How to refactor the C++ code to support the dynamic array size

Question

I need to get rid of fixed array size in C++/ODBC apps. Instead the hardcoded array size (ROWS =10 below) I want to pass the ROWS as the command-line argument. I know how to parse the command line. But how to adjust the following code?

    #define ROWS 10

    SQLINTEGER idata[ROWS]
    SQLCHAR cdata1[ROWS][256]
    SQLFLOAT fdata[ROWS]
    SQL_TIMESTAMP_STRUCT ts[ROWS]

    SQLSetStmtAttr(SQL_ATTR_ROW_ARRAY_SIZE, ROWS)

    SQLBindCol(1, &idata)
    SQLBindCol(2, cdata1)
    SQLBindCol(3, fdata)
    SQLBindCol(4, &ts)
    SQLExecDirect("query producing a result-set")

Update: I cannot modify the signature of SQLBindCol(..)

Let say I will create std::vector instead of SQLFLOAT fdata[ROWS] but how to pass it into SQLBindCol() which does not expect std::vector?

You cannot have variable length arrays in c++.The length of the array must be a compile time constant.You might want to consider using std::vector or std::array instead of the c-style arrays. — Alok Save
– Alok Save, Commented Feb 10, 2012 at 6:28
C++ doesn't provide variable-length arrays, but they certainly don't have to be compile-time constant. That's what new[] is for. e.g. SQLINTEGER * idata = new SQLINTEGER[rows] (and don't forget to delete[] later). The multidimensional array cdata1 is a bigger problem, because that can't be allocated dynamically. The best you can do is an array-of-arrays or a single new SQLCHAR[rows*256], one of which might work depending on how SQLBindCol works here. I don't know enough about ODBC to help more than that. — Derek Park
– Derek Park, Commented Feb 10, 2012 at 6:48

Georg Fritzsche · Accepted Answer · 2012-02-10 07:23:32Z

1

Using std::vector on fdata:

size_t rowCount = /* ... */;
std::vector<SQLFLOAT> fdata(rowCount);
// ...
SQLSetStmtAttr(SQL_ATTR_ROW_ARRAY_SIZE, rowCount);
// ...
SQLBindCol(3, &fdata[0]);
// ...

For cdata1, std::vector<SQLCHAR> cdata1(250 * rowCount); might work.

edited Feb 10, 2012 at 7:23

answered Feb 10, 2012 at 7:16

Georg Fritzsche

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Yavar · Accepted Answer · 2012-02-10 07:22:55Z

0

Use pointers.

Accept the ROWS argument from command line as you said. Allocate memory of desired size (ROWS argument that you get) using dynamic memory allocation by using malloc/new to all the variables you have declared like idata, cdata1 etc. That's it!

answered Feb 10, 2012 at 7:22

Yavar

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Mariusz · Accepted Answer · 2016-01-06 16:44:36Z

I haven't managed to make it work with a 2d dynamic array. But here's a solution for a dynamic 2d array using a 1d array

std::set<std::string> getResults(int columnIndex, HSTMT stmt) {
    std::set<std::string> results;
    RETCODE rc = 0;
    int rows = 250;
    int colsize = 500 + 1; //Need to increment colunm size by 1 because SQLBindCol add the terminator character "\0"

    SQLLEN *indicator1 = new SQLLEN[rows];
    char *buff = new char[rows*colsize];
    SQLSetStmtAttr(stmt, SQL_ATTR_ROW_ARRAY_SIZE, (SQLPOINTER)rows, 0);

    SQLUINTEGER     NumRowsFetched;
    SQLSetStmtAttr(stmt, SQL_ATTR_ROWS_FETCHED_PTR, &NumRowsFetched, 0);

    SQLBindCol(stmt, columnIndex, SQL_C_CHAR, buff, colsize, indicator1);
    while (rc = SQLFetch(stmt) == SQL_SUCCESS) {
        for (int i = 0; (SQLUINTEGER)i < NumRowsFetched; i++) {
            results.insert(&buff[i * colsize]);
        }
    }
    delete[] buff;
    return results;

}

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