory to create a Derived object via new...*/ );   p->init();   return p; }                      

This simplifies all the joe_user-like functions (a niggling), but more importantly, it reduces the gamble that whatsoever of them will create a Derived object without also calling init() on it.

            void joe_user() {   Base of operations::Ptr p = Base::create();   // ... }                      

If you're sufficiently clever and motivated, y'all tin can even eliminate the chance that someone could create a Derived object without too calling init() on it. An important pace in achieving that goal is to make Derived'south constructors, including its copy constructor, protected or private..

The next approach does not rely on a ii-stage initialization, instead using a second hierarchy whose only chore is to house fellow member functions foo() and bar(). This approach doesn't e'er work, and in particular it doesn't work in cases when foo() and bar() need to access the instance data declared in Derived, but information technology is conceptually quite elementary and clean and is unremarkably used.

Let's call the base course of this second hierarchy Helper, and its derived classes Helper1, Helper2, etc. The first stride is to move foo() and bar() into this second hierarchy:

            class Helper { public:   virtual void foo(int n) const = 0;   virtual double bar() const = 0; };  form Helper1 : public Helper { public:   virtual void foo(int n) const;   virtual double bar() const; };  class Helper2 : public Helper { public:   virtual void foo(int northward) const;   virtual double bar() const; };                      

Adjacent, remove init() from Base (since nosotros're no longer using the two-phase approach), remove foo() and bar() from Base and Derived (foo() and bar() are now in the Helper hierarchy), and change the signature of Base of operations'southward constructor and so it takes a Helper by reference:

            class Base of operations { public:   Base(const Helper& h);   // Remove init() since non using two-phase this time   // Remove foo() and bar() since they're in Helper };  course Derived : public Base { public:   // Remove foo() and bar() since they're in Helper };                      

We then define Base::Base of operations(const Helper&) and then it calls h.foo(42) and h.bar() in exactly those places that init() used to call this->foo(42) and this->bar():

            Base::Base(const Helper& h) {   // Almost identical to the body of the original Base::Base()   // except for the insertion of h.    // ...   h.foo(42);   h.bar();   ↑↑ // The h. occurrences are new   // ... }                      

Finally nosotros alter Derived's constructor to laissez passer a (perhaps temporary) object of an advisable Helper derived class to Base's constructor (using the init list syntax). For case, Derived would laissez passer an case of Helper2 if it happened to contain the behaviors that Derived wanted for functions foo() and bar():

            Derived::Derived()   : Base(Helper2())   // ← the magic happens here {   // ... }                      

Note that Derived can pass values into the Helper derived class's constructor, merely information technology must non laissez passer any data members that actually live inside the this object. While we're at it, let's explicitly say that Helper::foo() and Helper::bar() must not access data members of the this object, specially data members alleged in Derived. (Think about when those data members are initialized and you'll see why.)

Of course the choice of which Helper derived class could be made out in the joe_user-like role, in which case information technology would exist passed into the Derived ctor and then up to the Base ctor:

            Derived::Derived(const Helper& h)   : Base(h) {   // ... }                      

If the Helper objects don't need to hold any data, that is, if each is but a collection of its member functions, then you can merely pass static member functions instead. This might be simpler since it entirely eliminates the Helper hierarchy.

            class Base { public:   using FooFn = void (*)(int);  // blazon aliases simplify   using BarFn = double (*)();   //    the rest of the code   Base(FooFn foo, BarFn bar);   // ... };  Base::Base(FooFn foo, BarFn bar) {   // Almost identical to the body of the original Base::Base of operations()   // except the calls are made via function pointers.    // ...   foo(42);   bar();   // ... }                      

The Derived class is too easy to implement:

            class Derived : public Base { public:   Derived();   static void foo(int n); // the static is important!   static double bar();    // the static is of import!   // ... };  Derived::Derived()   : Base(foo, bar)  // ← pass the function-ptrs into Base of operations'due south ctor {   // ... }                      

Every bit earlier, the functionality for foo() and/or bar() can be passed in from the joe_user-like functions. In that case, Derived's ctor just accepts them and passes them up into Base'south ctor:

            Derived::Derived(FooFn foo, BarFn bar)   : Base(foo, bar) {   // ... }                      

A terminal approach is to utilize templates to "pass" the functionality into the derived classes. This is similar to the case where the joe_user-like functions choose the initializer-function or the Helper derived class, but instead of using function pointers or dynamic bounden, information technology wires the code into the classes via templates.

I'm getting the same affair with destructors: calling a virtual on my this object from my base of operations class's destructor ends upwards ignoring the override in the derived class; what's going on?

C++ is protecting you from yourself. What y'all are trying to do is very dangerous, and if the compiler did what you wanted, y'all'd be in worse shape.

For rationale of why C++ needs to protect you from that danger, brand sure yous understand what happens when a constructor calls virtuals on its this object. The situation during a destructor is analogous to that during the constructor. In particular, within the {body} of Base::~Base(), an object that was originally of blazon Derived has already been demoted (devolved, if y'all will) to an object of type Base. If you telephone call a virtual part that has been overridden in class Derived, the call will resolve to Base::virt(), not to the override Derived::virt(). Same goes for using typeid on the this object: the this object really has been demoted to type Base; information technology is no longer an object of blazon Derived.

Reminder to also read this.

Should a derived class redefine ("override") a member function that is not-virtual in a base class?

It's legal, simply it ain't moral.

Experienced C++ programmers will sometimes redefine a non-virtual function for efficiency (east.chiliad., if the derived class implementation tin can make better use of the derived class's resources) or to get around the hiding dominion. However the customer-visible effects must be identical, since non-virtual functions are dispatched based on the static type of the arrow/reference rather than the dynamic type of the pointed-to/referenced object.

What's the meaning of, Warning: Derived::f(char) hides Base::f(double)?

It means y'all're going to dice.

Hither'southward the mess yous're in: if Base declares a member function f(double x), and Derived declares a member function f(char c) (same proper noun but different parameter types and/or constness), then the Base f(double x) is "hidden" rather than "overloaded" or "overridden" (fifty-fifty if the Base f(double 10) is virtual).

            grade Base { public:   void f(double x);  // Doesn't affair whether or not this is virtual };  grade Derived : public Base of operations { public:   void f(char c);  // Doesn't matter whether or not this is virtual };  int main() {   Derived* d = new Derived();   Base* b = d;   b->f(65.3);  // Okay: passes 65.iii to f(double x)   d->f(65.3);  // Bizarre: converts 65.3 to a char ('A' if ASCII) and passes it to f(char c); does NOT call f(double 10)!!   delete d;   render 0; }                      

Hither's how y'all get out of the mess: Derived must have a using declaration of the hidden member role. For example,

            class Base { public:   void f(double ten); };  class Derived : public Base { public:   using Base of operations::f;  // This un-hides Base::f(double x)   void f(char c); };                      

If the using syntax isn't supported past your compiler, redefine the hidden Base member function(s), even if they are not-virtual. Normally this re-definition merely calls the hidden Base of operations member function using the :: syntax. Due east.yard.,

            course Derived : public Base { public:   void f(double x) { Base::f(x); }  // The redefinition merely calls Base of operations::f(double x)   void f(char c); };                      

Note: the hiding trouble also occurs if form Base declares a member function f(char).

Annotation: warnings are not part of the standard, and so your compiler may or may not give the above warning.

Note: aught gets hidden when you have a base-pointer. Call up well-nigh information technology: what a derived class does or does not practice is irrelevant when the compiler is dealing with a base of operations-arrow. The compiler might not even know that the particular derived form exists. Fifty-fifty if it knows of the being of some particular derived class, information technology cannot assume that a specific base-pointer necessarily points at an object of that particular derived class. Hiding takes place when you take a derived pointer, non when you have a base of operations arrow.

Why doesn't overloading piece of work for derived classes?

That question (in many variations) are commonly prompted past an example like this:

                          #include<iostream>     using namespace std;      class B {     public:         int f(int i) { cout << "f(int): "; return i+one; }         // ...     };      class D : public B {     public:         double f(double d) { cout << "f(double): "; return d+1.3; }         // ...     };      int main()     {         D* pd = new D;          cout << pd->f(2) << '\n';         cout << pd->f(2.3) << '\n';          delete pd;     }                      

which will produce:

                          f(double): three.3     f(double): 3.6                      

rather than the

                          f(int): 3     f(double): 3.vi                      

that some people (wrongly) guessed.

In other words, there is no overload resolution between D and B. Overload resolution conceptually happens in 1 scope at a time: The compiler looks into the telescopic of D, finds the single function double f(double), and calls it. Because it constitute a match, it never bothers looking farther into the (enclosing) scope of B. In C++, there is no overloading beyond scopes – derived class scopes are not an exception to this general rule. (Come across D&E or TC++PL4 for details).

But what if I want to create an overload set of all my f() functions from my base and derived class? That's hands done using a using-declaration, which asks to bring the functions into the scope:

                          class D : public B {     public:         using B::f; // make every f from B bachelor         double f(double d) { cout << "f(double): "; return d+1.3; }         // ...     };                      

Given that modification, the output volition be:

                          f(int): 3     f(double): 3.6                      

That is, overload resolution was applied to B's f() and D'south f() to select the most appropriate f() to phone call.

What does information technology mean that the "virtual tabular array" is an unresolved external?

If you become a link error of the course "Mistake: Unresolved or undefined symbols detected: virtual tabular array for course Fred," you probably accept an undefined virtual fellow member function in class Fred.

The compiler typically creates a magical data structure called the "virtual table" for classes that accept virtual functions (this is how it handles dynamic binding). Normally y'all don't take to know about it at all. Just if you forget to define a virtual function for class Fred, y'all will sometimes get this linker fault.

Here'due south the nitty gritty: Many compilers put this magical "virtual table" in the compilation unit that defines the first non-inline virtual function in the form. Thus if the kickoff non-inline virtual function in Fred is wilma(), the compiler will put Fred's virtual table in the same compilation unit of measurement where information technology sees Fred::wilma(). Unfortunately if you accidentally forget to define Fred::wilma(), rather than getting a Fred::wilma() is undefined, you may get a "Fred's virtual table is undefined". Lamentable only truthful.

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Source: https://isocpp.org/wiki/faq/strange-inheritance

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