Skip to main content

Always define virtual non-pure methods

The ISO C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined and compilers are not bound (by standards) to warn you if you don't follow this rule. Based on this assumption, GCC will only emit the implicitly defined constructors, the assignment operator, the destructor and the virtual table of a class in the translation unit that defines its first such non-inline method.

Therefore, if you fail to define this particular method, the linker complains. In case of gcc and ld (linker on Linux), the linker gives out an error message saying "undefined reference to `vtable for function_name' ". This error message is quite misleading. The solution is to ensure that all virtual methods that are not pure are defined. An exception to this rule is a pure-virtual destructor, which must be defined (empty body) in any case. Ch. 12, [class.dtor]/7.

Comments

Anonymous said…
Google and Sun horse trading
The Google-Sun collaboration has kept us busy the last two days-trying to figure out what was in the works and now deciphering the actual announcement.
Hey, this is a good blog here! I'm definitely going to bookmark you!

I have a guaranteed visitors site/blog. It pretty much covers guaranteed visitors related stuff.

Come and check it out if you get time :-)
Anonymous said…
Ch 12 of which book
Sumant said…
Ch. 12 of C++98 standards official draft
Astonshing blog. I relished in the site and you
know I will be going to it again! Surfing the internet
hepls me to find blogs that arfe just as good.
Please proceed to my blog when you find the time.
dat-girl said…
Sensational blog. I took pleasure in the site and I
will go back! Surfing online for blogs like this one
is worth my time.
You got me! I will check out your 1800contacts coupon code blog a.s.a.p!
outa-time said…
I surf the web looking for blogs like this one.
Your site was on point and will be back again! Awesome
blog.
Check out my coupon 1800contacts blog, you won't be sorry!
stunned said…
Fine blog. I found your site suitable for another
visit! And when I'm able to surf the web, I look for
blogs as great as your work.
I'm looking at the possibility of checking your coupon codes 1800contacts blog.
I surf the web looking for blogs like this one.
Your site was on point and will be back again! Awesome
blog.
Look who checking out my 1800contacts coupon codes blog?
Captivate blog. I surf the web for blogs this
nature.The site are wonderful and will be returned to
again!
My 1800contacts coupon codes blog, is something you need to peep out!
sceptic said…
Unique blog my friend, I can hardly wait to vist
this site again. I just worship the site its comes
from! Believe me in my extra time I'm consistently
looking up blogs like this.
I will give you access to jump to my 1800contacts web coupon code blog.
Excellent blog. Your site was great and will be
finding it again!  I surf the net for blogs like
yours.
Click on my michael jackson plastic surgery blog before its to late.
Excellent blog. It was so great and I bet I will
go back to it! I get to look online for blogs like
yours is a blessing.
Look who checking out my atlanta plastic surgery blog?
Exciting blog. Your site was amazing and will be
back again! I never get tired of looking for blogs
just like this one.
Go and click my plastic surgery las vegas blog.

Popular posts from this blog

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together.

It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array?

int native[3][4];
std::array<std::array<int, 3>, 4> arr;

No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is:

std::array<std::array<int, 4>, 3> arr;

That's quite annoying for two r…

Folding Monadic Functions

In the previous two blog posts (Understanding Fold Expressions and Folding Functions) we looked at the basic usage of C++17 fold expressions and how simple functions can be folded to create a composite one. We’ll continue our stride and see how "embellished" functions may be composed in fold expressions.

First, let me define what I mean by embellished functions. Instead of just returning a simple value, these functions are going to return a generic container of the desired value. The choice of container is very broad but not arbitrary. There are some constraints on the container and once you select a generic container, all functions must return values of the same container. Let's begin with std::vector.
// Hide the allocator template argument of std::vector. // It causes problems and is irrelevant here. template <class T> struct Vector : std::vector<T> {}; struct Continent { }; struct Country { }; struct State { }; struct City { }; auto get_countries…

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example.
struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a c…