Error: Variable length array of Non-POD element type 'string'

Question

Before beginning, I must first say that I have already looked into possible solutions for this error. Unfortunately, they all have to do with not using arrays, which is a requirement for my project. Also, I'm currently taking Intro to CS so my experience is next to none.

The purpose of the array is to gather names from a file. So in order to initialize array, I count the number of names and use it as the size. The problem is the error stated in the title, but I don't see a way around it while still using a 1D array.

main.cpp

    #include <iostream>
    #include <cstdlib>
    #include <fstream>
    #include <string>
    #include <iostream>
    #include "HomeworkGradeAnalysis.h"

    using namespace std;

    int main()
    {
        ifstream infile;
        ofstream outfile;
        infile.open("./InputFile_1.txt");
        outfile.open("./OutputfileTest.txt");

        if (!infile)
        {
            cout << "Error: could not open file" << endl;
            return 0;
        }

        string str;
        int numLines = 0;

        while (infile)
        {
            getline(infile, str);
            numLines = numLines + 1;
        }
        infile.close();
        int numStudents = numLines - 1;
        int studentGrades[numStudents][maxgrades];
        string studentID[numStudents];

        infile.open("./InputFile_1.txt");

        BuildArray(infile, studentGrades, numStudents, studentID);

        infile.close();
        outfile.close();
        return 0;
    }

HomeworkGradeAnalysis.cpp

    using namespace std;

    void BuildArray(ifstream& infile, int studentGrades[][maxgrades], 
            int& numStudents, string studentID[])
    {
        string lastName, firstName;
        for (int i = 0; i < numStudents; i++)
        {
            infile >> lastName >> firstName;
            studentID[i] = lastName + " " + firstName;
            for (int j = 0; j < maxgrades; j++)
                infile >> studentGrades[i][j];
            cout << studentID[i] << endl;
        }
        return;
    }

HomeworkGradeAnalysis.h

    #ifndef HOMEWORKGRADEANALYSIS_H
    #define HOMEWORKGRADEANALYSIS_H

    const int maxgrades = 10;

    #include <fstream>

    using namespace std;

    void BuildArray(ifstream&, int studentGrades[][maxgrades], int&, string studentID[]);
    void AnalyzeGrade();
    void WriteOutput();

    #endif

The text files are of simple format:

    Boole, George   98    105    0    0    0    100    94    95    97    100

Each line is like this, with varying number of students.

What is another method to which I can still stream the names of the students while still using an array?

Answer 1

An Array must be declared with a constant value, you can't use a variable. if you want to declared it using variables you must use a dynamically allocated array.

string studentID[numStudents]; //wrong

string *studentID = new string[numStudents]; //right

EDIT: Be sure to free the array once your finished with it

delete [] studentID

Answer 2

Variable-lengthed arrays is not standard feature of the language. You have to allocate on the heap or create a vector or use a constant instead.

Besides. I got this error message from Clang, whereas g++-4.9 does't give it to me and compilation is ok. So it's compiler-dependent.