Instantiation
The recently standardized Library Extension Technical Report 1 (TR1) includes a new class template called
std::tr1::function that generalizes the notion of a callable entity while mimicking pointers to functions in both syntax and semantics.
std::tr1::function can wrap any function object, pointer to function or pointer to a member function that can be called with the same arguments and return types.
A declaration of an std::tr1::function object includes the return type and the arguments of the target function:
#include <functional>
using std::tr1::function;
function <long (int, int)> func;
You can assign to
func any callable entity that returns
long and takes two arguments of type
int:
long add(int x, int y) { return x+y; }
func = &add //assign a target function
cout << func(1, 2) << endl; // calls add(1,2)
Unlike with ordinary pointers to functions, the return type and the arguments of the target function may differ from those of
func so long as there is an implicit conversion between those types. The following code listing demonstrates this property. It binds a function whose arguments are of type
long,
long to a
std::tr1::function object whose arguments are of type
int, int:
long subtract(long x, long y) { return x-y; }
func = &subtract //OK, implicit conversion of arguments
cout << func(2, 1) << endl; // output: 1
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