C++ Truths

I built a fun set of classes using C++ template meta-programming technique to count the number of times a member method of a class is invoked in a chain . These template classes implement a meta-programming technique called Counted Method Chaining . It counts the number of times a member function is called at compile-time and does not let programmer invoke it more than a used defiend limit. The limit is passed as a integer template parameter to the class to which to method belongs. It gives a compile time error only when method chaining is used. Otherwise it throws and exception. These set of classes use the C++ technique of co-variant return types. /// @brief A genereral case of the template where add function /// is public and therefore, it can be invoked. template <unsigned int C> class Base : public Base <C-1> { public: virtual Base <C-1> & add (void const *, size_t length) = 0; }; /// @brief The special case of the template when C (count) becomes ...

Puzzle: What will be the output of the following program? struct B { void foo () { printf ("B::foo"); } }; void foo () { printf("::foo"); } template <typename Base> struct Derived : public Base { void bar () { foo (); } }; int main (void) { Derived <B> d; d.bar(); return 0; } Answer: C++ compiler does not perform template argument dependent lookup because the rule is to lookup as many names as possible at the time of parsing the template the first time. It does not wait till the instantiation of it. So ::foo!

I came across another C++ idiom which is "useful if a function takes a large number of mostly default-able parameters". You can check it out here. It makes use of a fairly common technique called Method Chaining. I am thinking of putting together a list of C++ idioms, which are spread across web and also the idioms from popular books.

In my reading, I came across a cool, little technique in C++ which allows us to invoke a pre-method and a post-method automatically when a member method of the class is called. It makes use of so called "Double Application Of Smart Pointers" (see the second bullet). This technique might be useful to implement different popular things such as pre-conditions and post-conditions, the "Monitor Object" pattern (POSA2), pointcut models in AOP, the Decorator pattern (GOF) and also the Execute-Around-Method pattern.

I found it quite interesting to know that g++ compiler actually provides an option (-Weffc++) which enables warnings for constructs that violate guidelines in Effective C++!! From g++ manual (man g++) == Begin == Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ book: * Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory. * Item 12: Prefer initialization to assignment in constructors. * Item 14: Make destructors virtual in base classes. * Item 15: Have "operator=" return a reference to *this. * Item 23: Don’t try to return a reference when you must return an object. Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C++ book: * Item 6: Distinguish between prefix and postfix forms of increment and decrement operators. * Item 7: Never overload "&&", "││", or ",". When s...

Developing generic containers in C++ can become complex if you want to develope truly generic containers (as much as they can get). Relaxing the requirements on type T is the key behind developing truly generic containers. There a few C++ idioms to actually achieve the "lowest denominator" possible with requirements on type T. It is easy to come up with a generic stack which requires following operations defiend on type T: a default constructor, a copy constructor, a non-throwing destructor and a copy assignment operator. But thats too much! The requirements can be reduced to the folloing list: a copy constructor and a non-throwing destructor. To achieve this, a generic container should be able to allocate uninitialized memory and invoke constructor(s) only once on each element while "initializing" them. This is possible using following two techniques: 1. operator new: void * mem = operator new (sizeof (T) * NUMBER_OF_ELEMENTS); 2. construct helper using placemen...

Resource Acquisition catagory: *********************************** Lookup: Naming service, Trading service, Directory service Lazy Acquisition: Resource proxy, web browser image proxy and deferred image download, Java JIT compiler. Good for scalability and low latency. Eager Acquisition: Good for predictability, bandwidth pre-allocation, pre-connect. Partial Acquisition: Acquire resoruce in steps. Resource Lifecycle catagory: ******************************** Caching: for performance, identity of resources is important. Pooling: for boundedness, identity of resources is not important. Coordinator: To provide commit-or-rollback semantics. Similar to strong exception safety guarantee. Best example: two-phase commit protocol (prepare() and commit()) Resource Release catagory: ******************************* Leasing: Free up unused/invalid resources/service location references after some pre-determined lease time if not accessed. Renew lease if accessed in between. Evictor: Used with cachin...

C++ programming laguage is undergoing interesting language changes. The delegating constructor is a useful new addition which changes the semantics of throwing an exception from a constructor. As of now, exception raised during execution of any constructor of a class (including base-member initialization list), the destructor of that class is not invoked (for that object). This is because the object never really got created successfully. This will be now longer be true after C++0x is released!!! Below is a list of some of the upcoming language changes. * Nearly all of the Library Technical Report (TR1) * Auto type deduction * Delegating constructors * Right angle brackets * extern templates Source : Highlights of the April 2006 ISO C++ meeting (Herb Sutter)

I found some really cool one liner solutions to swap two integers without using a temporary variable. These were posted on "C/C++ Programming Puzzles" community on www.orkut.com . Some of them are listed here. 1. a+=b-=a=b-a; 2. a/=b=(a=a*b)/b; 3. a^=b^=a^=b; same as a=(b=(a=b^a)^b)^a; (Thanks to Robert Vukovic) 4. b=a+b-(a=b); Each individually are cool solutions. But all of them have one big problem. These are not portable! C standard says that when a value of an operand is changed in the same expression at some other place, then the result of the expression is undefined. Try running the program on gcc with -Wall option. All the above one liners (which are really cool to begin with!) are plagued by that. So I feel the most portable way to achieve the goal is: a = a xor b; b = a xor b; a = a xor b; (by Harpreet) Unfortunately it is not a one liner.

Double Checked Locking Pattern (DCLP) is used to initialize singleton only once in a multithreaded application and also to avoid cost of acquiring lock everytime singleton is accessed. DCLP should be considered in two different dimensions. (1) CPU instruction reordering (2) Multi-processor machines. In a single threaded application running on a modern CPU, use of volatile keyword in DCLP is really important. volatile keyword prevents any aggressive instruction reordering that modern CPUs might do. The static instance pointer of the singleton and the singleton by itself (both) should be volatile for the magic to work! volatile exists for special purposes: (1) the content of a volatile variable is “unstable” (can change by means unknown to the compiler), (2) all writes to volatile data are “observable” so they must be executed religiously, and (3) all operations on volatile data are executed in the sequence in which they appear in the source code. The first two rules ensure proper readi...

"Maximal munch" rule of C++ compiler tokenizer causes ">>" token to be indentified as a right shift operator. Basically, the tokenizer tries to match a string with longest possible "known" token. Because of this, in nested templates, X<Y<int>> can't be parsed sensibly as the ">>" is not split into two ">" (as desired by programmer). A simple solution is to put a space between two consecutive ">"s. This would have otherwise required context sensitive parsing. But C++0x standard promises to relieve programmers from this seemling vexing parse of C++. One of the few language changes in C++0x is to allow "X<Y<int>>! Another addition is "extern template" , which provides a syntax for telling the compiler that a specific template instantiation will be provided somewhere else. For more information please see: DDJ - More News From the Front C99 standard defines a new (not ne...

Exposing type information "out" from a class template appears to me as a good practice. For example, a class template Array<T> should define a trait which looks something like typedef T Array:TYPE in its public interface. Not only the parameter type, it should also expose any composite types used inside the template. There are several good reasons behind it. 1. Member functions of the class can make use of the traits. 2. You can avoid typing really long names using these quick typedefs. 3. Consistency. 4. Other classes can make use of the traits. For example, derived classes. Sometimes it is simply not possible to write a non-template derived class of a parameterized class if the parameterized base-class does not expose necessary traits. For example, template <typename T> class Array; template <typename T> class Access_Table { Array<T> array_; }; A new class AT_Adapter wants to adapt Access_Table<T> by adding get_array() functio...

My STL notes after reading Effective STL by Scott Meyer. For most of the points below, there are subtle important reasons for that. The reasons are not mentioned here as it would be too long. Please read Effective C++ by Scott Meyer. 1. Use member insert function instead of copy algorithm and inserter. 2. Minimize capacity: string (s).swap (s) Clear and minimize capacity : string ().swap (s) 3. Associative containers use equivalence (strict weak ordering) and not equality. Equivalence: !(w1 < w2) && !(w2 > w1) In strict weak ordering, equal values are not equivalent! Therefore, Comparison function should return false for equal values when equivalence is expected. 4. Use map::operator [] to modify/retrieve existing elements in the map Use map::insert to add new elements 5. Iterator can be implicitely converted to reverse_iterator. reverse_iterator to iterator: use member base() function. const_cast works for conversion from const_iterator to iterator o...

A very nice collection of wrappers is available on Stackoverflow . 1. Copy everything in the a container to std::cout (e.g. std::set<std::string> c; ) std::copy (c.begin(), c.end(), std::ostream_iterator <std::string> (std::cout, "\n")); 2. Clear vector v and minimize its capacity (potential number of elements it can hold without resizing). vector <std::string>().swap (v); Yes! It is a swap with an empty temporary!! 3. Remove all integers of value = 0xDeadBeef; std::erase (std::remove (c.begin(), c.end(), value), c.end()); This is known as erase-remove idiom. 4. Invoke a member function on all the elements of the container. std::for_each (c.begin(), c.end(), std::mem_fun (&Class::function)); 5. Copy one container to other std::copy(V.begin(), V.end(), L.begin()); 6. Fill an array with random numbers std::generate(V.begin(), V.end(), rand); 7. Read a file of integers in a set std::istream_iterator <int> data_begin (std::cin),...

The "Concurrency Revolution" is coming at us fast (to gulp us) whether you are ready for it or not! Please see a presentation (audio/video) by Hurb Sutter at parc. Our favorite language, C++, is also not far behind in providing standard concurrency support to the language/library. Please see: Threads and memory model for C++. I feel there is exciting time ahead and a lot of really cool stuff to continue this blog for many years to come!

A "thunk" appears to be an overloaded term in computer science. Use of the word "thunk" goes back to Algol 60 implementation. In general, as I understand it, thunk is a function which is used to replace something. More often than not, it is auto-generated. This "something" could be an expression (in a programming language) or an address. In programming language world, a known usecase is call-by-name. The mechanism of dynamic linking (.DLL/.so files) uses local thunks to invoke dynamic linker at run-time and replace the thunk with the actual function for all the later invocations of the function. Please see how this technique can be exploited to modify program behavior at run-time . In the C++ world, transparent replacement of addresses is very useful. An example is, adjusting pointers when virtual functions are invoked using one of the many possible base classes of the derived most class when multiple inheritance is involved. Depending upon which base cla...

There are several cool implementation techniques of C++ object layout, especially when multiple inheritance, virtual functions and virtual inheritance is involved. Understanding the consequences of these rather advanced concepts in the language on the implementation of the language is quite interesting. There are several "gotchas" involved. Example, it is highly recommended to invoke the virtual base class's constructor in the derived most class when there is a diamond shape inheritance hierarchy. It is a pretty eloborate subject so I won't duplicate the same thing here. I would just assemble some really good articles on the topic. Lippman's book "Inside The C++ Object Model" talks about it at length. Please see: C++ Object layout in multiple inheritance Adjustor thunks Multiple inheritance in C++ (Stroustrup)

This is a for constructors, which do non-trivial resource allocation and initialization using helper member functions in the class. Exceptions raised in such member functions should be propagated upto the constructor and not eat them. In other words, such member functions should throw exception if they can't satisfy their contract even though they provide strong exception safety guarantee. Exception propagated upto the constructor will be automatically propagated where the object is being constructed indicating failure in object construction. Otherwise, silently eating exceptions during object initialization will create an object in an inconsistent state. The scope in which it is created will never know that actually the object construction failed. If object construction can be reasonably performed even in the face of exception, then it need not be propagated out. Example, LStack: A Stack implemented as a linked-list. LStack::LStack (const LStack & s) { this->copy_all_nodes...

The canonical form of overloaded pre-increment and post-increment operators for class Foo are declared in the following way in C++. /// Preincrement operator Foo & operator++ (void); /// Postincrement operator const Foo operator++ (int); The int in the post-increment operator is obviously to disambiguate between post and pre forms of the operator. Then, why is the return type different? As many other C++ features, this one also has a subtle meaning! In C++, for ints, you can write ++i = k; but not, i++ = k; This is because, i++ results in a Rvalue to which you can't assign. Unlike i++, ++i results in a Lvalue to which you can assign which is infact ( incremented ) i itself. Not that ++i = k; has a great importance, but it is a fact that C++ has been designed that way. I would be interested in knowing the reason. One reason is that i++ = k; is not allowed is that it is just ambiguous. but ++i = k; is not ambiguous. A const in the return type is also required to disallow...

It is a C++ idiom in which inheritance and template type parameters are cleverly integrated to generate flexible types at compile-time. In CRTP, a template class inherits from its own template parameter. For example, template <typename Base> class MixinOne : public Base {}; This technique is also one of the ways of using "Mixins". Here class Mixin helps to "mix in" some useful functionality into any class type Base. For example, class Mixin can make class Base a singleton. Complete code example is here. In this case of CRTP, a hierarchy (a new class plus inheritance relationship) is created when Mixin template is instantiated. Thus hierarchy creation is differed till the instantiation of the template. This is pretty cool! Moreover, CRTP can appear in a different fashion in which a class passes itself as a template parameter to its base class. For example, class Derived: public MixinTwo<Derived> {}; The non-template class Derived inherits interface/stru...