C++ Member declaration not found

Question

I started in the language c++ a short time and I am doubtful in methods specializations, I have two header files, arrangement and student, I want to use 2 methods of arrangement in student, but when declaring the method with specialization the eclipse points out the error "Member not found"If anyone can help me by clarifying why I made the mistake, I’d appreciate it. Below are the Arrangement and Student codes respectively.

#define ARRANJO_H_

#include <iostream>

using namespace std;

namespace arranjo{

template <class T>
    class Arranjo {

    private:
    int tamanho;
    T * items;

    public:
    Arranjo(int tam) {
        this -> tamanho = tam;
        this -> items = new T[tamanho];
    }
    virtual ~Arranjo(){}

    virtual T get(int idx) {
        if (idx < 0 || idx >= tamanho) throw "Erro! Esse Indice não existe";
        return items[idx];
    }

    virtual void set(int idx, const T & item) {
        if (idx < 0 || idx >= tamanho) throw "Erro! Esse Indice não existe";
        this -> items[idx] = item;
    }

    virtual void exibir();

};

template<class T>
    void Arranjo<T>::exibir() {
    // exibir cada item numa linha da forma "<idx>: <item>"
    for(int i = 0; i < tamanho; i++) {
      cout << i << ": " << items[i] << "\n";
        }
    }
}
#endif /* ARRANJO_H_ */

#define ALUNO_H_

#include "arranjo.h"

using namespace arranjo;

class Aluno {

private:
    string nome;
    string mat;

public:
    Aluno() {}
    Aluno(const char * nome, const char * mat) : nome(nome), mat(mat) {}
    friend class Arranjo<Aluno>;
};

template<>
void Arranjo<Aluno>::set(int idx, const Aluno & aluno) {
// Ao declarar este método especializado o Eclipse CDT exibe a mensagem de erro Member declaration not found

}

template<>
void Arranjo<Aluno>::exibir() {
// Ao declarar este método especializado o Eclipse CDT exibe a mensagem de erro Member declaration not found
}

#endif /* ALUNO_H_ */