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

From: Stuart Dootson (stuart.dootson_at_[hidden])
Date: 2006-01-24 12:19:55

On 1/24/06, David A. Greene <greened_at_[hidden]> 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<void (int)> bound = boost::lambda::bind(&Test::foo,
> &tester, boost::lambda::_1);
>    Caller<boost::function<void (void)> >
> printer(boost::lambda::bind(bound, value));
>    //Caller<boost::function<void (void)> >
> 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 <boost/lambda/bind.hpp>
> #include <boost/function.hpp>
> #include <boost/shared_ptr.hpp>
>
> #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<void (int)> bound = boost::lambda::bind(&Test::foo,
> tester, boost::lambda::_1);
>    Caller<boost::function<void (void)> >
> 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