pointers as template parameters? - c++

Question

I have a container class, we'll call it
template <class T> CVector { ... }
I want to do something different with this class when T is a pointer type, e.g. something along the lines of:
template <class T*> CVector< SomeWrapperClass<T> >;
where SomeWrapperClass is expecting the type of the pointed to thing as its parameter. Unfortunately, this syntax doesn't quite work and with some digging, I haven't found a good way to get something like this working.
Why do it this way? I want to change, in a very large app, how some of our containers work when the type they're specialized on is a pointer vs. not a pointer - and ideally, i'd like to do it without changing the ~1,000 places in the code where there are things like CVector<Object*> vs CVector<int> or some such - and playing games with partial specializations seemed to be the way to go.
Am I on crack here?

Answer 1

If I understand you correctly, this might do what you want:
template<typename T>
class CVector { ... };
template<typename T>
class CVector<T*> : public CVector< SomeWrapperClass<T> > {
public:
// for all constructors:
CVector(...) : CVector< SomeWrapperClass<T> >(...) {
}
};
It adds an additional layer of inheritance to trick CVector<T*> into being a CVector< SomeWrapperClass<T> >. This might also be useful in case you need to add additional methods to ensure full compatibility between the expected interface for T* and the provided interface for SomeWrapperClass<T>.

Answer 2

This works just fine in C++...
#include <iostream>
template <class T>
class CVector
{
public:
void test() { std::cout << "Not wrapped!\n"; }
};
template <class T>
class CVector<T*>
{
public:
void test() { std::cout << "Wrapped!\n"; }
};
int main()
{
CVector<int> i;
CVector<double> d;
CVector<int*> pi;
CVector<double*> pd;
i.test();
d.test();
pi.test();
pd.test();
}

Answer 3

I don't think you can specialize a class using the syntax you describe... I don't know how that could possibly work. What you can do is specialize the class for pointers and re-implement its guts using the wrapper class around the raw pointers. I'm not sure if it will help, but this article describes specializing templates for pointers.

Answer 4

The Boost type traits library can help you achieve this. Check out the is_pointer type trait.
#include <boost/type_traits.hpp>
#include <iostream>
#include <vector>
using namespace std;
template <class T>
class CVector {
public:
void addValue(const T& t) {
values_.push_back(t);
}
void print() {
typedef boost::integral_constant<bool,
::boost::is_pointer<T>::value> truth_type;
for (unsigned int i = 0; i < values_.size(); i++)
doPrint(values_[i], truth_type());
}
private:
void doPrint(const T& t, const boost::false_type&) {
cout << "Not pointer. Value:" << t << endl;
}
void doPrint(const T& t, const boost::true_type&) {
cout << "Pointer. Value: " << *t << endl;
}
std::vector<T> values_;
};
int main() {
CVector<int> integers;
integers.addValue(3);
integers.addValue(5);
integers.print();
CVector<int*> pointers;
int three = 3;
int five = 5;
pointers.addValue(&three);
pointers.addValue(&five);
pointers.print();
}

Answer 5

I don't think templates are quite that flexible.
A very brute force approach would be to specialize for all of your pointer types...which defeats the problem of using templates.
Could you have a different CVector class that is used only for vectors of pointers?

Answer 6

I agree with rlbond's answer. I have modified it a little bit to suit your need. CVector can be a derived class of the CVector itself. You can then use different members and functions for it.
#include <iostream>
#include <string>
template <class T>
class CVector
{
public:
void test() { std::cout << "Not wrapped!\n"; }
void testParent() { std::cout << "Parent Called\n";}
};
template <class T>
class CVector<T*>:
public CVector<T>
{
public:
void test(std::string msg) { std::cout << msg; testParent(); }
};
int main()
{
CVector<int> i;
CVector<double> d;
CVector<int*> pi;
CVector<double*> pd;
i.test();
d.test();
pi.test("Hello\n");
pd.test("World\n");
system("pause");
}