Binding a Function Object
The same
std::tr1::function object may change its target, even if the original target and the new target have radically different implementations. In the following example, the object
func2 is first bound to the freestanding function
adjacent(). Later, the same
func2 is bound to a function object:
using namespace std;
bool adjacent(int x, int y){return x == y-1 || x == y+1;}
class Data
{
private:
vector <pair<int, int> > records;
public:
bool operator()(int x, int y)
{
records.push_back(make_pair(x, y));
return x < y;
}
};
function <bool(int, int)> func2;
func2 = &adjacent;
func2(2, 3); // calls adjacent(2, 3), returns true
Data rec;
func2 = ref(rec); //see note
func2 (5, 2)); // adds pair<5, 2> to rec.records
| Author's Note: The std::tr1::function specification makes use of another TR1 feature called reference wrapper classes. I will discuss reference wrappers in a future 10-Minute Solution, but for the time being, assume that ref() is a standard helper function that takes an argument of type X and returns an object whose interface simulates a X& object. |
You may also assign two std::tr1::function objects whose return types and arguments are different if there is an implicit conversion between those types:
func2=func;//OK, long return value is convertible to bool
Implicit Boolean Conversion
std::tr1::function defines a
Boolean conversion operator that enables you to use objects of this type in Boolean expressions:
std::tr1::function f<int (int,int>) f;
assert (!f); //OK, f is empty
f=&add;
assert (f); //OK, f isn't empty
void func(function <int (int,int>) &f1);
{
if(f1)
{
f1(2,3);
}
else
{
f1=&subtract;
f1(3,2);
}
}
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