No raw loops 3 – member functions

Loop over all the elements in a container and call a member function (with arguments) on each element

I want to introduce one more std::bind trick. In part 1 and part 2 the functions called were non-member functions and we were passing each element of the container into the function as an argument. In this installment we are going to call a member function on each element in the container. We’ll extend our set of declarations to include a member function on class T:

class T
{
public:
   void mf_int( int ) const;
   ...
};

std::vector< T > v_T;

We have a vector full of objects of type T, we want to call the member function mf_int on each object in that vector, and we have to pass an integer argument to mf_int (I am skipping the example where we don’t have to pass any additional arguments to the function).

Let’s start off with the “old style” solution:

Solution #0

int j( 7 );
for( std::vector< T >::iterator i( std::begin( v_T ) ); 
    i != std::end( v_T ); 
    ++i )
{
    i->mf_int( j );
}

The range-based for version of the solution is as expected:

Solution #1

for( T const& t : v_T )
{
    t.mf_int( j );
}

The lambda function version also holds no surprises:

Solution #2

std::for_each( 
    std::begin( v_T ), 
    std::end( v_T ), 
    [ j ]( T const& t )
{
    t.mf_int( j );
} );

The std::bind version looks a little different though:

Solution #3

using namespace std::placeholders;
std::for_each( 
    std::begin( v_T ), 
    std::end( v_T ), 
    std::bind( &T::mf_int, _1, j ) );

The std::bind statement includes something new. &T::mf_int refers to a member function of class T – the function T::mf_int. When std::bind sees a member function (as opposed to a regular function) as its first argument, it expects the second argument to be the object on which to call the member function. In this case since the second argument is _1, it will call the member function T::mf_int on every element of the container. There is an additional argument to std::bind (j) which will be passed as a parameter to T::mf_int.

Having said above that I was skipping the example where we don’t have to pass any additional arguments to the function I want to mention that example briefly in connection with std::mem_fn, another function adapter (std::bind is a function adapter).

std::mem_fn provides a shorter way to specify that you want to call a member function with no arguments. Let’s add a new declaration to class T:

class T
{
public:
   void mf_int( int ) const;
   void mf_void() const;
   ...
};

When we use std::mem_fn we don’t need to specify the placeholder _1:

std::for_each( 
    std::begin( v_T ), 
    std::end( v_T ), 
    std::mem_fn( &T::mf_void ) );

std::mem_fn only works for member functions taking no arguments.

We can also use adobe::for_each:

Solution #4

adobe::for_each( v_T, std::bind( &T::mf_int, _1, j ) );

Wrap up

When we’re using std::bind we always specify the function we are going to call as the first argument. If the function we are going to call is a member function we specify the object on which we are going to call the member function second. Any additional parameters follow.

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