understanding function object

[Wenjie]

Hello,


In the SGI STL documentation:
struct less_mag : public binary_function<double, double, bool> {
bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};

vector<double> V;
...
sort(V.begin(), V.end(), less_mag());

I am a little confused concerning 'less_mag()': does it construct
some less_mag object? Then what should be the constructor and could
it be written as 'less_mag'? If it is calling operator(), I don't
think so


Thanks for your enlightment,
Wenjie

 

[Alf P. Steinbach]

In the SGI STL documentation:
struct less_mag : public binary_function<double, double, bool> {
bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};

vector<double> V;
...
sort(V.begin(), V.end(), less_mag());

I am a little confused concerning 'less_mag()': does it construct
some less_mag object?
Yes.


Then what should be the constructor

It's the default constructor, which is generated by the compiler.

and could it be written as 'less_mag'?
No.


If it is calling operator(), I don't think so

The sort code calls the less_mag object's operator() which takes
two arguments, the values to be compared.

 

[Rolf Magnus]

Hello,


In the SGI STL documentation:
struct less_mag : public binary_function<double, double, bool> {
bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};

vector<double> V;
...
sort(V.begin(), V.end(), less_mag());

I am a little confused concerning 'less_mag()': does it construct
some less_mag object?
Yes.

Then what should be the constructor

Since the object is empty, the constructor shouldn't do anything.

and could it be written as 'less_mag'?

No. less_mag is a type, and you cannot pass types as function arguments.

If it is calling operator(), I don't think so

Well, you create an object of class less_mag, and sort() internally
calls the operator() on that object.

 

[Victor Bazarov]

In the SGI STL documentation:
struct less_mag : public binary_function<double, double, bool> {
bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};

vector<double> V;
...
sort(V.begin(), V.end(), less_mag());

I am a little confused concerning 'less_mag()': does it construct
some less_mag object?

Yes, a temporary object.

Then what should be the constructor and could
it be written as 'less_mag'?

I am not sure I understand the second part of the question. The
constructor's "name" is always the same as the class'. If you need
to define a parametrised constructor, then you write

struct less_mag : ... {

less_mag(int parameter) : blahblah(blah) {}

bool operator()( ...
};

If it is calling operator(), I don't
think so

No, it's not.

Victor