Using for_each and boost::bind with a vector of pointers

Question

I have a vector of pointers. I would like to call a function for every element, but that function takes a reference. Is there a simple way to dereference the elements?

Example:

MyClass::ReferenceFn( Element & e ) { ... }

MyClass::PointerFn( Element * e ) { ... }

MyClass::Function()
{
    std::vector< Element * > elements;
    // add some elements...

    // This works, as the argument is a pointer type
    std::for_each( elements.begin(), elements.end(),
                   boost::bind( &MyClass::PointerFn, boost::ref(*this), _1 ) );

    // This fails (compiler error), as the argument is a reference type
    std::for_each( elements.begin(), elements.end(),
                   boost::bind( &MyClass::ReferenceFn, boost::ref(*this), _1 ) );
}

I could create a dirty little wrapper that takes a pointer, but I figured there had to be a better way?

Is there a reason you're using boost::ref(*this)? I just use: boost::bind(&MyClass::ReferenceFn, this, _1) and it works fine. — Mark Ingram
– Mark Ingram, Commented Sep 21, 2010 at 11:57

Johannes Schaub - litb · Accepted Answer · 2010-03-10 01:05:47Z

15

You could use boost::indirect_iterator:

std::for_each( boost::make_indirect_iterator(elements.begin()), 
               boost::make_indirect_iterator(elements.end()),
               boost::bind( &MyClass::ReferenceFn, boost::ref(*this), _1 ) );

That will dereference the adapted iterator twice in its operator*.

answered Mar 10, 2010 at 1:05

Johannes Schaub - litb

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4 Comments

Steve Jessop Over a year ago

+1, although for this case I prefer BOOST_FOREACH(Element *e, elements) this->ReferenceFn(*e);. C++ may be usable as a functional language, but not as a concise functional language...

Steve Jessop Over a year ago

And the Python would be for e in elements: self.ReferenceFn(e). It's heart-breaking.

Johannes Schaub - litb Over a year ago

For C++0x it will be for(auto *e : elements) ReferenceFn(*e);. Sweet :)

Steve Jessop Over a year ago

I'm considering starting to call it C++JamTomorrow.

Drew Dormann · Accepted Answer · 2013-01-09 21:43:29Z

3

It looks like you could also use the Boost.Lambda library.

// Appears to compile with boost::lambda::bind
    using namespace boost::lambda;
    std::for_each( elements.begin(), elements.end(),
                   bind( &MyClass::ReferenceFn, boost::ref(*this), *_1 ) );

But I agree with the commenters about preferring BOOST_FOREACH. The for_each "algorithm" does practically nothing useful, and what it does, range-based for loop can do for you with a much smaller effort.

edited Jan 9, 2013 at 21:43

Drew Dormann

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answered Mar 10, 2010 at 8:18

UncleBens

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