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 and
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(
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
#include
#include
#include
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 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 v;
generate_n(inserter(v, v.begin()), 10,
boost::bind(Builder(),*this) );
print(v);
}
void f2()
{
/* A more compact, but substantially identical form */
set 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 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 v;
generate_n(inserter(v, v.begin()), 10,
boost::lambda::bind(
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 v;
generate_n(inserter(v, v.begin()), 10,
boost::bind(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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