Size of dynamic array does not match the value submitted - c++

Question

I'm learning dynamic memory and something isn't quite going right. I have a function that accepts a number as input and is supposed to make an array of that size.
class Doctor {
public:
Doctor();
void fillOut();
void listPatients();
void patientReset();
~Doctor();
private:
string name;
int numPatients;
string *patientList;
};
Doctor::Doctor() {
name = "";
numPatients = 0;
patientList = new string[numPatients];
}
(Most relevant code in the 3rd code block).
void Doctor::fillOut()
{
string buffer = "";
string buffer2 = "";
size_t found;
bool valid = false;
int numP = 0;
int tester = 0;
bool validNum = false;
while(!valid)
{
cout << "Enter doctor name: ";
getline(cin, buffer);
found = buffer.find_first_of("1234567890!##$%^&*()-=_+/<>?;':][");
if(string::npos == found)
{
name = buffer;
valid = true;
}
}
while (!validNum)
{
cout << "\nEnter number of patients: ";
buffer = "";
getline(cin, buffer);
buffer2 = buffer;
stringstream ss(buffer);
if(ss >> tester)
{
stringstream ss2(buffer2);
ss2 >> numP;
validNum = true;
}
else
{
cout << "Not a number. Please try again." << endl;
}
}
patientList = new string[numP];
cout << patientList->size() << endl;
for(int i = 0; i < (numP + 0); i++)
{
valid = false;
while(!valid)
{
cout << "\nEnter patient " << (i + 1) << ": ";
getline(cin,buffer);
found = buffer.find_first_of("1234567890!##$%^&*()-=_+,./<>?;':][");
if(string::npos == found)
{
*(patientList + i - 0) = buffer;
//patientList[i-1] = buffer;
valid = true;
}
else
{
valid = false;
}
}
}
}
I then try to display the contents of the list.
void Doctor::listPatients()
{
cout << "size: " << patientList->size() << endl;
cout << "\nDoctor: " << name << endl;
for(int i = 0; i < (patientList->size() - 1); i++)
{
cout << "Patient " << (i+1) << ": " << patientList[i] << endl;
}
cout << "end patients" << endl;
}
But for some reason the number I submit as the size isn't the size of the array. For example, in fillOut() I have the function output the size. It outputs the size as 0 every time. Then in listPatients(), I have something that prints the size again, to verify I'm doing it right. If I initially input 3, it has outputs 5 in this function.
I'm completely mystified.

Answer 1

The line patientList->size() is equivalent to patientList[0].size(), the length of the initial string in your patientList array. At the point where you have just allocated your array, the result is always zero; in other instances, it is the length of the first string.
A preferred way of making containers in C++ is std::vector or std::array. In your case using std::vector is more appropriate, because your code allocates the patientList dynamically.

Answer 2

Ok, I found your problem. You are declaring an array of strings. But a raw array is really just a pointer to the contents of the array. When you call ->size, it dereferences that pointer, which points to the first string in the array and tells you the size of that string.
If you want an array that actually knows its size, use std::array.

Answer 3

OK so you declare patientList as a string* but when you call size() on this you are actually calling it on the first item in the array you have created. The only reason your code compiled is because string has a size() function and as this is the type contained in your array you got away with it because patientList actually points to the item at the start of the array (a string). In general an array does not have a size() function.
One other problem you will have though is here in your loop
for(int i = 0; i < (patientList->size() - 1); i++) {
^
^
cout << "Patient " << (i+1) << ": " << patientList[i] << endl;
}
this will cause your loop to terminate one before the end of patientList. You should use
for(int i = 0; i < patientList->size(); i++) {
cout << "Patient " << (i+1) << ": " << patientList[i] << endl;
}
You would be better off using a std::vector here to put your strings in. Also be careful here
if(string::npos == found) {
*(patientList + i - 0) = buffer;
//patientList[i-1] = buffer;
valid = true;
}
else {
valid = false;
}
because you would only be assigning to your array on the occasions when your if statement is true - the indices in the array when it is false are left uninitialised. I would honestly start again from scratch and use a vector of strings.