about STL sort

[Magcialking]

I wanna sort a pointer vector with the algorithm "sort" in STL like
this:

"
int compare(vector< string*>::itera tor a,vector<string *>::iterator b){
return (**a)<(**b);
}

sort(iter,iter_ end,compare);
"
but this dosen't work,and I really don't know why.
Could anyone show me how to use this correctly?

[red floyd]

Magcialking wrote:

I wanna sort a pointer vector with the algorithm "sort" in STL like
this:

"
int compare(vector< string*>::itera tor a,vector<string *>::iterator b){
return (**a)<(**b);
}

should be:

int compare(const string *a, const string *b)
{
return *a < *b;
}

sort(iter,iter_ end,compare);
"
but this dosen't work,and I really don't know why.
Could anyone show me how to use this correctly?

[Nate Barney]

Magcialking wrote:

I wanna sort a pointer vector with the algorithm "sort" in STL like
this:

"
int compare(vector< string*>::itera tor a,vector<string *>::iterator b){
return (**a)<(**b);
}

sort(iter,iter_ end,compare);
"

You don't specify the type of iter and iter_end, but I am going to
assume that they're of type vector<string*> ::iterator.

>
but this dosen't work,and I really don't know why.
Could anyone show me how to use this correctly?

The problem is that your comparison function takes iterators as
parameters, when it should take values. Additionally, it should return
a bool. For example:

bool compare(string *a,string *b) {
return (*a) < (*b);
}

Another thing you might consider is writing a generic dereferencing
binary predicate adapter:

#include <functional>

template <typename It,typename Pred>
class deref_pred : public std::binary_fun ction<It,It,boo l>
{
public:

deref_pred() {}
deref_pred(cons t Pred &pred) : pred(pred) {}

bool operator()(cons t It &a,const It &b) const
{ return pred(*a,*b); }

private:

Pred pred;
};

Then you would call sort like:

std::sort(iter, iter_end,
deref_pred<std: :string*,std::l ess<std::string ());

This is a bit more work on the front end, but it's much easier to reuse.

Hope this helps,
Nate

[Magcialking]

Another thing you might consider is writing a generic dereferencing

binary predicate adapter:

#include <functional>

template <typename It,typename Pred>
class deref_pred : public std::binary_fun ction<It,It,boo l>
{
public:

deref_pred() {}
deref_pred(cons t Pred &pred) : pred(pred) {}

bool operator()(cons t It &a,const It &b) const
{ return pred(*a,*b); }

private:

Pred pred;
};

Then you would call sort like:

std::sort(iter, iter_end,
deref_pred<std: :string*,std::l ess<std::string ());

it seems that deref_pred is just a wrapping paper,the real compare
function need to be pass to it when used?so, what's deref_pred doing
here?

and I am also confuse about the inheritance here:public
std::binary_fun ction<It,It,boo l>,
what's it for?

[Nate Barney]

Magcialking wrote:

>#include <functional>

template <typename It,typename Pred>
class deref_pred : public std::binary_fun ction<It,It,boo l>
{
public:

deref_pred() {}
deref_pred(cons t Pred &pred) : pred(pred) {}

bool operator()(cons t It &a,const It &b) const
{ return pred(*a,*b); }

private:

Pred pred;
};

Then you would call sort like:

std::sort(iter ,iter_end,
deref_pred<std: :string*,std::l ess<std::string ());

it seems that deref_pred is just a wrapping paper,the real compare
function need to be pass to it when used?so, what's deref_pred doing
here?

It dereferences the pointers/iterators passed as arguments, which allows
it to work on containers of pointers/iterators, like the
std::vector<std ::string*contai ner you have. The reason for the Pred
parameter is that you can change the predicate to, say, std::greater, if
you wish to reverse the sort.

and I am also confuse about the inheritance here:public
std::binary_fun ction<It,It,boo l>,
what's it for?

This base class does nothing other than define a few typedefs that allow
it to be used more easily with other parts of the STL.

See http://www.sgi.com/tech/stl/functors.html, specifically the third
paragraph of the Description section.

Nate

[Magcialking]

Nate Barney 写�：

Magcialking wrote:

#include <functional>

template <typename It,typename Pred>
class deref_pred : public std::binary_fun ction<It,It,boo l>
{
public:

deref_pred() {}
deref_pred(cons t Pred &pred) : pred(pred) {}

bool operator()(cons t It &a,const It &b) const
{ return pred(*a,*b); }

private:

Pred pred;
};

Then you would call sort like:

std::sort(iter, iter_end,
deref_pred<std: :string*,std::l ess<std::string ());

it seems that deref_pred is just a wrapping paper,the real compare
function need to be pass to it when used?so, what's deref_pred doing
here?

It dereferences the pointers/iterators passed as arguments, which allows
it to work on containers of pointers/iterators, like the
std::vector<std ::string*contai ner you have. The reason for the Pred
parameter is that you can change the predicate to, say, std::greater, if
you wish to reverse the sort.

and I am also confuse about the inheritance here:public
std::binary_fun ction<It,It,boo l>,
what's it for?

This base class does nothing other than define a few typedefs that allow
it to be used more easily with other parts of the STL.

See http://www.sgi.com/tech/stl/functors.html, specifically the third
paragraph of the Description section.

Nate

OK, I got it. Thanks a lot!

[Gernot Frisch]

"Magcialkin g" <ma********@163 .comschrieb im Newsbeitrag
news:11******** **************@ m73g2000cwd.goo glegroups.com.. .

>I wanna sort a pointer vector with the algorithm "sort" in STL like
this:

"
int compare(vector< string*>::itera tor a,vector<string *>::iterator
b){
return (**a)<(**b);
}

sort(iter,iter_ end,compare);

struct MySort
{
bool operator() (const string*& p1, const string*& p2)
{
return *p1 < *p2;
}
};

int main()
{
std::vector<std ::string*vec;
// fill vector
std::sort(vec.b egin(), vec.end(), MySort() );
}

Question: Why are you holding a string* instead of a string in your
vector?

[Magcialking]

Gernot Frisch 写�：

"Magcialkin g" <ma********@163 .comschrieb im Newsbeitrag
news:11******** **************@ m73g2000cwd.goo glegroups.com.. .

I wanna sort a pointer vector with the algorithm "sort" in STL like
this:

"
int compare(vector< string*>::itera tor a,vector<string *>::iterator
b){
return (**a)<(**b);
}

sort(iter,iter_ end,compare);

struct MySort
{
bool operator() (const string*& p1, const string*& p2)
{
return *p1 < *p2;
}
};

int main()
{
std::vector<std ::string*vec;
// fill vector
std::sort(vec.b egin(), vec.end(), MySort() );
}

Question: Why are you holding a string* instead of a string in your
vector?

Cause I think this will save time than to push string itself into a
vector.

[Jerry Coffin]

In article <11************ *********@i3g20 00cwc.googlegro ups.com>,
ma********@163. com says...

[ ... ]

Question: Why are you holding a string* instead of a string in your
vector?

Cause I think this will save time than to push string itself into a
vector.

A string object isn't usually a _whole_ lot more than a wrapper around a
pointer and a couple of size_t's. Depending on implementation, it may
also include a _small_ array of elements, but it's still usually pretty
fast to copy around and such.

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Gernot Frisch]

Question: Why are you holding a string* instead of a string in your
vector?

Cause I think this will save time than to push string itself into a
vector.
And you know you have to delete the strings themselfes some day?? The
string you forgot, because you put it's pointer in the vector?
It's OK if you do, but are you aware if it?