C++ File Reading Library – ANSI and Unicode

1. A pointer can be re-assigned:

   int x = 5;
   int y = 6;
   int *p;
   p = &x;
   p = &y;
   *p = 10;
   assert(x == 5);
   assert(y == 10);
   

   A reference cannot be re-bound, and must be bound at initialization:

   int x = 5;
   int y = 6;
   int &q; // error
   int &r = x;
   

2. A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.

   int x = 0;
   int &r = x;
   int *p = &x;
   int *p2 = &r;
   
   assert(p == p2); // &x == &r
   assert(&p != &p2);
   

3. You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

   int x = 0;
   int y = 0;
   int *p = &x;
   int *q = &y;
   int **pp = &p;
   
   **pp = 2;
   pp = &q; // *pp is now q
   **pp = 4;
   
   assert(y == 4);
   assert(x == 2);
   

4. A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

   /* the code below is undefined; your compiler may optimise it
    * differently, emit warnings, or outright refuse to compile it */
   
   int &r = *static_cast<int *>(nullptr);
   
   // prints "null" under GCC 10
   std::cout
       << (&r != nullptr
           ? "not null" : "null")
       << std::endl;
   
   bool f(int &r) { return &r != nullptr; }
   
   // prints "not null" under GCC 10
   std::cout
       << (f(*static_cast<int *>(nullptr))
           ? "not null" : "null")
       << std::endl;
   

   You can, however, have a reference to a pointer whose value is nullptr.

5. Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.

6. A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..

7. References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)

8. Const references can be bound to temporaries. Pointers cannot (not without some indirection):

   const int &x = int(12); // legal C++
   int *y = &int(12); // illegal to take the address of a temporary.
   

   This makes const & more convenient to use in argument lists and so forth.

You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:

ExcelLibrary

This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.

It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.

ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.

You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.

There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.

Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)

Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License

Here some example code for ExcelLibrary:

Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:

//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");

//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;

//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();

//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();

adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();

//Add the table to the data set
ds.Tables.Add(dt);

//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);

Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.