Assignment Library

Copyright © 2003-2006 Thorsten Ottosen

Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see http://www.boost.org/LICENSE_1_0.txt).

Table of Contents

• Introduction
• Tutorial
  • Function operator+=()
  • Function operator()()
  • Function list_of()
  • Function map_list_of()
  • Function tuple_list_of()
  • Functions repeat(), repeat_fun() and range()
  • Functions ref_list_of() and cref_list_of()
  • A "complicated" example
  • Functions ptr_push_back(), ptr_push_front(), ptr_insert() and ptr_map_insert()
  • Function ptr_list_of()
• Reference
  • Headers
  • Standard containers
  • Functions list_of() and map_list_of()
  • Functions repeat(), repeat_fun() and range()
  • Class list_inserter
  • Function make_list_inserter()
  • Customizing argument list sizes
• Exceptions and exception-safety
• Extending the library
• Supported libraries
• Examples
• Portability
• History and Acknowledgment
• References

Introduction

There appear to be few practical uses of operator,().
Bjarne Stroustrup, The Design and Evolution of C++

The purpose of this library is to make it easy to fill containers with data by overloading operator,() and operator()(). These two operators make it possible to construct lists of values that are then copied into a container:

• A comma-separated list:

  vector<int> v; 
  v += 1,2,3,4,5,6,7,8,9;

• A parenthesis-separated list:

  map<string,int> m; 
  insert( m )( "Bar", 1 )( "Foo", 2 );

These lists are particularly useful in learning, testing, and prototyping situations, but can also be handy otherwise. The library comes with predefined operators for the containers of the standard library, but most functionality will work with any standard compliant container. The library also makes it possible to extend user defined types so for example a member function can be called for a list of values instead of its normal arguments.

Tutorial

Within two minutes you should be able to use this library. The main components are explained in these sections:

• Function operator+=()
• Function operator()()
• Function list_of()
• Function map_list_of()
• Function tuple_list_of()
• Functions repeat(), repeat_fun() and range()
• Functions ref_list_of() and cref_list_of()
• A "complicated" example
• Functions ptr_push_back(), ptr_push_front(), ptr_insert() and ptr_map_insert()
• Function ptr_list_of()

Function operator+=()

Function operator()()

#include <boost/assign/list_inserter.hpp> // for 'push_front()'
#include <boost/assert.hpp> 
#include <string>
#include <utility>
using namespace std;
using namespace boost::assign; // bring 'push_front()' into scope
 
{
    typedef pair< string,string > str_pair;
    deque<str_pair> deq;
    push_front( deq )( "foo", "bar")( "boo", "far" ); 
    BOOST_ASSERT( deq.size() == 2 );
    BOOST_ASSERT( deq.front().first == "boo" );
    BOOST_ASSERT( deq.back().second == "bar" );
}   

If operator()() is too cumbersome to use with eg. push_front()we can also say

deque<int> di;    
push_front( di ) = 1,2,3,4,5,6,7,8,9;
BOOST_ASSERT( di.size() == 9 );    
BOOST_ASSERT( di[0] == 9 );    

Just to make it perfectly clear, the code above is not restricted to the standard containers, but will work with all standard compliant containers with the right member function. It is only operator+=() that has been restricted to the standard containers.

Function list_of()

Please notice that list_of() can even convert to a boost::array<T,sz> and see also the list of supported libraries .

Note that the type returned by list_of() (and its variants) has overloaded comparison operators. This allows you to write test code such as BOOST_CHECK_EQUAL( my_container, list_of(2)(3)(4)(5) );.

Function map_list_of()

Function tuple_list_of()

Functions repeat(), repeat_fun() and range()

If you just need to insert an iterator-range somewhere in the list, the member function range() provides just what you want. It is based on Boost.Range, so you can pass all the ranges supported by that library. For example

#include <boost/assign/list_inserter.hpp> // for 'push_back()'
#include <boost/assign/list_of.hpp>       // for 'list_of()' and 'ref_list_of()'
#include <boost/assert.hpp>

using namespace std;
using namespace boost::assign;
 
{
    vector<int> v, v2;
    v  = list_of(1)(2)(3);
    v2 = list_of(0).range(v).range(v.begin(),v.end())(4);
    // v2 = [0,1,2,3,1,2,3,4]
    BOOST_ASSERT( v2.size() == 8u );

    push_back( v ).range(v2)(5);
    // v = [1,2,3,0,1,2,3,1,2,3,4,5]
    BOOST_ASSERT( v.size() == 12u ); 

    //
    // create a list of references, some coming from a container, others from the stack 
    //
    int x = 0;
    int y = 1;
    BOOST_ASSERT( ref_list_of<10>(x).range(v2)(y).size() == 10u );
}

Functions ref_list_of() and cref_list_of()

A "complicated" example

Functions ptr_push_back(), ptr_push_front(), ptr_insert() and ptr_map_insert()

#include <boost/assign/ptr_list_inserter.hpp> // for 'ptr_push_back()', 'ptr_insert()' and 'ptr_push_front()'
#include <boost/assign/ptr_map_inserter.hpp>  // for 'ptr_map_insert()'
#include <boost/ptr_container/ptr_deque.hpp>
#include <boost/ptr_container/ptr_set.hpp>
#include <boost/ptr_container/ptr_map.hpp>

//
// Example class
//
struct Foo
{
    int i;
    
    Foo() : i(0)
    { }
    Foo( int i ) : i(i)
    { }
    Foo( int i, int ) : i(i)
    { }
    Foo( const char*, int i, int ) : i(i)
    { }

    virtual ~Foo()
    {}
};

struct Bar : Foo
{
    Bar()
    { }
    
    Bar( int i ) : Foo( 42 )
    { }
};

//
// Required by ptr_set<Foo>
//
inline bool operator<( Foo l, Foo r )
{
    return l.i < r.i;
}

 
using namespace boost;
using namespace boost::assign;

int main()
{
    ptr_deque<Foo> deq;
    ptr_push_back( deq )()();
    BOOST_ASSERT( deq.size() == 2u );
    ptr_push_back<Bar>( deq )()(); // insert 'Bar' objects
    BOOST_ASSERT( deq.size() == 4u );
    ptr_push_front( deq )( 3 )( 42, 42 )( "foo", 42, 42 );
    BOOST_ASSERT( deq.size() == 7u );

    ptr_set<Foo> a_set;
    ptr_insert( a_set )()( 1 )( 2, 2 )( "foo", 3, 3 );
    BOOST_ASSERT( a_set.size() == 4u );
    ptr_insert( a_set )()()()();
    BOOST_ASSERT( a_set.size() == 4u ); // duplicates not inserted
    ptr_insert<Bar>( a_set )( 42 ); // insert a 'Bar' object 
    BOOST_ASSERT( a_set.size() == 5u );

    ptr_map<int,Foo> a_map;
    ptr_map_insert( a_map )( 1 )( 2, 2 )( 3, 3, 3 )( 4, "foo", 4, 4 );
    ptr_map_insert<Bar>( a_map )( 42, 42 ); // insert a  'Bar' object
}
    

For ptr_map_insert() the first argument arg1 in an argument tuple (arg1,arg2,...,argN) is used to construct a key; this means that the first argument need only be convertible to the key_type of the container. The rest of the arguments are used to construct the mapped object.

Function ptr_list_of()

#include <boost/assign/ptr_list_of.hpp>
#include <boost/ptr_container/ptr_deque.hpp>

using namespace boost;
using namespace boost::assign;

{
    ptr_deque<Foo> deq;
    deq = ptr_list_of<Foo>( 42 )()()( 3, 3 )( "foo", 2, 1 );
    BOOST_CHECK( deq.size() == 5 );
}    
    

That is all; now you are ready to use this library.

Reference

It is worth noticing the way the library is implemented. A free-standing function (eg. push_back() or operator+=()) returns a proxy object which is responsible for the insertion or the assignment. The proxy object does the insertion or assignment by overloading operator,() and operator()() and by calling the "insert" function from within those operators. The "insert" function is typically stored in the proxy object by using boost::function.

Often overloading of operator,() is discouraged because it can lead to surprising results, but the approach taken in this library is safe since the user never deals with objects that have overloaded operator,() directly. However, you should be aware of this:

The expressions in a comma-separated list no longer follow the rules of the built-in comma-operator. This means that the order of evaluation of expressions in a comma-separated list is undefined like when one specifies a list of function arguments.

Most of the code in this document use int in the examples, but of course it works for arbitrary types as long as they are Copy Constructible. The inserted data need not be constant data, but can be variables or data returned from functions; the only requirement is that the type of the data is convertible to the type stored in the container.

All forwarding is done by passing objects by const reference. Originally arguments were passed by value (and still is in tuple_list_of()). One thing to remember is that references can be passed by using boost::ref.

Everything is put in namespace boost::assign.

More details can be found below:

• Headers
• Standard containers
• Functions list_of() and map_list_of()
• Functions repeat(),repeat_fun() and range()
• Class list_inserter
• Function make_list_inserter()
• Customizing argument list sizes

Headers

Standard containers

Functions list_of() and map_list_of()

These two functions are used to construct anonymous list which can be converted to any standard container and boost::array<T,sz>. The object returned by the two functions is guaranteed to have the interface described below.

namespace boost  
{
namespace assign
{
    template< class T >
    class Implementation-defined
    {
    public:
        const_iterator  begin() const;
        const_iterator  end() const;
        
        template< class U >
        Implementation-defined& operator,( U u );
        
        // inserts default-constructed object
        Implementation-defined& operator()();  

        template< class U >
        Implementation-defined& operator()( U u );
        
        template< class U, class U2 >
        Implementation-defined& operator()( U u, U2 u2 );

        //
        // and similarly up to 5 arguments
        //
        
        //
        // Convert to a 'Container'. 'Container' must have a constructor 
        // which takes two iterators.  
        //
        template< class Container >
        operator Container() const; 

        //
        // Convert to a container adapter like 'std::stack<>'.
        //
        Convertible-to-adapter to_adapter() const;
        
        //
        //
        // Convert to eg. 'boost::array<T,std::size_t>'. If the  
        // assigned variable is too small, 
        // a assignment_exception is thrown.
        // If the assigned variable it is too big, the rest of the 
        // values are  default-constructed.
        //
        template< template <class,std::size_t> class Array, class U, std::size_t sz > 
        operator Array<U,sz>() const;
    };
    
    //
    // Comparison operators. 'op' can be <,>,<=,>=,==,!=
    //
    template< class Range >
    bool op( const Implementation-defined&, const Range& );
    template< class Range >
    bool op( const Range&, const Implementation-defined& );
    
    template< class T >
    Implementation-defined   list_of();

    template< class T >
    Implementation-defined   list_of( T t );
    
    template< class T, class U, class U2 >
    Implementation-defined   list_of( U u, U2 u2 );
    
    template< class T, class U, class U2, class U3 >
    Implementation-defined   list_of( U u, U2 u2, U3 u3 );

    template< class T, class U, class U2, class U3, class U4 >
    Implementation-defined   list_of( U u, U2 u2, U3 u3, U4 u4 );
  
    template< class T, class U, class U2, class U3, class U4, class U5 >
    Implementation-defined   list_of( U u, U2 u2, U3 u3, U4 u4, U5 u5 );

    template< class Key, class T >
    Implementation-defined   map_list_of( Key k, T t )
    {
        return list_of< std::pair<Key,T> >()( k, t );
    }
} // namespace 'assign'
} // namespace 'boost'  

Functions repeat(), repeat_fun() and range()

Class list_inserter

Synopsis

namespace boost
{
namespace assign
{
    template< Function, Argument = void > 
    class list_inserter
    {
        Function fun;
        
    public:
        explicit list_inserter( Function fun );
        
        // conversion constructor
        template< class Function2, class Arg >
        list_inserter( const list_inserter<Function2,Arg>& );
        
    public:
        template< class U >
        list_inserter& operator,( U u );
        
        template< class U >
        list_inserter& operator=( U u );
        
        // calls 'fun()' with default-constructed object
        list_inserter& operator()();
        
        template< class U >
        list_inserter& operator()( U u );
        
        template< class U, class U2 >
        list_inserter& operator()( U u, U2 u2 )
        {
           //
           // if 'Argument' is 'void'
           //     fun( u, u2 );
           // else
           //     fun( Argument( u, u2 ) );
           //
           return *this;
        }

        //
        // similarly up to 5 arguments
        //
    };
    
    template< class C >
    list_inserter< ... > push_back( C& );
      
    template< class C >
    list_inserter< ... > push_front( C& );

    template< class C >
    list_inserter< ... > insert( C& );

    template< class C >
    list_inserter< ... > push( C& );
      
} // namespace 'assign'
} // namespace 'boost'

Notice how the arguments to operator,() and operator()() are passed differently to fun depending of the type of Argument. So if we only pass one template argument to list_inserter, we can forward "arbitrary" argument lists of functions. If we pass two template arguments to list_inserter we can construct types with "arbitrary" constructors.

And because a reference to list_inserter is returned, we can chain argument list together in a very space-efficient manner.

Function make_list_inserter()

A simple "constructor" function for list_inserter. A typical use of this function is to call it with the result of boost::bind() which in general returns some unreadable and weird class template.

namespace boost 
{
namespace assign
{  
    template< class Function >
    list_inserter<Function>  make_list_inserter( Function fun )
    {
        return list_inserter<Function>( fun );
    } 
}
}    

Customizing argument list sizes

This library uses the boost Preprocessor Library to implement overloaded versions of operator()() and list_of(). By default you can call these functions with five arguments, but you can also customize this number by defining a macros before including a header from this library:

#define BOOST_ASSIGN_MAX_PARAMS 10
#include <boost/assign.hpp>

Exceptions and exception-safety

The exception guarantees by the library is the same as guarantee as the guarantee of the function that is forwarded to. For standard containers this means that the strong guarantee is given for a single insertions and that the basic guarantee is given for many insertions (provided that the object being copied gives the basic guarantee).

The functions may throw standard exceptions like std::bad_alloc. Note however that, unfortunately, the standard does not guarantee allocation-failures in standard containers to be reported by std::bad_alloc or exceptions derived from std::exception.

Class assignment_exception

The exception is thrown by the conversion operator in the proxy object returned from list_of().

namespace boost 
{
namespace assign
{
    class assignment_exception : public std::exception
    {
    public:
        explicit assignment_exception( const char* what ); 
        virtual const char* what() const throw();
    };
}   
}  

Extending the library

It is very simple to make the library work with new classes. This code shows how to use operator+=() with a container:

template< class V, class A, class V2 >
inline list_inserter< assign_detail::call_push_back< std::vector<V,A> >, V > 
operator+=( std::vector<V,A>& c, V2 v )
{
    return make_list_inserter( assign_detail::call_push_back< std::vector<V,A> >( c ) )( v );
}

template< class C >
class call_push_back
{
    C& c_;
public:

    call_push_back( C& c ) : c_( c )
    { }
    
    template< class T >
    void operator()( T r ) 
    {
        c_.push_back( r );
    }
};

An alternative way would be to use boost::function and boost::bind() in combination. However, in this case one must remember that it is illegal to take the address of a function in the standard library.

Calling a function with more that one argument can be very useful too. This small example shows how we take advantage of this functionality:

//  
// A class representing emails
//
class email
{
public:
    enum address_option
    {
        check_addr_book,
        dont_check_addr_book
    };
    
private:

    typedef std::map< std::string,address_option >  address_map;
    
    //
    // Store list of persons that must be cc'ed
    //
    mutable address_map cc_list;
        
    //
    // This extra function-object will take care of the 
    // insertion for us. It stores a reference to a 
    // map and 'operator()()' does the work. 
    //
    struct add_to_map
    {
        address_map& m;
    
        add_to_map( address_map& m ) : m(m)        
        {}
    
        void operator()( const std::string& name, address_option ao )
        {
            m[ name ] = ao; 
        }
    };

public:
    
    //
    // This function constructs the appropriate 'list_inserter'.
    // Again we could have use 'boost::function', but it is
    // trivial to use a function object.
    //
    // Notice that we do not specify an extra template
    // parameter to 'list_inserter'; this means we forward
    // all parameters directly to the function without 
    // calling any constructor.
    //
    list_inserter< add_to_map >
    add_cc( std::string name, address_option ao )
    {
        //
        // Notice how we pass the arguments 'name' and 'ao' to
        // the 'list_inserter'.
        //
        return make_list_inserter( add_to_map( cc_list ) )( name, ao );
    }
};

//
// Now we can use the class like this:
//
email e;
e.add_cc( "Mr. Foo", email::dont_check_addr_book )
        ( "Mr. Bar", email::check_addr_book )
        ( "Mrs. FooBar", email::check_addr_book );  

Examples

• email_example.cpp
• my_vector_example.cpp
• multi_index_container.cpp
• array.cpp
• list_of.cpp
• std.cpp
• list_inserter.cpp
• list_of_work_around.cpp

Supported libraries

1. boost::array
2. boost::multi_index_container
3. Boost.Pointer Container

Portability

Library has been successfully compiled and tested with MVC++ 7.1, GCC 3.2 (under Cygwin) Comeau 4.3.3

There are known limitation on platforms not supporting templated conversion operators. The solution is to call certain member functions on the object returned by list_of():

{
    using namespace std;
    using namespace boost;
    using namespace boost::assign;
    
    vector<int>         v = list_of(1)(2)(3)(4).to_container( v );
    set<int>            s = list_of(1)(2)(3)(4).to_container( s );  
    map<int,int>        m = map_list_of(1,2)(2,3).to_container( m );
    stack<int>         st = list_of(1)(2)(3)(4).to_adapter( st );
    queue<int>          q = list_of(1)(2)(3)(4).to_adapter( q ); 
    array<int,4>        a = list_of(1)(2)(3)(4).to_array( a );
}     

Notice how one must supply the functions with an argument so the right return type can be deduced.

Some standard libraries are also broken. One problem is that insert() might not work:

map<int,int> next; 
insert( next )(1,2)(2,3); // compile-time error 

map<int,int> next = map_list_of(1,2)(2,3);

History and Acknowledgment

The idea for an assignment/initialization library is not new. The functionality of this library resembles Leor Zolman's STL Container Initialization Library a great deal, but it does not rely on string parsing to achieve its goals.

The library is non-intrusive and puts only a minimum of requirements on its supported classes. Overloading operator comma is sometimes viewed as a bad practice [1]. However, it has been done with success in eg. the Generative Matrix Computation Library and Blitz to initialize matrices (see [2]) and [3]). The Initialization Library overloads the comma operator in a safe manner by letting free standing functions return an object that is responsible for the initialization. Therefore it takes explicit action from the programmer to begin using the overloaded operator,().

There has recently been some discussion about enhancing the language to support better initialization (see [4]).

Special thanks goes to

• Leor Zolman for our many discussion that eventually led to this library.
• Tom Brinkman for being review manager.
• Joaquín Muñoz for vc6/vc7 portability.
• Pavel Vozenilek for his countless suggestions, improvements and portability fixes.
• Rene Rivera for Code Warrior portability.

References

1. Scott. Meyers, "More Effective C++", Item 7, Addison Wesley, 1996
2. K. Czarnecki and U.W. Eisenecker, "Generative programming", Addison-Wesley, 2000
3. http://www.oonumerics.org/blitz/
4. Gabriel Dos Reis and Bjarne Stroustrup, "Generalized Initializer Lists", 2003

(C) Copyright Thorsten Ottosen 2003-2006