On Mon, Jun 22, 2009 at 6:31 PM, Steven Watanabe<watanabesj@gmail.com> wrote:

AMDG

Zachary Turner wrote:

...

It seems very simple, but it's not compiling.

int translate(const std::string& str) {   return 0; }

//all required using declarations are present but omitted for brevity // options_description options; variables_map values; int i;

options.add_options()   ("test", value<std::string>()->notifier(var(i) = translate(_1)));

I get the following error:

'translate': cannot convert parameter 1 from 'boost::lambda::placeholder1_type' to 'std::string'.  No user defined conversion operator available that can perform this conversion, or the operator cannot be called.

Is this type of usage unsupported for some reason?

It's unsupported because C++ doesn't allow Boost.Lambda to magically create overloads of every function that you might want to use with it.  Not to mention that if this usage did work, then trying to pass a lambda functor to a function would be ambiguous with trying to compose a larger lambda expression. Use bind.  http://tinyurl.com/nkhuez.

Thanks. On a related note, if a lambda placeholder refers to a structure, is there any way to access its members short of using bind and making a fake function to perform the access? For example struct foo { int x; int y; }; struct bar { int x, int y; }; foo f; options.add_options() ("test", value<bar>()->notifier( var(f.x) = _1.x))); is what I want. I've tried a variety of different things, most promising seemed to be options.add_options() ("test", value<bar>()->notifier( var(f.x) = bind(&bar::x, _1))); but this doesn't work either. The only way I was able to get it to work was to make a function void copy_value(foo& dest, const bar& src) { dest.x = src.x; } and then doing options.add_options() ("test", value<bar>()->notifier( bind(copy_value, f, _1)); Is this my only hope?

On Tue, Jun 23, 2009 at 2:52 PM, Steven Watanabe<watanabesj@gmail.com> wrote:

AMDG

Zachary Turner wrote:

...

Thanks.  On a related note, if a lambda placeholder refers to a structure, is there any way to access its members short of using bind and making a fake function to perform the access?

For example

struct foo { int x; int y; }; struct bar { int x, int y; };

foo f;

options.add_options()   ("test", value<bar>()->notifier(      var(f.x) = _1.x)));

is what I want.  I've tried a variety of different things, most promising seemed to be

options.add_options()   ("test", value<bar>()->notifier(      var(f.x) = bind(&bar::x, _1)));

but this doesn't work either.

It ought to work.  What's the error?

The following works for me:

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

struct foo { int x; int y; }; struct bar { int x; int y; };

int main() {   foo f = { 1, 2 };   const bar b = { 3, 4 };   (boost::lambda::var(f.x) = boost::lambda::bind(&bar::x, boost::lambda::_1))(b);   std::cout << f.x << std::endl; }

Ok so the problem is that I was using boost::bind instead of boost::lambda::bind. Should only boost::lambda::bind be used inside lambda expressions? And why isn't there just a single bind that works in all cases?