Lambda Expression in c++ std::copy_if - c++

Question

Can somebody explain or help me why this isnt working?
std::vector<std::shared_ptr<Publication> > Bibliography::givePubWithIFHigherThan(float value) const
{
Publication *p;
std::vector<std::shared_ptr<Publication>> highIFPubs(publications);
auto checkIF = std::mem_fun(p->IFHigherThan(value));
auto last = std::copy_if(publications.begin(), publications.end, highIFPubs.begin(),
[=] (std::shared_ptr<Publication> p)
{
return checkIF(*p, value);
});
return highIFPubs;
}
class Publication
{
public:
Publication(std::string aTitle, int aYear, std::string anID);
virtual bool IFHigherThan(float value) const {return false;};
private:
};
class Paper : public Publication
{
public:
Paper(std::string aTitle, int aYear, std::string aJournal, float aImpactFactor);
bool IFHigherThan(float value) const {return value < impactFactor;};
private:
};
At the moment i get this error,
no matching function for call to 'mem_fun(bool)'
auto checkIF = std::mem_fun(p->IFHigherThan(value));
^

Answer 1

std::mem_fun is a depracated helper function that will probably be soon removed from the standard library. std::mem_fn would be a better choice.
Moreover, if you want to use std::mem_fn, std::mem_fun or std::bind with a function, then you pass in a pointer to function, not a call expression, so instead of:
auto checkIF = std::mem_fun(p->IFHigherThan(value));
use:
auto checkIF = std::mem_fn(&Publication::IFHigherThan);
Alternatively, don't use any wrapper, just directly call the selected member function:
auto last = std::copy_if(publications.begin(), publications.end(), highIFPubs.begin(),
[=] (std::shared_ptr<Publication> p)
{
return p->IFHigherThan(value);
});
There is one more logical error you have in your code:
std::vector<std::shared_ptr<Publication>> highIFPubs(publications.size());
should be:
std::vector<std::shared_ptr<Publication>> highIFPubs;
and then instead of:
auto last = std::copy_if(publications.begin(), publications.end()
, highIFPubs.begin(),
// ~~~~~~~~~~~~~~~~~^
you should be using std::back_inserter:
auto last = std::copy_if(publications.begin(), publications.end()
, std::back_inserter(highIFPubs),
// ~~~~~~~~~~~~~~~~~^
as you don't actually know how many elements will the resultant vector have.