need help on #include doesn't seem to be working - c++

Question

So I have a class called HPStack and I have to include it in my main class etc. However I get a "In File included from" error, what could be causing this?
Also my string objects also have errors I have have no idea why, the error is: "Unable to identifier string".
I'm new the C++ so any help would be appreciated, thanks in advance.
The error I am getting (I think) are these:
error: expected unqualified-id before "namespace"
error: expected `,' or `;' before "namespace"
error: expected namespace-name before ';' token
error: `<type error>' is not a namespace
Im not sure what I am missing but that isn't telling me much.
Here is my code: The class.h file.
#ifndef HPSTACK_H
#define HPSTACK_H
class HPStack {
public:
HPStack();
void push(double);
double pop();
double peek();
private:
double register_[4];
}
#endif
The class.cpp file.
#include "HPStack.h"
#include <cstdlib>
HPStack::HPStack() : register_{}{
}
double HPStack::push(double x) {
for (int i = 2; i >= 0; i--) {
if (isdigit(register_[i])) {
register_[i] = register_[i + 1];
}
register_[0] = x;
}
}
double HPStack::pop() {
return register_[0];
for (int i = 0; i < 3; i++) {
register_[i] = register_[i + 1];
}
}
double HPStack::peek() {
return register_[0];
}
And my main file:
#include "HPStack.h"
#include <cstdlib>
#include <string>
#include <sstream>
#include <iostream>
using namespace std;
int main() {
HPStack stack;
string line;
while (getline(cin, line)) {
stringstream expression(line);
string token;
while (expression >> token) {
if (isdigit(token[0])) {
stack.push(atof(token.data()));
} else if (token == "+") {
double x = stack.pop();
double y = stack.pop();
double z = (y + x);
stack.push(z);
}
}
cout << stack.peek();
}

Answer 1

The error is, I'm guessing, because of this line:
double register_[4] = {};
You can not initialize class members when declaring them.
If your compiler is new enough to support C++11 features, you can use an initializer list with the constructor:
HPStack::HPStack()
: register_{}
{
}
Otherwise you have to initialize the array manually in the constructor.
And as I noted in a comment, using register_ - 2 makes no sense as it returns a pointer so the index variable i will be way beyond the end of the array.
And using register_ - 1 as the condition in the pop loop makes even less sense, as it will always be non-zero and therefore always true and the loop will loop forever.

Answer 2

You're missing the ; at the end of the class definition:
class HPStack {
...
}; // <== This semicolon is required