C++ Blog

Functional Programming & Boost.Lambda Programming

Posted in boost, templates by Umesh Sirsiwal on December 26, 2009

I will like to take a break from Boost.Spirit related topic to talk about Functional Programming. Understanding of Functional Programming is essential for understanding how Spirit related actions are implemented. Typical object oriented programming paradigm combines mutable data and set of algorithms which operates on data. In contrast FP avoids mutable data or state and emphasizes on application of functions. From Wikipedia:

In practice, the difference between a mathematical function and the notion of a “function” used in imperative programming is that imperative functions can have side effects, changing the value of already calculated computations. Because of this they lack referential transparency, i.e. the same language expression can result in different values at different times depending on the state of the executing program. Conversely, in functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) both times. Eliminating side-effects can make it much easier to understand and predict the behavior of a program, which is one of the key motivations for the development of functional programming.

STL provides set of algorithms which can be viewed as FP. For example std::copy will always copy source to destination.

The next concept in FP is called currying, technique in which a function is applied to its arguments one at a time, with each application returning a new function that accepts the next argument. The STL function bind1st, bind2nd and binary_compose can be used for currying

All funcitonal programming languages are based on Lambda Calculus introduced in 1930s which is based on anonymous functions and currying. You can read more on it here.

Boost.Lambda and Boost.Phoenix introduce formal system of funcitonal programming and lambda expressions to C++. I typically use Boost.Lambda. I only use Phoenix when using Spirit since they are more closely integrated. They share a lot of similar concept. This post will discuss Boost.Lambda. The next post will use these concepts to introduce Phonix as applied to Boost.Spirit.

Let us start with a simple lambda expression _1 = 1 is definition of an anonymous function which takes one argument and sets it to 1. So to initialize a variable to 1 you could write:

(_1 + 1)(i);

or if you wanted to initialize all variables in a container to 1, you would write something like:

std::vector v;

std::for_each(v.begin(), v.end(), _1=1);

or to print everyobject in a container:

std::for_each(v.begin(), v.end(), std::cout << _1);

What if you wanted to define a function with calls a function with one argument and assigns results to the same argument:

That function will be _1 = bind(foo, _1);

Bind is generalized version of std::bind1st and bind2nd. With the above expression in place one can write:

(_1 = bind(foo, _1))(i);

Which will be equivalent to:

i = foo(i);

So with for_each one can write:

std::for_each(v.begin(), v.end(); _1 = bind(foo, _1));

_1 in the above examples are called placeholders which are equlvalent of lambda in the lambda expression. The above can be generalized with larger number of placeholders. For example one can write: (_1 + _2) to create a function which adds its two arguments. Boost Lambda defines up to 3 place holders. Higher order functions can be constructed using currying.

A function definition which used _2 takes 2 arguments and one which uses _3 takes 3 arguments.

For example _3 = 1, takes 3 argument and

(_3 = 1)(k) will have compile time errors.

(_3 = 1)(i,j,k) is good and is equivalent to k = 1.

Above, we are using simple expressions to create anonymous functions which do not have side effects and depend only on their arguments. As alluded earlier BLL also provides bind expression as generalization of bind1st and bind2nd. With BLL bind expression it is possible to bind any method with up to 9 arguments for delayed executions. It can target C++ class members, or simple functions as target. The syntax used is similar to bind1st and bind2nd.

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Boost.Spirit : Complete Parser Design

Posted in boost, templates by Umesh Sirsiwal on December 25, 2009

Note that this post applies to Spirit.Classic or 2.0.

In the last post we discussed how to write a simple parser using Spirit. Most real life parsers are a lot more complex requiring several rules and combining them to create a full grammar. Spirit provides support to declare a full grammar. Let us create a parser which looks for all mailto: tag with associated e-mail address in an input. All Spirit grammar are derived from grammar class. The following is definition of grammar:

 struct my_grammar : public grammar<my_grammar>
        template <typename ScannerT>
        struct definition
            rule<ScannerT>  r;
            definition(my_grammar const& self)  { r = /*..define here..*/; }
            rule<ScannerT> const& start() const { return r; }

The my_grammar is derived from grammar class using curiously recurring template pattern (CRTP). For those who are new to CRTP, it was introduced by Jim Copelien and Wikipedia has some details on it. Using CRTP it is possible to achieve effect of dynamic polymorphism without taking cost of virtual function. More on CRTP another day.

Each grammar class must define another nested structure called definition. The definition must have a function called start which returns starting parser rule. So let us start writing rules for e-mail address parser. The mailto rule can be written as:

     mailTo = "mailto:" >> emailAddress;

From our previous post, emailAddress can be written as:

     emailAddress =  lexeme_d[ +alnum_p >> '@' >> +alnum_p >> *('.' >> +alnum_p)];

Of course the full input has things other than the mailto: tags. So we must skip other characters. You do that as follows:

       r = mailTo | anychar_p;

The above rule is saying that try matching the input with mailTo and if it does not match mailTo rule, match it against any character parser.

So combining these two the grammar constructor now can be written as:

      definition(my_grammar const& self)  { 
            r = mailTo | anychar_p;
            mailTo = "mailTo:" >> emailAddress;
            emailAddress = lexeme_d[ +alnum_p >> '@' >> +alnum_p >> *('.' >> +alnum_p)];

First thing one notices is that the rule “r” refers to rules “emailAddress” and “mailTo” before they are initialized. It works because the rules are held by reference and not by value. The referred rule can be initialized anytime. This does complicate programming a little bit. It means that it is user’s responsibility to make sure that the referred rules never go out of scope. So emailAddress cannot be local to definition. Typically, one declares all rules as part of the definition call definition. The full grammar now becomes:

 struct my_grammar : public grammar<my_grammar>
        template <typename ScannerT>
        struct definition
            rule<ScannerT>  r;
            rule<ScannerT>  emailAddress mailTo;
            definition(my_grammar const& self)  { 
                r = mailTo | anychar_p;
                mailTo = "mailTo:" >> emailAddress;
                emailAddress = lexeme_d[ +alnum_p >> '@' >> +alnum_p >> *('.' >> +alnum_p)];
            rule<ScannerT> const& start() const { return r; }

OK! now we have the grammar. Let us use it.

   my_grammar g;
    if (parse(first, last, g, space_p).full)
        cout << "parsing succeeded\n";
        cout << "parsing failed\n";

Here first and last are iterators pointing to first and the last characters in the input.

Note that the above grammar matches exactly one e-mail address or any other character. This can be easily be modified to match all e-mail addresses.

The new rule will be:

r = *(mailTo | anychar_p);

C++ Concept Part 3

Posted in c++0x, templates by Umesh Sirsiwal on January 25, 2009

This is the last post in series of posts on C++0x concepts. In this we will look at C++0x concepts. The C++0x standard for concepts is almost identical to ConceptC++. The syntax are almost identical. Significant parts of ConceptC++ have become part of the draft standard.

One of the things we have not looked at the definition of concept map. In the previous example of sort, the template takes ForwardIterator as an argument. The article did not discuss what makes a forward iterator. Can we use an int* as a forard iterator? We have also said that ForwardIterator must has value_type argument. int* does not have a member value_type. But we want to pass int* as parameter to sort so that for example, we can sort an array of integers.

C++0x provides a way to solve this problem called concept_map. Using concept_map we can define what constitute ForwardIterator.

template ;
	concept_map ForwardIterator {
                                    // T*'s value_type is T
		typedef T value_type;

Using above syntax we are saying that any T”*’s value_type is T. This removes need to wrap int* in another container.

In all C++0x concept provides solution for one of the biggest ongoing frustration with generic programming with C++. It will provide early and clear compilation errors for template users and authors both and bind them in template parameter contract.

Other C++0x sources on the web

C++0x Concepts Contd..

Posted in c++0x, templates by Umesh Sirsiwal on January 18, 2009

In previous article we had discussed BCCL. This post continues to discuss Concept with ConceptC++. The ConceptC++ was the playground for language level Concept ideas which eventually became part of currently proposed C++0x standard. In this post we take in-depth view to ConceptC++. These extensions were implemented by modifying GCC.


Unlike the Boost Concept Check Library (BCCL), which is implemented purely using standard C++ constructs, these extensions are implemented by extending C++ language. By changing the C++ definition, it is possible to do things which was not possible otherwise. In particular, remember that with BCCL, it was not possible to make template parameter restriction part of template or function definition. With ConceptC++, this becomes now possible. In addition to enforcing restriction on instantiation of template, ConceptC++ also imposes restrictions on template implementation and makes sure that it only uses template parameter’s facility defined in the contract.

To understand ConceptC++, let us consider sum template which adds two parameters and returns the value:

template <typename T>
T sum(T a, T b){
   return a+b;

With Concept C++ we can rewrite this to:

template <CopyConstructable T>
T sum(T a, T b){
   return a+b;

This says T must be copy constructable. If we compile this template using ConceptC++ (even without any instantiation) we get the following error:

error: no match for 'operator+' in 'a+b'

That is because the template definition did not require T to implement operator+ but is trying to use it. This forces template signature to accurately define the template signature as:

template <CopyConstructable T>
  requires Addable<T>
T sum(T a, T b){
   return a+b;

In other words ConceptC++ puts restrictions on both template instantiation and template definition and binds them in a contract with respect to template parameter. This is what function, class or template definition, etc. are supposed to be.

We have been using magic words like CopyConstructable, Addable, etc. What are they? Are they new language features or are provided by library? What if we wanted to implement our own custom concepts? How will we go about that.

The C++ language addition provides capability to define new concepts while the library provides set of standard concepts. It is rather easy to build your own concept. For example let us look at definition of Addable concept:

auto concept Addable<typename T> {
  T operator+(T x, T y);

How simple can that be? The above definition is saying that the Addable concept implies that the template parameter must implement operator+ which takes two parameters of type T and returns T.

With this background, let us modify out example sort template. In case of sort, the template parameters are iterators and value pointed to by the iterator must implement less than operator. With ConceptC++ you can specify these restrictions as follows:

#include <concepts>
template<std::ForwardIterator T>
    requires LessThanComparable<T::value_type>
void sort(T b, T e)

As you can see ConceptC++ provides natural way of definining restrictions on template parameters allowing compilers to perform early error checking.


The ConceptC++ implements its functionality by reducing compile speed. However, just like BCCL the ConceptC++ does not incur any run time overhead.


C++ Proxy Template

Posted in templates by Umesh Sirsiwal on January 14, 2009

Earlier we had discussed Java Proxy class and had looked for ways to develop similar facility with C++. It turns out there is a rather easy way to provide such a facility. If you have used smart pointers, you have already used one form C++ proxy template. Smart pointers essentially provide a proxy facility. Bjarne Stroustrup covered this subject in this paper.

The key to the wrapper definition is that the fact that all objects created on the stack are eventually destroyed when the object goes out of the scope. For example:

class Test{
    void method(){
         X a;
       // Implementation


In the above case the X’s destructor is called when Test::method returns due to return call or exception.

Now let us use this knowledge to define the Wrap class. Let us first define the prefix, suffix and the wrapped class:

#include <iostream>
 using namespace std;
 void prefix() { cout << "prefix\n" }
 void suffix() { cout << "suffix\n"; }

// The person class looks like this:

class Person{
  std::string mName;
  Person(std::string pName): mName(name){}
  void printName(){
     std::cout << mName << std::endl;


Now we can define the Wrap class as the following:

template <class T >
class Wrap {
     T * p ;
             Wrap (T * pp ) :p (pp) { }
             Call_proxy <T> operator ->() {
                   prefix ();
                   return Call_proxy<T>(p);
template <class T >
class Call_proxy {
       T * p ;
       public :
              Call_proxy (T * pp ) :p (pp ){ }
              ˜Call_proxy () {
                     suffix ();
               T * operator ->() {
                          return p ;

We can now wrap any object  of type T in the Wrap class. For example: we can take class Person and wrap it like Wrap<Person> person(new Person);

Any dereferencce of person using -> operator, results in call to operator-> of Wrap class. The method calls the prefix function , creates a new object of type Call_proxy containing the pointer to object. The method is called. On return from the method, Call_proxy is destroyed which implicitly results in call to suffix().

Let us test it. the following code snippet:

Wrap<Person> person(new Person("test"));

should print:


Rather straightforward and elegant to use. But, syntatic sugar to implement.

Note that suffix will be called even if the method returned due to exception.  This is better than Java, where the programmer has to take explicit action to make sure suffix is called in case of exception.

The Stroustrup’s paper also covers other issues like ownership, parametrization etc.  In most cases you may not need to worry about those. If you need to, take a look at the paper.

Happy proxying!

Template or not to Template

Posted in templates by Umesh Sirsiwal on January 10, 2009

As I was learning C++ over an year ago,  I bought Modern C++ Design by Andrei Alexandrescu. As I read through the book, I was simply amazed by power offered by templates. Experience with the book was followed by Boost libraries. Specifically boost lambda and what it could provide to programmer. These went way beyond traditional STL use of templates. This excitement with C++ templates was followed by my experimentation with using templates in my own projects.  This post summarizes  experiences with templates.

  • Templates provide power with efficiency: Templates provide a lot of power without compromising efficiency. The Andrei’s book is full of examples of such power. A quick look inside the boost library provides examples of such power. Look at simple construct like boost::any which allows you to hold any type of data in it so that you can create a container/array which can hold boolean, integers, your various classes, etc. all at the same time. You can also look at a lot more complex construct like boost::lambda library which brings lambda programming to C++. Recently the boost has added boost::proto discussed in a seperate post on this site. The power really exists. Using these libraries can make your programming life a lot simpler. It is natural tendency to write your own templates on the same line. Should you or shouldn’t you?
  • Templates are not well understood: The templates are not well understood yet. Most experienced C++ programmers find it hard to understand how to program templates. At present templates are a the stage as Object Oriented Programming was several years ago. The compilers have progressed to state where they can correctly compile templates. However, the power of templates is yet to be distilled in to developer community at large. So if you decide to use templates, please be patient and give plenty of time for developers to learn and get up to speed.
  • Template debugging is horrifying experience for first time users: Error messages produced by compiler are horrifying large and complex. For simplest of errors, one gets 3 pages long error message with actual error who knows where. Once you get used to of debugging compilation issues you can generally figure out the error but it takes time to get used to of those errors. Template adaption will go a long way if the compilers start producing better error messages. But if you think compiler was the only issue just wait till you get to debuggers. The debuggers have not yet found a good way to deal with templates. Last but not least is compile time with templates. With most compilers, templates have to be included in header files. A simple change in the template requires compilation of all of your code which uses templates. This is like going backward on libraries.

In my experience, templates offer a lot of power and flexibility without compromising efficiency. The compilers can mostly deal with templates correctly. However debugger, compiler errors, and ways to reduce compile time can go a very long way in helping wider adaptation of templates. But if you are one of those early adapters and technology enthusiast, go ahead experiment and adapt templates and it will go a long way in helping your projects.

If you are going to use templates, you should start off by reading C++ FAQ Lite on template.