Template для функций С++

class Tree
{
public:
   ....
   template <typename X> X getEl() {return static_cast<X>;};
   ...
};

class TreeDouble : public Tree
{
public:
    ...
    [B]template<double> double getEl()[/B]
    {
        return element<double>;
    }
       ....
};

//--------------------------------------------------------------------------------------------------
class Tree
{
public:
    template <typename X> X getEl() { return static_cast<X>; };
};
//--------------------------------------------------------------------------------------------------
class TreeDouble : public Tree
{
public:
    template<typename T>
    T getEl(T a)
    {
        std::cout << "T getEl(T a) = ";
        return a;
    }
 
    template<>
    double getEl<double>(double a)
    {
        std::cout << "double getEl<double>(double a) = ";
        return a;
    }
 
    template<>
    int getEl<int>(int a)
    {
        std::cout << "int getEl<int>(int a) = ";
        return a;
    }
 
};
//--------------------------------------------------------------------------------------------------
int main()
{
    setlocale(LC_ALL, "Russian");
 
    TreeDouble Test;
 
    std::cout << Test.getEl(3.14) << std::endl;
    std::cout << Test.getEl(3) << std::endl;
 
    system("pause");
    return 0;
}
//--------------------------------------------------------------------------------------------------

static_cast<X>(выражение, которое приводится к типу X);

template <typename T> T func ()
{
    return T(0);
}
.....
double st = func();

double st = func<>();

double st = func<double>();

template <typename T> T func ()
{
    return T(0);
}
 
......
double st = func<decltype(st)>();