C++: using deque member functions inside a class stack - c++

Question

I am having trouble with stacking values for a deque object defined in a template class stack. When initializing a class stack object and use the push_back() member function of the STL deque class on the initialized object and read the number of elements inside the stack I get zero, yet the terminal told me it pushed values into it. It looks as follows:
Stack header file
#ifndef STACK_HH
#define STACK_HH
#include <iostream>
#include <deque>
template<class T>
class Stack {
public:
Stack(): s() { // <-- creating an empty deque s
s.size() ;
nitems() ;
empty() ;
}
~Stack() {
nitems() ;
empty() ;
}
//Member functions
int nitems() { return s.size() ; }
bool empty() { return (s.size()==0) ; }
void push_back(T c) {
s[s.size()] = c ;
}
T back() {
return s[s.size()] ;
}
void pop_back() {
if (empty()) {
std::cout << "Stack::pop() Error: stack is empty" << std::endl ;
}
}
private:
std::deque<T> s ;
};
#endif
Main script
#include <iostream>
#include <deque>
#include "Stack.h"
using namespace std ;
// Main
int main() {
int LEN = 10;
Stack<double> s ;
// Write doubles into Stack
for (int i=0 ; i<LEN ; i++) {
cout << "pushing value " << i*i << " in stack s" << endl ;
s.push_back(i*i) ;
}
// Count doubles in fifo
cout << s.nitems() << " value(s) in stack" << endl ;
return 0 ;
}
I was wondering if I should define the deque push_back(), back() and pop_back() functions differently in my Stack header file. Something like void deque < T > ::push_back() and similar for the other ones. But this did not solve it as I already was expecting yet using s.pushback() did not work either.
Any help in solving this problem is kindly appreciated.
EDIT:
Thank you all for the help.

Answer 1

You can indeed use directly the deque methods like that:
template<class T>
class Stack {
public:
Stack(): s() { // <-- creating an empty deque s
}
~Stack() {
}
//Member functions
int nitems() const { return s.size() ; }
bool empty() const { return s.empty() ; }
void push_back(const T& c) {
s.push_back(c) ;
}
const T& back() {
return s.back();
}
void pop_back() {
if (empty()) {
std::cout << "Stack::pop() Error: stack is empty" << std::endl ;
}
s.pop_back();
}
private:
std::deque<T> s ;
};

Answer 2

Why not std::stack?
Assigning to s[s.size()] doesn't add new elements:
Notes
Unlike std::map::operator[], this operator never inserts a new element into the container.
From http://en.cppreference.com/w/cpp/container/deque/operator_at.
Use s.push_back(new_element) to add new element.
Last element is s[s.size() - 1] or, better, s.back().
s.size(), s.size() and empty() in constructor and destructor do nothing.