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Showing posts from 2007

using std::copy on std::map iterator pair

Sometimes it is useful to be able iterate over all the elements of a std::map using standard algorithms like std::copy(), std::count(), std::min_element(), std::max_element(). These standard functions do not work out of the box using std::map::iterator. For example, if you want to print all the elements of a map to standard output, you can't use the following popular copy-ostream_iterator idiom.

std::map <std::string, int> m;
std::copy (m.begin(), m.end(), std::ostream_iterator<int>(std::cout, "\n"));
// does not compile

This is because value_type of the map::iterator is a pair. In other words, if iter is a map<T,U>::iterator then *iter gives pair<T,U> and not U. If we could somehow get hold of pair::second (i.e. type U) instead of pair<T,U> all the above mentioned algorithms can be used out of the box.

The approach I took to solve this problem is to write an iterator adaptor that behaves likes any general bidirectional_iterator. In general, t…

New book on modern C++ idioms

For quite some time now, I am hoping to come across a book on modern C++ idiom as comprehensive as the Purple book by James Coplien! Finally, I got bored of the long wait and a thought came to my mind - This is Web 2.0 and open knowledge era. Why don't I start writing myself? and ask the world to contribute to it? So here we go! A new free, open book on modern C++ idioms called More C++ Idioms is taking shape on And everyone's invited!

const overload functions taking char pointers

Always have two overloaded versions of functions that take
char * and const char * parameters. Declare (but don't define if not needed)
a function that takes const char* as a parameter when you have defined
a function that accepts a non-const char* as a parameter.

#include <iostream>
#include <iomanip>

static void foo (char *s) {
std::cout << "non-const " << std::hex << static_cast <void *>(s) << std::endl;
static void foo (char const *s) {
std::cout << "const " << std::hex << static_cast <void const *>(s) << std::endl;

int main (void)
char * c1 = "Literal String 1";
char const * c2 = "Literal String 1";
foo (c1);
foo (c2);
foo ("Literal String 1");
//*c1 = 'l'; // This will cause a seg-fault on Linux.
std::cout << c2 << std::endl;

return 0;

Because of default conversion rule from string literal to char *,
the call to foo using in-pl…

Future of C++ track @ ACCU

Recently held ACCU 2007 conference had a track dedicated to C++ called: Future of C++ Track. I have listed the presentations in that track and some other related presentations for a quick reference.

* An Outline of C++0x (Bjarne Stroustrup)
* Generalizing Constant Expressions in C++ (Jens Maurer)
* C++0x Initilaisation: Lists and a Uniform Syntax (Bjarne Stroustrup)
* An Introduction to the Rvalue Reference in C++0x (Howard Hinnant)
* C++ has no useful purpose (Russel Winder)
* C++ Modules (David Vandevoorde)
* Support for Numerical Programming in C++ (Walter Brown)
* C++ Threads (Lawrence Crowl)
* Standard Library report (Alisdair Meredith)
* Concepts: An Introduction to Concepts in C++0x (Doug Gregor) (OOPSLA06 paper)

iterator tags idiom and compile-time algorithm selection

Compile-time algorithm selection can be done using function overloading and traits idiom. It is a quite useful technique in generic programming. In this technique usually a set of "selector" types are used to select one among many overloaded functions. Note that the number of parameters to these overloaded functions is same but there is a "selector" type at the end of the parameter list. A use of that technique based on standard defined iterator tags is shown here.

Use of std::bad_exception

std::bad_exception is a useful device to handle unexpected exceptions in a C++ program , which works in conjunction with unexpected_handler. unexpected_handler and terminate_handler are a traditional way of dealing with unknown exceptions. std::set_unexpected () and std::set_terminate () functions let us register custom handlers instead of standard handlers, which usually abort the program. More information can be found here.

Assuming g++ 4.0.2 complies with the standard in this area, I verified that if function f() throws an exception that is not listed in its exception specification list, our custom function pointed to by the unexpected_handler is invoked if we have set one. If our custom handler rethrows whatever unknown exception caused the unexpected() to be invoked, following things happen.

* If the exception specification of f() included the class std::bad_exception, unexpected() will throw an object of type std::bad_exception, and the C++ run time will search for another ha…

Boost C++ Idioms

This time lets take a brief look at some nifty C++ idioms in the Boost peer-reviewed libraries. We will talk about Boost Base-from-Member idiom, Boost Safe bool idiom, Boost Named External Argument idiom, Boost Non-member get() idiom, Boost Meta-function wrapper idiom, Boost Iterator pair idiom, and the Boost Mutant idiom.

Boost Base-from-Member idiom
This idiom is used to initialize a base class from a data-member of the derived class. It sounds contradictory to the rules of C++ language and that is the reason why this idiom is present. It basically boils down to pushing the parameter data member in a private base class and put that private base class before the dependent base class in the derivation order. A generalization of the technique can be found here.

Boost Safe bool idiom
Many authors have talked about the evils of type conversion functions defined in a class. Such functions allow the objects of that type to participate in nonsensical expressions. One good example is in standard…

new, delete, custom memory allocation, and exception safety

This post will hopefully push you ahead to a higher level of expertise in memory management mechanics of C++ and exception safety. Here we go...

Some warm-up:

Consider a function
void func (T*, U*);
int main() {
func (new T, new U); // line 1
Calling the function like in line 1 is a bad idea because parameter evaluation sequence is not standard and therefore it memory allocation of second new (it could T or U) fails, we have a blatant memory leak.

What to do? Lets use our silver bullet: auto_ptr! but it fails too for the same reasons. Consider a function
void func (std::auto_ptr <T>, std::auto_ptr <U>);

Now it is possible that, even before any auto_ptrs are constructed, new may throw or a constructor may throw and all efforts go in vain. Details can be found in Hurb Sutter's More Excetional C++: Item 20-21.

So we should separate the allocation and the function call.
auto_ptr <T> t1 = new T; auto_ptr <U> u1 = new U;
func (t1, u1);

That's no better either!

Now …

Non-Virtual Interface (NVI) idiom and the design intent

Assuming that the philosophy of Non-Virtual Interface (NVI) idiom is strictly adhered I came up with a table that summarizes the design intent in terms of access modifiers and virtuality. Strict adherence of NVI allows separation of the interface of a class into two distinct interfaces: client interface (public non-virtual) and subclass interface (protected virtual/non-virtual). Such a structure helps mitigate the Fragile Base Class (FBC) Interface problem if discipline is followed. Its only downside is a little bit of code bloat. More about this approach of resolving FBC can be found here.

virtual but not pure
pure virtual without body
pure virtual with body

Clients expect substitutability. Extension points are
encapsulated.  Subclasses should stay away from this if an equivalent
protected function using NVI is given.
Clients expect substitutability. Extension points are
visible. Subclasses can optionally extend  them but assuming NVI is
place, subclasse…

Virtuality is an implementation detail !

There are many ways of achieving separation of interface from implementation:

1. Using Interface Definition Language (IDL) as in CORBA
2. Bridge Pattern (GoF)
3. Pimpl idiom
4. Handle/Body idiom
5. Envelope/Letter idiom
6. Template Method pattern (GoF)

The Template Method way of achieving this is a bit tricky. It basically boils down to a few class design guidelines based on access modifiers, virtuality and language expressibility. It is also known as Non-Virtual Interface (NVI) idiom.

1. Prefer to make interfaces non-virtual, using Template Method.
2. Prefer to make virtual functions private.
3. Only if derived classes need to invoke the base implementation of a virtual function, make the virtual function protected.
4. A base class destructor should be either public and virtual, or protected and non-virtual.

For more information please see: Virtuality and Virtually Yours.
A few more elemental base class idioms can be found here.
A comparison of the above approaches can be found in thisACCU art…

Forgotton friend: pointer/reference to an array

The name of an array "degenerates" into a pointer to the first element of the array. For example,
int Array [10];
int *p = Array;

there is a quite a bit loss of information here. Specifically, such a decay loses its type information and specifically, its dimensions. The type of Array is "An integer array of 10 integers." So we lost the word "array" and we also lost the length (10) of the array.

An advantage of using pointer to an array is that this type information is retained. int (*q)[10] = &Array; It declares a pointer q to to an array of 10 integers. So whats the big deal? Compiler has this information and it is happy to let us take advantage of it.

template <int p, int q, int r>
int average (int (&array)[p][q][r]) { ... }

main () {
int q [][2][2] = {
{ {1, 2}, {3, 4} },
{ {5, 6}, {7, 8} }
average (a); /// This call knows the dimensions of the array.
This type information can be exploited to implement the average function using template meta-…

Service Configurator pattern and singleton deletion

The Service Configurator pattern is about linking in additional functionality in an application dynamically by means of an API such as dlsym/dlopen provided by the OS/run-time linker. Almost all popular platforms provide this functionality. This post is about programmatically, dynamically loaded libraries (hence forth called simply library)(e.g., .DLL, .so) that contain sigletons.

One of my previous posts talks about singleton deletion. If the sinlgleton in such a library is a static instance then it is not deleted untill the library is closed using appropriate API such as dlclose. This can be quite undesirable or awkward at times. All the destruction techniques based on static variables fail if you want to destroy the singleton without unloading the library. A possible variant of the singleton in such a case is given below.

class Singleton {
Singleton ();
static Singleton *instance () {
if (instance_ == 0)
instance_ = new Singleton;
return instance_;
static void operator d…

swap using XOR

In one of my previous posts I described a few ways of swapping two numbers using xor operation in a sinlge line. The discussion is not correct or rather incomplete in the sense that the algorithm needs to be guarded by an if condition. The algorithm that exercises proper care of sequence points is as below:

swap (int &a, int &b) {
if (&a != &b) {
a ^= b;
b ^= a;
a ^= b;
} }

It is crutial that the two number must not be located at the same address. The two numbers must have different identities. The algorithm works even though the values are the same.

Publications by C++ experts

Ownership idioms, The Move Constructor idiom and auto_ptr

There are properties of objects that are relevant beyond mere identity of the object. One of such properties is Ownership! Who owns the object? Can you transfer the ownership? Questions like these become quite important when you are implementing Resource Acquisition Is Initialization (RAII) idiom. Tom Cargill describes [in PLoPD2] a set of ownership patterns: "Creator as a Sole Owner", "Sequence of Owners", and "Shared Ownership". Lets take a look at the idioms for each one of them.

Creator as a sole owner can be simply implemented using const auto_ptr idiom by Hurb Sutter. Sequence of owners come into play when ownerships moves from one guy to the other. (a.k.a. Move Semantics). Standard auto_ptrs use move semantics. Implementation of move semantics appears to be a juicy issue as far as std::auto_ptrs are concerned. See Meyer's errata like notes on his book More Effective C++ and Sutter's GotW #25 articles. M. D. Wilson's article on Move Const…

The Policy Clone idiom

Modern C++ library design is heavily based on templates. Highly reusable, flexible and extensible classes can be built using policy based class design techniques. (See "Modern C++ Design" by Andrei Alexandrescu.) Sometimes, the host class of the policies need to make an exact replica of one of its policies but instantiated with different type parameter. Unfortunately, the writer of the host class template does not know the template name to instantiate beforehand. This situation is quite analogous to the situation in the Factory Method (GoF) pattern where type of the object to be created is not known apriori and therefore, the object creation is simply delegated to the derived object.

In the world of templates, although the problem is conceptually is quite similar, the mechanism to solve it is a different because we are interested in a "clone" type and not an object. The Policy Clone idiom comes handy here. A member template struct (called rebind) is used to pass a d…

The singleton universe!

Singleton pattern appears to be very easy to understand because in its simplest form it can be implemented in minutes with the help of well defined language idioms. But in fact, it is one of the most complex patterns I have ever read about! If you dig around a little bit, there is a whole slew of information/articles available on this nifty pattern talking about its limitations and their solutions. Here is a collection of some of the amazing articles.

The Singleton pattern - Gamma et al.
To kill a Singleton - John Vlissides
The Monostate pattern (The Power of One (More C++ Gems) - Steve Ball and John Crawford)
Implementing Singletons (Chapter 6, Modern C++ Design) - Andrei Alexandrescu
Double Checked Locking Pattern (DCLP) - Doug Schmidt
C++ and the perils of DCLP - Scott Meyers, Andrei Alexandrescu
DCLP is broken in Java - Bacon et al.
Object Lifetime Manager pattern - Levine, Gill, and Doug Schmidt