[Boost-users] Inconsistency between boost::bind and boost::lambda::bind

From: Gerardo Lamastra (gerardo.lamastra_at_[hidden])
Date: 2006-05-08 09:35:29

Hello everybody,

I've started to use the functional programming facilities offered
by boost, and they're really impressive.

To begin with, I've started using both; I've seen however, that
when I #include both <boost/bind.hpp> and <boost/lambda/bind.hpp>
I must use full namespace names to address lambda placeholders.
In other words, instead of writing
for_each(container.begin(),
         container.end(),
         cout << _1 << constant('\n'));
I've to write
for_each(container.begin(),
         container.end(),
         cout << boost::lambda::_1 << boost::lambda::constant('\n'));
also if I've written a using boost::lambda declaration; I know,

I can simplify this using a namespace alias, but I was trying to
keep the code less clobber as I can, because it has to be used
for teaching/example purposes.
So, my first question would be: where could I find an exhaustive
comparison between these two facilities? And, is there any indication
on what to prefer, also related to future development and/or inclusion
in C++ standards (If I remember well, boost::lambda has been proposed
in tr1?)

The second question points at a small inconsistency that I believe
I've found while doing some experiments: boost::bind and
boost::lambda::bind offer similar construct, but the former works
while the latter fails in the example included below.

I'm using boost_1_33_1, with g++ -v (provided below)
Target: i586-mandriva-linux-gnu
Configured with: ../configure --prefix=/usr --libexecdir=/usr/lib
--with-slibdir=/lib --mandir=/usr/share/man --infodir=/usr/share/info
--enable-shared --enable-threads=posix --disable-checking
--enable-languages=c,c++,ada,f95,objc,java
--host=i586-mandriva-linux-gnu --with-system-zlib --enable-long-long
--enable-__cxa_atexit --enable-clocale=gnu
--disable-libunwind-exceptions --enable-java-awt=gtk
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
--enable-gtk-cairo --disable-libjava-multilib
Thread model: posix
gcc version 4.0.1 (4.0.1-5mdk for Mandriva Linux release 2006.0)

The situation is the following: if I try to create a nullary
lamnda function with boost::lambda::bind()
like this:

        generate_n(inserter(v, v.begin()), 10,
                   boost::lambda::bind<Slave*>(
                      boost::lambda::new_ptr<Slave>(),*this) );

the compiler fails to compile: it tries to pass the this pointer
as a const reference somehow, and then it fails claiming that
a Slave(const Master& m) cannot be found; If I try to add one,
it fails with a similar error.

If I substitute the boost::lambda::bind() with a boost::bind(),
it compiles & works perfectly.

Thanks for the attention,
-Gerardo Lamastra

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#include <set>
#include <algorithm>
#include <iterator>
#include <iostream>

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/construct.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/bind.hpp>

using namespace std;
using namespace boost::lambda;

class Master;

struct Slave {
    static int globalId;
    Master& master;
    int id;
    Slave(Master& m) : master(m), id(++globalId) {};
};

int Slave::globalId = 0;

class Master {
    struct Builder {
        typedef Slave* result_type;
        Slave* operator()(Master& master) { return new Slave(master); };
    };

    static Slave* build(Master& master) { return new Slave(master); }

public:

    void print(set<Slave*> s)
    {
        for_each(s.begin(), s.end(), (cout << constant("S: ") 
                 << (boost::lambda::_1) ->* &Slave::id) << '\n' );
    }
    
    void f1()
    {
        /* This is the most trivial thing I've tried,
           but I need to write builder explicitely;
        */
        set<Slave*> v;
        generate_n(inserter(v, v.begin()), 10, 
                   boost::bind(Builder(),*this) );
        print(v);
    }

    void f2()
    {
        /* A more compact, but substantially identical form */
        set<Slave*> v;
        generate_n(inserter(v, v.begin()), 10, 
                   boost::bind(&Master::build,*this) );
        print(v);
    }

    void f3()
    {
        /*
          Does not work; it seems that the object
          passed to the compiler is a const Master&
          instead of just a Master&, it is like
          that the boost::lambda::bind() adds a
          const where that is not required...
          I tried to substitute the Master& in
          the Slave() constructor with a const Master&
          but it does not work either
        */

        /*
          set<Slave*> v;
          generate_n(inserter(v, v.begin()), 10, 
                     boost::lambda::bind(&Master::build,*this) );
          print(v);
        */
    }

    void f4()
    {
        /* The same issue applies here...
         * This has been my original experiment, which
         * motivated all this trouble...
         */
        /*
        set<Slave*> v;
        generate_n(inserter(v, v.begin()), 10,
                   boost::lambda::bind<Slave*>(
                      boost::lambda::new_ptr<Slave>(),*this) );
        print(v);
        */
    }
    
    void f5()
    {
        /* A work-around that I come up with,
         * mixing boost::bind && boost::lambda::new_ptr
         */

        set <Slave*> v;
        generate_n(inserter(v, v.begin()), 10,
           boost::bind<Slave*>(boost::lambda::new_ptr<Slave>(),*this) );
        print(v);
    }
};

int main()
{
    Master m;
    cout << "F1: " << endl; m.f1();
    cout << "F2: " << endl; m.f2();
    cout << "F5: " << endl; m.f5();
}

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