Re: Creating a function object based on a constructor
On Dec 7, 3:28 am, ber <bpa...@gmail.com> wrote:
I somehow arrived at this question when writing some code to process
DNA sequences:
I have a class that is constructed by providing two arguments and I
would like to construct a vector of objects of this class using a
varying first argument (found by iteration over an appropriate
container) and a fixed second argument.
I tried several alternatives but, so far, the closer I got to what I
would like to do is captured by the following code.
#include <algorithm>
#include <functional>
#include <vector>
#include <string>
using namespace std;
class BP
{
public:
char c;
int q;
BP() : c(' '), q(0) { }
BP(const char i_c, const int i_q) : c(i_c), q(i_q) { }
BP(const BP & x) : c(x.c), q(x.q) { }
};
BP createBP(const char c, const int q)
{
return BP(c, q);
}
int main(void)
{
string s("actg");
vector<BP> r10(4, BP()), r20(4, BP());
transform(s.begin(),s.end(),r10.begin(),bind2nd(ptr_fun(createBP),10));
transform(s.begin(),s.end(),r20.begin(),bind2nd(ptr_fun(createBP),20));
}
My main problem is that I would like to use something *like* ptr_fun
to create in line a function object that represents the constructor
taking two arguments and then bind the second one to whatever I'd
like.
Because constructors do not have names, it is not possible to pass a
constructor to a function like std::mem_fun and have it return a
function object..Nor can I think of any routines in the Standard
Library that would create a suitable function object in this case.
Nonetheless, I did think of two alternative approaches that might be
worth considering.
One approach would be to make BP() a function object - capable of
creating BP objects on its own (in much the same way that DNA makes
copies of itself):
class BP
{
char c;
int q;
public:
BP() : c(' '), q(0) { }
BP(const char& i_c, const int& i_q) : c(i_c), q(i_q) { }
BP(const BP & x) : c(x.c), q(x.q) { }
typedef char first_argument_type;
typedef int second_argument_type;
typedef BP result_type;
BP operator()( char c, int i_a) const
{
return BP(c, i_a);
}
};
and then call std::transform() like so:
transform(s.begin(), s.end(), r10.begin(), bind2nd(BP(), 10));
A less intrusive solution would be to replace createBP() with a more
generic routine:
template <class T, class P1, class P2>
inline T
Create( P1 p1, P2 p2)
{
return T(p1, p2);
}
and then call transform() like so:
transform(s.begin(), s.end(),
r10.begin(), bind2nd(ptr_fun(Create<BP, char, int>), 10));
Even if Create() is only used to create BP objects - the fact that
Create() could be used for other types makes the function more
"elegant" (at least in my view) than the single-purpose createBP()
routine.
Greg
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