[Boost-users] if_then_else_return (boost::lambda) question

Subject: [Boost-users] if_then_else_return (boost::lambda) question
From: Jonathan Leonard (v-jleonard_at_[hidden])
Date: 2009-02-18 14:30:32

Greetings!

I'm fairly new to boost and have a little problem with if_then_else_return. 
Can anyone tell me the problem with the following short program?  [It seems 
that my 'thenBranch' and 'elseBranch' functions are never called and 
if_then_else_return' always returns true (as if the 'then' branch were 
called)].

Many thanks in advance for any help you may offer!

P.S. (A more general C++ q:  A little improvement I'd like to make is to 
drop the ITERATORTYPE type argument from IfNotEndThenElseReturn and use 
CONTAINERTYPE::iterator instead, but the compiler won't allow that.  Anyone 
know why?).  Thanks again!

============================================

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/if.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/function.hpp>
#include <functional>
#include <list>

using namespace boost::lambda;

template<typename RET>
RET InvokeReturn(boost::function<void ()> f, const RET& retVal)
{
    printf("in InvokeReturn!\n");
    f();
    return retVal;
}

bool InvokeReturnTrue(boost::function<void ()> f)
{
    return InvokeReturn(f, true);
}

bool InvokeReturnFalse(boost::function<void ()> f)
{
    return InvokeReturn(f, false);
}

boost::function<bool ()>
MakeReturnTrue(boost::function<void ()> f)
{
    printf("in MakeReturnTrue!\n");
    return boost::lambda::bind(&InvokeReturnTrue, f);
}

boost::function<bool ()>
MakeReturnFalse(boost::function<void ()> f)
{
    printf("in MakeReturnFalse!\n");
    return boost::lambda::bind(&InvokeReturnFalse, f);
}

template<typename CONTAINERTYPE, typename ITERATORTYPE>
bool
IfNotEndThenElseReturn(const ITERATORTYPE& it,
    CONTAINERTYPE& container, boost::function<void ()> thenBranch,
    boost::function<void ()> elseBranch)
{
    printf("in IfNotEndThenElseReturn!\n");
    std::not_equal_to<ITERATORTYPE> ne;

    return boost::lambda::if_then_else_return(
        boost::lambda::bind(&std::not_equal_to<ITERATORTYPE>::operator(),
            ne, it, container.end()),
        MakeReturnTrue(thenBranch),
        MakeReturnFalse(elseBranch))();
}

void thenBranch()
{
    printf("in then branch!\n");
}

void elseBranch()
{
    printf("in else branch!\n");
}

int _tmain(int argc, _TCHAR* argv[])
{
    printf("beginning execution!\n");

    std::list<int> l;
    l.push_back(8);
    l.push_back(7);
    l.push_back(6);
    l.push_back(5);

    std::list<int>::iterator it = l.end();
    std::list<int>::iterator it2 = l.begin();

    bool b = IfNotEndThenElseReturn(it, l, bind(&thenBranch), 
bind(&elseBranch));
    printf("b is %s\n", b ? "true" : "false");

    bool b = IfNotEndThenElseReturn(it2, l, bind(&thenBranch), 
bind(&elseBranch));
    printf("b is %s\n", b ? "true" : "false");

    printf("finished!\n");
 return 0;
}