C++ creating an array in a temporary class - c++

Question

Newbie question! I've started a small project that loads integer values from a file into an array. (The array needs to be accessed at random which is why i've chosen an array and not a vector.)
To load the data values from the file I've created a Load/Save class. The load function reads the first line of the file which gives us the total number of entries the array needs to have, then it will fill the array with the rest of the values in that file.
This load object is only created temporarily, I want to give the data to the program and then delete the object.
What is the best way to achieve this? should I create the array in main() and pass the load object a reference, in which case how can I create the array so it can be re-sized for the amount of data that needs loading..?
here is the load/save class:
class FileIOClass {
public:
FileIOClass();
int ReadFile(char*);
private:
};
this is the cpp code for the class:
FileIOClass::FileIOClass() {
}
int FileIOClass::ReadFile(char* file_name) {
string line;
ifstream file;
file.open(file_name, ios::in);
cout << "Loading data...\n";
int num_x, num_y, array_size;
bool machine_header = false;
if (file.is_open()) {
while(getline(file, line)) {
if (line.size() && machine_header == false) {
// Load machine header information
file >> num_x;
file >> num_y;
file >> array_size;
machine_header = true; // machine header has now been read, set this to true.
}
else {
// this is where i want to load the data from the file into an array.
// the size of the array should be equal to the value in array_size.
}
}
cout << "Loading complete!\n";
}
else {cout<<"File did not open!\n";}
file.close();
return 0;
}
and here is the main.cpp so far:
int main(int argc, char** argv)
{
FileIOClass data_in;
data_in.ReadFile(argv[1]);
return 0;
}
there will be several other classes that will process the data thats contained in the array.
I bet there are a load of odd newbie mistakes in this code - please feel free to point them out, better to learn these things now.
Thanks all!
Ian.

Answer 1

Somthing like this might be good:
vector<int> myVector(array_size);
for(int i=0; file && i<array_size; i++) {
file >> myVector[i];
}

Answer 2

As long as you've already decided on using a class to read the file, storing the data within the class seems reasonable. add a member to this class to store the data:
class FileIOClass {
public:
FileIOClass();
int ReadFile(char*);
unsigned int operator [](int i) const {return m_data[i];}
int size(void) { return m_data.size(); }
private:
std::vector<int> m_data;
};
and insert data into this member in your ReadFile method:
while(getline(file, line)) {
int pos = 0;
if (line.size() && machine_header == false) {
// Load machine header information
file >> num_x;
file >> num_y;
file >> array_size;
m_data.resize(array_size);
machine_header = true; // machine header has now been read, set this to true.
}
else {
file >> m_data[pos++];
// this is where i want to load the data from the file into an array.
// the size of the array should be equal to the value in array_size.
}
}
notice that I overloaded the [] operator, so you can use your class like this:
int main(int argc, char** argv)
{
FileIOClass data_in;
data_in.ReadFile(argv[1]);
if (data_in.size() >= 1)
cout << data_in[0];
return 0;
}

Answer 3

The array needs to be accessed at random which is why i've chosen an array and not a vector.
C++ vectors allow efficient random access (they are arrays under the hood). Use std::vector unless you have profiled your code and found them inefficient for what you are doing.