boost::bind() binds extra arguments?

Question

Does boost::bind() bind binds extra arguments as it seems passing a bind function with no arguments into one expecting an argument double works fine? If I were to write out the bind function explicitly, what should that be?

struct MyClass
{
    void f() 
    {
        std::cout << "f()" << std::endl;
    }
};

void bar( const boost::function<void(const double&)>& f )
{
    f( 1.0 );
}

int main()
{
    MyClass c;

    // why this compiles
    bar( boost::bind( &MyClass::f, &c ) );

    // what should I write if I want to create the binded function explicitly before pass into bar?
    // boost::function<void(const double&)> f = boost::bind( ... boost::bind( &MyClass::f, &c ), ?? )
    bar( f );

}

Answer 1

This is by design, unbound parameters (e.g. 1.0) passed when invoking the bind-expression are just ignored.

boost::function<void(const double&)> f = boost::bind(&MyClass::f, &c);
bar(f);

would do nicely for explicit assignment of the bind expression.

Update to the comment:

Remember, two guidelines:

• function<...> has a fixed signature

• bind expressions do not have a fixed signature. The whole purpose of bind is to change the signature. This includes e.g.

  • adding state to fill in for formal parameters dropped from the signature or
  • adding parameters, to be ignoring by not binding them to the target callable
  • changing the argument/return type by using implicit conversions
  • even changing the order in which parameters are bound to the target callable, while the signature could technically be unchanged.

So while you cannot assign different func<...> types to eachother, you can always bind the one signature to the other.

Here's a more complete demonstration that shows the limits of what you can do with function and bind, and why (how it behaves): Live On Coliru:

#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <iostream>
#include <cassert>

int foo0()            { return 0; }
int foo1(int)         { return 1; }
int foo2(int,int)     { return 2; }
int foo3(int,int,int) { return 3; }

int main()
{
    boost::function<int()>            func0;
    boost::function<int(int)>         func1;
    boost::function<int(int,int)>     func2;
    boost::function<int(int,int,int)> func3;

    // "straight" assignment ok:
    // -------------------------
    func0 = foo0;                          assert (0 == func0());
    func1 = foo1;                          assert (1 == func1(-1));
    func2 = foo2;                          assert (2 == func2(-1,-1));
    func3 = foo3;                          assert (3 == func3(-1,-1,-1));

    // "mixed" assignment not ok:
    // --------------------------
    // func0 = foo1;                       // compile error
    // func3 = foo2;                       // compile error
    // func1 = func2;                      // compile error, just the same
    // func2 = func1;                      // compile error, just the same

    // SOLUTION: you can always rebind:
    // --------------------------------
    func0 = boost::bind(foo3, 1, 2, 3);    assert (func0() == 3);
    func3 = boost::bind(foo1, _3);         assert (func3(-1,-1,-1) == 1);
    func3 = boost::bind(foo2, _3, _2);     assert (func3(-1,-1,-1) == 2);
    // same arity, reversed arguments:
    func3 = boost::bind(foo3, _3, _2, _1); assert (func3(-1,-1,-1) == 3);

    // can't bind more than number of formal parameters in signature:
    // --------------------------------------------------------------
    // func3 = boost::bind(foo1, _4);      // in fact, the bind is fine, but assigning to `func3` fails
}

All asserts pass. You can try what the compiler says when you uncomment the lines that don't compile.

Cheers