Re: [Boost-users] [lambda] bind w/shared_ptr<Object> [Was: "Invalid use of void expression"]

On 1/24/06, David A. Greene wrote:

David A. Greene wrote:

...

I'm having some trouble with BLL and Boost.Function in this simple testcase:

I worked around the problem using Boost.Function:

int main(void) { Test tester; int value = 10;

boost::function bound = boost::lambda::bind(&Test::foo, &tester, boost::lambda::_1); Caller > printer(boost::lambda::bind(bound, value)); //Caller > printer(boost::lambda::bind(boost::lambda::bind(&Test::foo, &tester, boost::lambda::_1), value));

printer.bar();

return(0); }

I have a couple of questions and another problem, however.

I had to pass "&tester" to the first bind expression because g++ complained about passing "tester" as "const Test &." Is passing by reference supposed to work?

Only if you use a reference wrapper - i.e. you can use boost::ref(tester) or boost::lambda::var(tester) rather than &tester

Also, this workaround gets rid of the error but is the original code supposed to work as written or did I do something incorrectly?

Finally, the new problem, closer to the actual code I'm working with:

#include #include #include

#include <iostream>

class Test { public: void foo(int a) { std::cout << "a is " << a << std::endl; }; };

template<typename Function> class Caller { private: Function func;

public: Caller(Function f) : func(f) {};

void bar(void) { func(); }; };

int main(void) { boost::shared_ptr<Test> tester(new Test); int value = 10;

boost::function bound = boost::lambda::bind(&Test::foo, tester, boost::lambda::_1); Caller > printer(boost::lambda::bind(bound, value));

printer.bar();

return(0); }

If you're using a shared_ptr, you'll either need to use tester.get() directly to get a Test* to pass into the bind expression bind(&Test::foo, tester.get(), value) or something like bind(&Test::foo, bind(boost::shared_ptr<Test>::get, tester), value) to delay the call to tester.get() until invocation of printer.bar(). Stuart Dootson