Re: [Boost-users] Questions regarding lambda expressions

From: Matyas W Egyhazy (megyha80_at_[hidden])
Date: 2006-09-04 14:34:05

Jens,

I have attached a full example now.  I have included my questions and 
error snips from the compiler output within the source.

> I'm wondering why you call _1 on the functor in all, especially the last 
> example. 
Probably because I do not know what I am doing.  I still get an error whether I do that or not.  See attached example.

> What's the derefenrencing for? Are you sure the first one works 
> as you intend? Or does it merely compile?
scoped_ptr<T>::get returns a T* so I dereference it before operator<< is called.  The behavior works as intended, the operator<< for T is called.

Thanks,

Matt


#include <algorithm>
#include <iostream>
#include <vector>
#include <string>

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/scoped_ptr.hpp>
#include <boost/scoped_array.hpp>

using namespace std;
using namespace boost;
using namespace boost::lambda;
int main(int argc, char** argv)
{
    vector<char> t(10);
    scoped_array<scoped_ptr<int> > v(new scoped_ptr<int>[10]);

    //I know this is not typical to 
    //initialize this way
    for (int k(0); k < 10; ++k)
    {
        v[k].swap(scoped_ptr<int>(new int(k)));
    }

    //Works
    fill(t.begin(), t.end(), 'b');
    for_each(t.begin(), t.end(), cout << _1 << '\n');

    /* Doesnt work?
    for_each(t.begin(), t.end(), cout << _1 << endl);
    Error	1	error C2784: 'std::basic_ostream<_Elem,_Traits> 
    &std::operator <<(std::basic_ostream<_Elem,_Traits> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : 
    could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 
    'const boost::lambda::lambda_functor<T>'	d:\source\c\testlambda\testlambda\main.cpp	29	
    */

    cout << endl;

    int q(0);
    
    //Works
    /*
        0
        1
        2
        3
        4
        5
        6
        7
        8
        9
    */
    for (int i(0); i < 10; ++i)
    {
        cout << *(v[i].get()) << endl;
    }

    cout << endl;

    //Doesnt work 
    //for_each(&v[0], &v[0] + 10, cout << *bind(&scoped_ptr<string>::get, _1) << '\n');
    /*
    Error	1	error C2665: 'boost::lambda::function_adaptor<Func>::apply' : 
    none of the 2 overloads could convert all the argument types	
    D:\boost_1_33_1\boost\lambda\detail\actions.hpp	87	
    */

    //Doesnt work 
    //for_each(&v[0], &v[0] + 10, cout << *bind(&scoped_ptr<string>::get, _1)(_1) << '\n');
    /*
    Error	1	error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : 
    cannot access private member declared in 
    class 'std::basic_ios<_Elem,_Traits>'	
    D:\Microsoft Visual Studio 8\VC\include\ostream	581	
    */

    //Works, how does it know to bind correctly?
    /*
        0
        1
        2
        3
        4
        5
        6
        7
        8
        9

    */
    for_each(&v[0], &v[0] + 10, cout << *(&scoped_ptr<string>::get, _1) << '\n');

    return 0;
}