Sorting a vector of a class - c++

Question

I have class called "UltrasoundTemplate". These UltrasoundTemplate objects contain an int parameter, which shows when they where defined (something like a time stamp). And I have a class called "UltrasoundTarget" which contains a vector of UltrasoundTemplate's.
I add UltrasoundTemplates to the vector with push_back(ultrasoundTemplate).
Now I want to sort the vector by the order of time stamps instead of the order I added them to the vector.
I found a lot of answers in google, which all show me the same solution, but obviously I'm still doing something wrong. Here are the code snippets I think are necessary for finding a solution:
ultrasoundTemplate.h
class UltrasoundTemplate
{
public:
UltrasoundTemplate(/*...*/);
int getVolumePos() { return volume_; }
private:
int volume_;
};
ultrasoundTarget.h
//the sort algorithm
struct MyTemplateSort {
bool operator() ( UltrasoundTemplate t1, UltrasoundTemplate t2){
int it1 = t1.getVolumePos();
int it2 = t2.getVolumePos();
if (it1 < it2)
return true;
return false;
}
};
class UltrasoundTarget
{
public:
UltrasoundTarget(/*...*/);
vector<UltrasoundTemplate> getTemplates() { return USTemplateVector_; }
private:
vector<UltrasoundTemplate> USTemplateVector_;
};
FMainWindow.cpp
void FMainWindow::match_slot()
{
int i;
//here I get the name of the target I'm looking for
QTreeWidgetItem *item = targetInfoWidget_->treeWidget->currentItem();
int index = targetInfoWidget_->treeWidget->indexOfTopLevelItem(item);
QString itemToAppendName = item->text(0);
for(i = 0; i < USTargetVector.size(); i++){
if(USTargetVector.at(i).getName() == itemToAppendName) {
//here I try to sort
MyTemplateSort tmpltSrt;
std::sort(USTargetVector.at(i).getTemplates().begin(),
USTargetVector.at(i).getTemplates().end(), tmpltSrt);
break;
}
}
As an example: I define Template1 in Volume(0), Template2 in Volume(70) and Template3 in Volume(40). The order now is (Template1, Template2, Template3) but I want it to be (Template1, Template3, Template2). But this code is not doing it.
If there's Information missing, just tell me and I'll provide more code.
Thanks alot.

Answer 1

Your getTemplates() method returns by value, making a mess here:
std::sort(USTargetVector.at(i).getTemplates().begin(),
USTargetVector.at(i).getTemplates().end(), tmpltSrt);
You are sorting an incompatible iterator range. You can fix that particular problem by returning a reference:
vector<UltrasoundTemplate>& getTemplates() { return USTemplateVector_; }
It is common practice to add a const overload to such a method:
const vector<UltrasoundTemplate>& getTemplates() const { return USTemplateVector_; }
You can also modify your comparison functor to avoid unnecessary copies (and for general readability and const correctness):
struct MyTemplateSort {
bool operator() const ( const UltrasoundTemplate& t1, const UltrasoundTemplate& t2)
{
return t1.getVolumePos() < t2.getVolumePos();
}
};
This will require that you make getVolumePos() a const method, which it should be anyway:
class UltrasoundTemplate
{
public:
...
int getVolumePos() const { return volume_; }
...
};
Note that is is not generally good practice to provide references to the private data of a class. If possible, you should find a way to remove that from the UltraSoundTarget interface. You could, for instance, expose a pair of iterators, and/or give the class a sort method.

Answer 2

juanchopanza answer is correct, the problem is the way you are returning the vector from UltrasoundTarget. Just to touch another topic, maybe it would be nice to change a little the designing of your implementation. As UltrasoundTarget is a container of Ultrasound's, it makes sense to implement the sort as a method of this class, this way you have direct access to USTemplateVector_ and will save unecessary copies. Something like:
class UltrasoundTarget
{
public:
UltrasoundTarget(/*...*/);
vector<UltrasoundTemplate> getTemplates() { return USTemplateVector_; }
void sort();
private:
vector<UltrasoundTemplate> USTemplateVector_;
};
void UltrasoundTarget::sort()
{
std::sort(USTemplateVector_.begin(), USTemplateVector_.end(), tmpltSrt);
}
void FMainWindow::match_slot()
{
int i;
//here I get the name of the target I'm looking for
QTreeWidgetItem *item = targetInfoWidget_->treeWidget->currentItem();
int index = targetInfoWidget_->treeWidget->indexOfTopLevelItem(item);
QString itemToAppendName = item->text(0);
for(i = 0; i < USTargetVector.size(); i++){
if(USTargetVector.at(i).getName() == itemToAppendName)
{
//here I try to sort
MyTemplateSort tmpltSrt;
USTargetVector.at(i).sort();
break;
}
}