[C++] - Errore di compilazione.. ma dove?

[Enumaelish]

Questo è il nuovo main:

#include <iostream>
#include "Pila.h"
#include <string>

using namespace std;

int main() {
string str;

cout << "La parola che verrà inserita e: Rosa.\n";


Pila<char> p(7);

p.push('a');
p.push('s');
p.push('o');
p.push('R');
 
str.append(p.pop());
str.append(p.pop());
str.append(p.pop());
str.append(p.pop());

cout << "La parola inserita è:" << str << ".\n";

}

e questo è il nuovo errore

J:\eclipse\workspace\Pila\main.cpp:18: error: invalid conversion from `char' to `char*'
J:\eclipse\workspace\Pila\main.cpp:18: error:   initializing argument 1 of `void Pila<T>::push(T) [with T = char*]'
J:\eclipse\workspace\Pila\main.cpp:19: error: invalid conversion from `char' to `char*'
J:\eclipse\workspace\Pila\main.cpp:19: error:   initializing argument 1 of `void Pila<T>::push(T) [with T = char*]'
J:\eclipse\workspace\Pila\main.cpp:20: error: invalid conversion from `char' to `char*'
J:\eclipse\workspace\Pila\main.cpp:20: error:   initializing argument 1 of `void Pila<T>::push(T) [with T = char*]'
J:\eclipse\workspace\Pila\main.cpp:21: error: invalid conversion from `char' to `char*'
J:\eclipse\workspace\Pila\main.cpp:21: error:   initializing argument 1 of `void Pila<T>::push(T) [with T = char*]'

Aiuto...