Function Object
Many of the STL algorithms have a parameter which is
a function, for_each( ) and transform( )
being prime examples. That functional parameter doesn't have to be
an actual function. It can be an object that behaves like a function.
Such an object is called a "function object" or "functor".
A function is something that can be called using the parenthese and
passing arguments -- myFunction( arg1, arg2). We can make
objects behave this way by overloading operator( ).
But why would we want to do this?
Function objects are like "smart functions".
- They can have abilities beyond just operator( ).
- They can save information between function calls (have a "state").
- You can have separate copies of each function by
instantiating more than one function object. Each function object could
have a separate state.
- A function object can initialize itself (in its constructor).
Below is an example used to add an integer value
<![CDATA[
class Add
{
public:
// const
Add (int value)
: m_value( value )
{ }
void operator( ) (int& n ) const
{
n += m_value;
}
private:
int m_value; // value to add
};
int main ( )
{
set<int> iSet;
// insert some data
for (int i = 1; i < 10; i++)
iSet.insert( i );
// print elements
// 1 2 3 4 5 6 7 8 9
// create a function which adds 42 to its parameter
Add add42( 42 );
// add 42 to each element of the set
for_each( iSet.begin(), iSet.end(), add42 );
// print the elements
// 43 44 45 46 47 48 49 50 51
};
]]>
In this example, a random number generator class remembers
and uses the last number generated to create the next one.
<![CDATA[
class RNG // Random Number Generator
{
public:
RNG (unsigned int seed = 1)
: m_lastValue( seed )
{ };
unsigned int operator( ) ()
{
// modify last Value for new value
m_lastValue = some computation using m_lastValue
return m_lastValue;
}
private:
unsigned int m_lastValue;
};
int main ( )
{
RNG rng ( 42 );
for (int i = 0; i < 10; i++)
cout << rng() << endl;
}
]]>
Last Modified: Monday, 20-Nov-2006 15:15:10 EST