C++ single template specialisation with multiple template parameters
Solution 1
You have to provide a partial specialization of the class template B
:
template <typename I>
class B<float, I>
{
public:
void someFunc();
};
template <typename I>
void B<float, I>::someFunc()
{
...
}
You can also just define someFunc
inside the specialization.
However, if you only want to specialize a function, and not a class do e. g.
template <typename F, typename I>
void someFunc(F f, I i) { someFuncImpl::act(f, i); }
template <typename F, typename I>
struct someFuncImpl { static void act(F f, I i) { ... } };
// Partial specialization
template <typename I>
struct someFuncImpl<float, I> { static void act(float f, I i) { ... } };
But you can't specialize a function template without this trick.
Solution 2
Although you can totally specialize member functions of a class template, you cannot _partially specialize member functions. - Andrei Alexandrescu
Partial Class specialization is explained by the other posters.
You can, however, use overloading:
template <class T, class U> T fun(U obj); // primary template
template <class U> void Fun<void, U>(U obj); // illegal pertial
// specialization
template <class T> T fun (Window obj); // legal (overloading)
If you want to go deep into this, you can read about this issue in depth in "Modern C++ Design" by A. Alexandrescu.
tauran
Updated on July 26, 2021Comments
-
tauran almost 3 years
Hallo!
I would like to specialise only one of two template types. E.g.
template <typename A, typename B> class X
should have a special implementation for a single functionX<float, sometype>::someFunc()
.Sample code:
main.h:
#include <iostream> template <typename F, typename I> class B { public: void someFunc() { std::cout << "normal" << std::endl; }; void someFuncNotSpecial() { std::cout << "normal" << std::endl; }; }; template <typename I> void B<float, I>::someFunc();
main.cpp:
#include <iostream> #include "main.h" using namespace std; template <typename I> void B<float, I>::someFunc() { cout << "special" << endl; } int main(int argc, char *argv[]) { B<int, int> b1; b1.someFunc(); b1.someFuncNotSpecial(); B<float, int> b2; b2.someFunc(); b2.someFuncNotSpecial(); }
Compilation fails for
class B
. Is it true, that this is not possible in C++ in this way? What would be the best workaround?[edit]
template <float, typename I> void B<float, I>::someFunc();
leads to main.h:26: error: ‘float’ is not a valid type for a template constant parametertemplate <typename I> void B<float, I>::someFunc();
leads to main.h:27: error: invalid use of incomplete type ‘class B’And I'm using gcc.
[edit]
I don't want to specialise the whole class, as there are other functions that don't have a specialisation.