Close, but not quite what I am after. I would like to be able to define a function, say, cos, and have it accept a functor, be it a var_type, and to return another one. So I could write expressions like result = ( cos( _1 + 2 ) + 2 )( value ), for example. It would be nice if I could not have to write bind() where-ever I want do this, otherwise the expressions will rapidly become unreadable. TIA - Peter On 12/10/2007, Stuart Dootson <stuart.dootson@gmail.com> wrote:

On 12/10/2007, Peter Waller <peter.waller@gmail.com> wrote:

...

I wish to write some lambda functions to have this sort of behaviour.

#include <lambda/lambda.hpp> #include <lambda/bind.hpp> #include <function.hpp>

using namespace boost::lambda; using namespace boost;

#include <iostream> using std::cout; using std::endl;

#include <cmath>

int main( int argc, char *argv[] ) { float value; var_type<float>::type theValue( var( value ) );

// Ignore this line #define cos(theta) bind( unlambda(bind( cos, _1 )), theta )

function<float ( float )> myfunction = ( cos( theValue ) - 1 ); value = 2; cout << "The result = " << myfunction(1) << endl; return 0; }

Namely, the cos( value ) part.

The code above works, but I feel that using a #define is a little messy, and seems hard to generalise to functions with more arguments. I was wondering if there was a general, better way of turning a given function into one whose execution is delayed.

Thanks in advance,

- Peter

This seems to work:

double value = 3.14159 / 2.0; boost::function<float(void)> fn = bind(static_cast<double(*)(double)>(&cos), var(value)); std::cout << fn() << std::endl; value = 0.0; std::cout << fn() << std::endl;

At least, the output of 'fn()' changes when 'value' changes, which (as far as I could tell) was what you wanted?

The static_cast is needed to select the appropriate 'cos' overload.

HTH

Stuart Dootson _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users