c++ problem with temporany reference

[alessio211734]

Hello,

I have the following problem. I have this method:

void holeFilling::updateNearFaces( tri::Allocator<CMeshO>:ointerUpdater<CMeshO::FacePointer> pu, std::set<CMeshO::FacePointer> & faces )
{
std::set<CMeshO::FacePointer>::iterator it;
for (it=faces.begin();it!=faces.end();++it)
{
//CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update((*it));
}
}

where the function Update is defined as:

template<class SimplexPointerType>
class PointerUpdater
{
void Update(SimplexPointerType &vp)
{
//if(vp>=newBase && vp<newEnd) return;
if(vp<oldBase || vp>oldEnd) return;
assert(vp>=oldBase);
assert(vp<oldEnd);
vp=newBase+(vp-oldBase);
if(!remap.empty())
vp = newBase + remap[vp-newBase];
}

with the microsoft compiler with no problem while with mingw I get an error.
....holeFilling.cpp:1471: error: no matching function for call to 'vcg::tri::Allocator<CMeshO>:ointerUpdater<CFaceO*>::Update(CFaceO* const&)'
if (pu.NeedUpdate()) pu.Update((*it));
^
if I replace the code so:

CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update(fp);
compile on mingw but its different because I need to change the pointer (*it).

Thanks in advance.


Ale.

 

[Alf P. Steinbach]

Hello,

I have the following problem. I have this method:

void holeFilling::updateNearFaces( tri::Allocator<CMeshO>:ointerUpdater<CMeshO::FacePointer> pu, std::set<CMeshO::FacePointer> & faces )
{
std::set<CMeshO::FacePointer>::iterator it;
for (it=faces.begin();it!=faces.end();++it)
{
//CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update((*it));
}
}

where the function Update is defined as:

template<class SimplexPointerType>
class PointerUpdater
{
void Update(SimplexPointerType &vp)
{
//if(vp>=newBase && vp<newEnd) return;
if(vp<oldBase || vp>oldEnd) return;
assert(vp>=oldBase);
assert(vp<oldEnd);
vp=newBase+(vp-oldBase);
if(!remap.empty())
vp = newBase + remap[vp-newBase];
}

with the microsoft compiler with no problem while with mingw I get an error.
...holeFilling.cpp:1471: error: no matching function for call to 'vcg::tri::Allocator<CMeshO>:ointerUpdater<CFaceO*>::Update(CFaceO* const&)'
if (pu.NeedUpdate()) pu.Update((*it));
^
if I replace the code so:

CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update(fp);
compile on mingw but its different because I need to change the pointer (*it).

You can't change the stored member of a std::set, because that can
change whether it is logically a member, without the set's knowledge.

A std::set guarantees that its values are unique. After a change, if
that were allowed, the values might not be unique.

One way to resolve this: you can remove the value from the set, and add
the new value to the set.

Another way to resolve it: use a different data structure, one that
permits duplicates, such as std::vector.

How reasonable any solution is depends on your purpose with this, but
just to recap the problem: a set offers a uniqueness guarantee that
could easily be broken if you could just arbitrary modify set elements.


Cheers & hth.,

- Alf

 

[Victor Bazarov]

I have the following problem. I have this method:

void holeFilling::updateNearFaces( tri::Allocator<CMeshO>:ointerUpdater<CMeshO::FacePointer> pu, std::set<CMeshO::FacePointer> & faces )
{
std::set<CMeshO::FacePointer>::iterator it;
for (it=faces.begin();it!=faces.end();++it)
{
//CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update((*it));
}
}

where the function Update is defined as:

template<class SimplexPointerType>
class PointerUpdater
{
void Update(SimplexPointerType &vp)
{
//if(vp>=newBase && vp<newEnd) return;
if(vp<oldBase || vp>oldEnd) return;
assert(vp>=oldBase);
assert(vp<oldEnd);
vp=newBase+(vp-oldBase);
if(!remap.empty())
vp = newBase + remap[vp-newBase];
}

with the microsoft compiler with no problem while with mingw I get an error.
...holeFilling.cpp:1471: error: no matching function for call to 'vcg::tri::Allocator<CMeshO>:ointerUpdater<CFaceO*>::Update(CFaceO* const&)'
if (pu.NeedUpdate()) pu.Update((*it));
^
if I replace the code so:

CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update(fp);
compile on mingw but its different because I need to change the pointer (*it).

Your problem is that you're trying to update the value in 'std::set' in
place. It is not allowed. The set keeps its values sorted, so a change
in any value can disrupt the sorting. What you need to do is to
determine the new value and re-insert it into the set after removing the
one that doesn't need to be there anymore.

Technically, the set iterator returns a reference to a const value when
you dereference it. Microsoft cuts some corners (disable language
extensions to prohibit it from cutting corners), and that's why it
appears to work. In fact, your code has undefined behavior.

Change your loop

for (it=faces.begin();it!=faces.end();++it)
{
//CMeshO::FacePointer fp=(*it);
if (pu.NeedUpdate()) pu.Update((*it));
}

to something like this

if (pu.NeedUpdate())
{
std::vector<CMeshO::FacePointer> updated;
for (it = faces.begin(); it != faces.end() // note no ++it
{
CMeshO::FacePointer fp = *it;
pu.Update(fp);
updated.push_back(fp);
it = faces.erase(it); // this is where 'it' changes
}
for (int i = 0; i < updated.size(); ++i)
faces.insert(updated);
}

V

 

[alessio211734]

You can't change the stored member of a std::set, because that can

change whether it is logically a member, without the set's knowledge.



A std::set guarantees that its values are unique. After a change, if

that were allowed, the values might not be unique.

thanks very much!

 

[Anand Hariharan]

)

with the microsoft compiler with no problem while with mingw I get an error.

Alf and Victor have resolved your problem; just wanted to add that a reasonably new compiler from Microsoft (VS 2010 and later) would reject that code.

- Anand