STL, std::transform and member functions

[Alex]

I don't know why this works:

class C {
public:
C(int x, int y) : a(x), b(y) {}
int a,b;
};

float avg(const C* c)
{
return ( (c->a) + (c->b) )/2.0;
}

class V {
void foo()
{
vector<C*> v;
v.push_back(new C(3,4));
v.push_back(new C(1,2));
v.push_back(new C(3,3));
transform(v.begin(),v.end(),ostream_iterator<float>(cout," "), avg);
}
};

And this doesn't:

class C {
public:
C(int x, int y) : a(x), b(y) {}
int a,b;
};

class V {
float avg(const C* c)
{
return ( (c->a) + (c->b) )/2.0;
}
void foo()
{
vector<C*> v;
v.push_back(new C(3,4));
v.push_back(new C(1,2));
v.push_back(new C(3,3));
transform(v.begin(),v.end(),ostream_iterator<float>(cout," "), avg);
// Compiler error here
}
};

How can I fix the 2nd?

 

[Dietmar Kuehl]

I don't know why this works:

[example of using 'std::transform()' with a namespace level function]

And this doesn't:

class C {
public:
C(int x, int y) : a(x), b(y) {}
int a,b;
};

class V {
float avg(const C* c)
{
return ( (c->a) + (c->b) )/2.0;
}
void foo()
{
vector<C*> v;
v.push_back(new C(3,4));
v.push_back(new C(1,2));
v.push_back(new C(3,3));
transform(v.begin(),v.end(),ostream_iterator<float>(cout," "), avg);
// Compiler error here
}
};

Well, first of all, your "avg" member function cannot be found
because it is nested into a class. You need to refer to this
function as 'V::avg'. However, this still does not work as this
is a method which requires an additional parameter: the object of
type 'V' you are operating on. Although the 'avg()' method does
not actually access any members (actually, there are none), the
object is still required. Thus, you need to create a functor and
bind an object:

V vobj;
std::transform(..., std::bind1st(std::mem_fun(&V::avg), &vobj));

Instead, you might want to create a functor yourself, e.g.
something like this:

struct avg {
float operator()(C* obj) const { return ... }
};

which is then passed as

std::transform(..., avg());

to the 'std::transform()' function.

 

[Daniel T.]

I don't know why this works:

class C {
public:
C(int x, int y) : a(x), b(y) {}
int a,b;
};

float avg(const C* c)
{
return ( (c->a) + (c->b) )/2.0;
}

class V {
void foo()
{
vector<C*> v;
v.push_back(new C(3,4));
v.push_back(new C(1,2));
v.push_back(new C(3,3));
transform(v.begin(),v.end(),ostream_iterator<float>(cout," "), avg);
}
};

And this doesn't:

class C {
public:
C(int x, int y) : a(x), b(y) {}
int a,b;
};

class V {
float avg(const C* c)
{
return ( (c->a) + (c->b) )/2.0;
}
void foo()
{
vector<C*> v;
v.push_back(new C(3,4));
v.push_back(new C(1,2));
v.push_back(new C(3,3));
transform(v.begin(),v.end(),ostream_iterator<float>(cout," "), avg);
// Compiler error here
}
};

How can I fix the 2nd?

try making the 'avg' member-function in the second case static...