Specialization of template functions, C ++

Question

I specialize in functions, and I tried to create a small storage object with a specialized print function. Here is my code:

#ifndef STORAGE_HPP_ #define STORAGE_HPP_ #include <iostream> using namespace std; template <typename T, int size> class Storage { //LINE 16 ERROR public: Storage():arr(new T*[size]){}; ~Storage() { for (int i = 0; i < size; i++) { delete arr[i]; } delete[] arr; } void push(T obj, int i) { arr[i] = new T(obj); } void print() { for (int i = 0; i < size; i++) { cout << *arr[i]; } cout << endl; } private: T** arr; }; template <typename T, int size> void Storage<int,size>::print() //LINE 38 ERROR { for (int i = 0; i < size; i++) { cout << (char) *arr[i]; } cout << endl; } #endif /* STORAGE_HPP_ */ 

And I get this error:

 ../Storage.hpp:38:63: error: invalid use of incomplete type class Storage<int, size> ../Storage.hpp:9:1: error: declaration of 'class Storage<int, size>' 

So, the first question: is it possible to implement specialized functions inside a class? I tried but got an error. Secondly, why am I getting the error I made? Thanks!

EDIT: I tried something new, as someone suggested here. I change the print inside the class only void print() , and I embedded it outside so that I can overload the function. Here:

 template <typename T, int size> void Storage<T,size>::print() { for (int i = 0; i < size; i++) { cout << *arr[i]; } cout << endl; } template <typename T, int size> void Storage<int,size>::print() //ERROR HERE { for (int i = 0; i < size; i++) { cout << *arr[i]; } cout << endl; } 

Now I get invalid use of incomplete type 'class Storage<int, size>' Where I wrote ERROR HERE (obviously!) I understand that this is a general solution, right? And why am I getting this error?

Answer 1

The problem is that you are trying to use the partial specialization of the entire class without specifying a partially specialized class.

If print itself was a function template, the situation would be different, because you can really specialize function templates. However, your design only has the whole class as a template.

This means that template <typename T, int n> class Storage<T, n> and template <int n> class Storage<int, n> are completely different, unrelated classes. Thus, you must first define the last class:

 template<int n> class Storage<int, n> { // define everything }; template<int n> void Storage<int, n>::print() { /* implement */ } 

Consider that the partial specialization Storage<int, n> may be a completely different class from the main template and may have completely different member functions. The compiler does not know this until you define this class.

---

Following sbi comment, here is one idea:

 //... in the class definition template<typename S, int m> friend void print_helper(const Storage<S, m> &); template<int m> friend void print_helper(const Storage<int, m> &); void print() { print_helper(*this); } 

 // outside: template <typename S, int size> void print_helper(const Storage<S, size> & s) { // ... } template <int size> void print_helper(const Storage<int, size> & s) { // ... } 

Instead of friend you can also create a static helper function template, but this can add a lot of bloating code, since you will have two static function templates for the class type, and not just two globally.

Answer 2

Partial specializations should always use the full template, as they also define the template.

So this will not work:

 template <int size> void Storage<int,size>::print() 

Full specializations of member functions can be performed off-line for individual member functions because they define functions.

So this will work:

 template <> void Storage<int,44>::print() 

And you cannot declare / define any (partial) specializations in the main template.

Answer 3

I do not think that you can use partial specialization according to one method. You can use only full specialization.

Question: why do you want to use an array with the specified size? Why not just use std :: vector and enable the push () function dynamically (using std :: vector :: push_back ())?

Answer 4

The only mistake I see is the extraneous typename T in your specialization. It should be:

 template <int size> void Storage<int,size>::print() {...}