Yoh0xFF - 9 months ago 46

C++ Question

`template <class T> struct greater : binary_function <T, T, bool> {`

bool operator() (const T& x, const T& y) const {

return x > y;

}

};

I found this definition of "Function object class for greater-than inequality comparison" in STL library.

Can somebody please explain to me how this code works and compiles?

Answer Source

```
template <class T> // A template class taking any type T
// This class inherit from std::binary_function
struct greater : binary_function <T, T, bool>
{
// This is a struct (not a class).
// It means members and inheritens is public by default
// This method defines operator() for this class
// you can do: greater<int> op; op(x,y);
bool operator() (const T& x, const T& y) const {
// method is const, this means you can use it
// with a const greater<T> object
return x > y; // use T::operator> const
// if it does not exist, produces a compilation error
}
};
```

here is the definition of `std::binary_function`

```
template <class Arg1, class Arg2, class Result>
struct binary_function {
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
};
```

this allows you to access the types defining the binary_function

```
greater<int> op;
greater<int>::result_type res = op(1,2);
```

which is equivalent to

```
std::result_of<greater<int>>::type res = op(1,2);
```