Using STL algorithms (specifically std::sort) from within a templated class

Question

I've declared a template class MyContainer as bellow, then created an instance of it of type DataType1. The DataType1 class provides a friend function "DataSpecificComparison" which is used by std::sort to compare DataType1 objects. The program compiled and sorted correctly.

I then defined a class called DataType2, gave it a friend implementation of "DataSpecificComparison" and used it to create another instance of MyContainer.

I am now unable to compile the program as a "C2914: 'std::sort' : cannot deduce template argument as function argument is ambiguous" compile time error is reported.

How can a developer specify that the DataSpecificComparison binary predicate is to take arguments of template type T*? Or is there another way around this issue?

template <class T>
class MyContainer
{
private: 
    vector<T*> m_vMyContainerObjects;
    ....

public:
    ....
    void SortMyContainerObjects()
    {
        std::sort(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(), DataSpecificComparison)
    }
}


class DataType1
{
    ....
    friend bool DataSpecificComparison(const DataType1 * lhs, const DataType1 * rhs)
}

class DataType2
{
    ....
    friend bool DataSpecificComparison(const DataType2* lhs, const DataType2* rhs)
}

Answer 1

Did you try defining DataSpecificComparison as template with bunch of specializations and giving it the type?

template<T>
bool DataSpecificComparison(const T* t1, const T* t2)
{
    // something non compilable here
}

template<> bool DataSpecificComparison<Data1>(const Data1* t1, const Data1* t2)
{
    // return *t1 < *t2;
}

....
void SortMyContainerObjects()
{
    std::sort(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(), DataSpecificComparison<T>)
}
....

Answer 2

Templating DataSpecificComparison should work. You can also specifically call the proper std::sort template, but it's a bit cumbersome:

template <class T>
class MyContainer
{
private: 
    vector<T*> m_vMyContainerObjects;
    typedef bool (*compsT)(T, T); 

public:
    ....
    void SortMyContainerObjects()
    {
        std::sort<std::vector<T*>::iterator, compsT>(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(), DataSpecificComparison);
    }
}

Answer 3

You can use a temporary local function pointer variable of the required type to select the correct overload of DataSpecificComparison:

void SortMyContainerObjects()
{
    typedef bool (*comparer_t)(const T*, const T*);
    comparer_t cmp = &DataSpecificComparison;
    std::sort(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(), cmp);
}

Here the compiler can deduce that you want to use the DataSpecificComparison overload that matches the comparer_t type, which resolves the ambiguity.

Answer 4

template<typename T>
struct DataSpecificComp : public binary_function<T, T, bool>
{
public:
    bool operator()(const T* lhs, const T* rhs)
    {
        return *lhs < *rhs;
    }
};

call the sort function as shown below:

sort(vi.begin(), vi.end(), DataSpecificComp<int>());

Answer 5

sth already gave a correct answer, but there's also a direct alternative based on the same principle:

void SortMyContainerObjects()
{

    std::sort(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(),
       static_cast<bool (*comparer_t)(const T*, const T*)>(&DataSpecificComparison));
}

This uses essentially the same mechanism. The cast forces overload resolution to happen before the Template Argument Deduction for std::sort.

Answer 6

I'd prefer something along the following lines: by default it compares objects with less_than (so you wouldn't have to remember to provide a function with a funny name), and there's an overload that allows giving your own comparison functor (again, value-based):

#include <vector>
#include <algorithm>
#include <functional>

template <class T, class Func>
struct indirect_binary_call_type: public std::binary_function<const T*, const T*, bool>
{
    Func f;
    indirect_binary_call_type(Func f): f(f) {}
    bool operator()(const T* a, const T* b) const
    {
        return f(*a, *b); 
    } 
};

template <class T, class Func>
indirect_binary_call_type<T, Func> indirect_binary_call(Func f)
{
    return indirect_binary_call_type<T, Func>(f);
}

template <class T>
class MyContainer
{
private: 
    std::vector<T*> m_vMyContainerObjects;

public:
    void Sort()
    {
        Sort(std::less<T>());
    }
    template <class Func>
    void Sort(Func f )
    {
        std::sort(m_vMyContainerObjects.begin(), m_vMyContainerObjects.end(), indirect_binary_call<T>(f));
    }

};

int main()
{
    MyContainer<int> m;
    m.Sort();
    m.Sort(std::greater<int>());
}