Let’s say you have the following SQL table:

CREATE TABLE stock (
    item CHAR(30) NOT NULL,
    num BIGINT NOT NULL,
    weight DOUBLE NOT NULL,
    price DECIMAL(6,2) NOT NULL,
    sdate DATE NOT NULL,
    description MEDIUMTEXT NULL)

You can create a C++ structure corresponding to this table like so:

sql_create_6(stock, 1, 6,
    mysqlpp::sql_char, item,
    mysqlpp::sql_bigint, num,
    mysqlpp::sql_double, weight,
    mysqlpp::sql_decimal, price,
    mysqlpp::sql_date, sdate,
    mysqlpp::Null<mysqlpp::sql_mediumtext>, description)

This declares the stock structure, which has a data member for each SQL column, using the same names. The structure also has a number of member functions, operators and hidden data members, but we won’t go into that just now.

The parameter before each field name in the sql_create_# call is the C++ data type that will be used to hold that value in the SSQLS. MySQL++ defines C++ types for almost every data type MySQL understands, of the form sql_*. We’re using them for all of the columns above for reasons given in Section 3.6, “C++ Equivalents of SQL Column Types”. While you could use plain old C++ data types for most of these columns (long int instead of mysqlpp::sql_bigint, for example) the MySQL++ equivalents have several advantages.

Consider the description field definition above. It uses two MySQL++ features in combination to express a data type that has no exact C++ type equivalent. The difference between SQL’s MEDIUMTEXT and C++’s std::string type is small, so MySQL++’s typedef just aliases the two. However, there’s no equivalent of SQL’s “null” in the C++ type system. MySQL++ offers the Null template, which bridges this difference between the two type systems. For more on this topic, see Section 3.7, “Handling SQL Nulls”.

The general format of this set of macros is:

sql_create_#(NAME, COMPCOUNT, SETCOUNT, TYPE1, ITEM1, ... TYPE#, ITEM#)

Where # is the number of member variables, NAME is the name of the structure you wish to create, TYPEx is the type of a member variable, and ITEMx is that variable’s name.

The COMPCOUNT and SETCOUNT arguments are described in the next section.

The sql_create_# macro adds member functions and operators to each SSQLS that allow you to compare one SSQLS instance to another. These functions compare the first COMPCOUNT fields in the structure. In the example above, COMPCOUNT is 1, so only the item field will be checked when comparing two stock structures.

This feature works best when your table’s “key” fields are the first ones in the SSQLS and you set COMPCOUNT equal to the number of key fields. That way, a check for equality between two SSQLS structures in your C++ code will give the same results as a check for equality in SQL.

COMPCOUNT must be at least 1. The current implementation of sql_create_# cannot create an SSQLS without comparison member functions.

Because our stock structure is less-than-comparable, you can use it in STL algorithms and containers that require this, such as STL’s associative containers:

std::set<stock> result;   
query.storein(result);
cout << result.lower_bound(stock("Hamburger"))->item << endl;

This will print the first item in the result set that begins with “Hamburger.”

The third parameter to sql_create_# is SETCOUNT. If this is nonzero, it adds an initialization constructor and a set() member function taking the given number of arguments, for setting the first N fields of the structure. For example, you could change the above example like so:

sql_create_6(stock, 1, 2,
    mysqlpp::sql_char, item,
    mysqlpp::sql_bigint, num,         
    mysqlpp::sql_double, weight,  
    mysqlpp::sql_decimal, price,  
    mysqlpp::sql_date, sdate,
    mysqlpp::Null<mysqlpp::sql_mediumtext>, description)
    
stock foo("Hotdog", 52);

In addition to this 2-parameter constructor, this version of the stock SSQLS will have a similar 2-parameter set() member function.

The COMPCOUNT and SETCOUNT values cannot be equal. If they are, the macro will generate two initialization constructors with identical parameter lists, which is illegal in C++. You might be asking, why does there need to be a constructor for comparison to begin with? It’s often convenient to be able to say something like x == stock("Hotdog"). This requires that there be a constructor taking COMPCOUNT arguments to create the temporary stock instance used in the comparison.

This limitation is not a problem in practice. If you want the same number of parameters in the initialization constructor as the number of fields used in comparisons, pass 0 for SETCOUNT. This suppresses the duplicate constructor you’d get if you used the COMPCOUNT value instead. This is most useful in very small SSQLSes, since it’s easier for the number of key fields to equal the number of fields you want to compare on:

sql_create_1(stock_item, 1, 0, mysqlpp::sql_char, item)

Let’s put SSQLS to use. This is examples/ssqls1.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>
#include <vector>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    const char* db = 0, *server = 0, *user = 0, *pass = "";
    if (!parse_command_line(argc, argv, &db, &server, &user, &pass)) {
        return 1;
    }

    try {                       
        // Establish the connection to the database server.
        mysqlpp::Connection con(db, server, user, pass);

        // Retrieve a subset of the stock table's columns, and store
        // the data in a vector of 'stock' SSQLS structures.  See the
        // user manual for the consequences arising from this quiet
        // ability to store a subset of the table in the stock SSQLS.
        mysqlpp::Query query = con.query("select item,description from stock");
        vector<stock> res;
        query.storein(res);

        // Display the items
        cout << "We have:" << endl;
        vector<stock>::iterator it;
        for (it = res.begin(); it != res.end(); ++it) {
            cout << '\t' << it->item;
            if (it->description != mysqlpp::null) {
                cout << " (" << it->description << ")";
            }
            cout << endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {
        // Handle bad conversions; e.g. type mismatch populating 'stock'
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

Here is the stock.h header used by that example, and by several others below:

#include <mysql++.h>
#include <ssqls.h>

// The following is calling a very complex macro which will create
// "struct stock", which has the member variables:
//
//   sql_char item;
//   ...
//   Null<sql_mediumtext> description;
//
// plus methods to help populate the class from a MySQL row.  See the
// SSQLS sections in the user manual for further details.
sql_create_6(stock,
    1, 6, // The meaning of these values is covered in the user manual
    mysqlpp::sql_char, item,
    mysqlpp::sql_bigint, num,
    mysqlpp::sql_double, weight,
    mysqlpp::sql_double, price,
    mysqlpp::sql_date, sdate,
    mysqlpp::Null<mysqlpp::sql_mediumtext>, description)

This example produces the same output as simple1.cpp (see Section 3.2, “A Simple Example”), but it uses higher-level data structures paralleling the database schema instead of MySQL++’s lower-level generic data structures. It also uses MySQL++’s exceptions for error handling instead of doing everything inline. For small example programs like these, the overhead of SSQLS and exceptions doesn’t pay off very well, but in a real program, they end up working much better than hand-rolled code.

Notice that we are only pulling a single column from the stock table, but we are storing the rows in a std::vector<stock>. It may strike you as inefficient to have five unused fields per record. It’s easily remedied by defining a subset SSQLS:

sql_create_1(stock_subset,
  1, 0,
  string, item)
  
vector<stock_subset> res;
query.storein(res);
// ...etc...

MySQL++ is flexible about populating SSQLSes.^[9] It works much like the Web, a design that’s enabled the development of the largest distributed system in the world. Just as a browser ignores tags and attributes it doesn’t understand, you can populate an SSQLS from a query result set containing columns that don’t exist in the SSQLS. And as a browser uses sensible defaults when the page doesn’t give explicit values, you can have an SSQLS with more fields defined than are in the query result set, and these SSQLS fields will get default values. (Zero for numeric types, false for bool, and a type-specific default for anything more complex, like mysqlpp::DateTime.)

In more concrete terms, the example above is able to populate the stock objects using as much information as it has, and leave the remaining fields at their defaults. Conversely, you could also stuff the results of SELECT * FROM stock into the stock_subset SSQLS declared above; the extra fields would just be ignored.

We’re trading run-time efficiency for flexibility here, usually the right thing in a distributed system. Since MySQL is a networked database server, many uses of it will qualify as distributed systems. You can’t count on being able to update both the server(s) and all the clients at the same time, so you have to make them flexible enough to cope with differences while the changes propagate. As long as the new database schema isn’t too grossly different from the old, your programs should continue to run until you get around to updating them to use the new schema.

There’s a danger that this quiet coping behavior may mask problems, but considering that the previous behavior was for the program to crash when the database schema got out of synch with the SSQLS definition, it’s likely to be taken as an improvement.

SSQLS can also be used to add data to a table. This is examples/ssqls2.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    const char* db = 0, *server = 0, *user = 0, *pass = "";
    if (!parse_command_line(argc, argv, &db, &server, &user, &pass)) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(db, server, user, pass);

        // Create and populate a stock object.  We could also have used
        // the set() member, which takes the same parameters as this
        // constructor.
        stock row("Hot Dogs", 100, 1.5, 1.75,
                mysqlpp::sql_date("1998-09-25"), mysqlpp::null);

        // Form the query to insert the row into the stock table.
        mysqlpp::Query query = con.query();
        query.insert(row);

        // Show the query about to be executed.
        cout << "Query: " << query << endl;

        // Execute the query.  We use execute() because INSERT doesn't
        // return a result set.
        query.execute();

        // Retrieve and print out the new table contents.
        print_stock_table(query);
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {  
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

That’s all there is to it!

There is one subtlety: MySQL++ automatically quotes and escapes the data when building SQL queries using SSQLS structures. It’s efficient, too: MySQL++ is smart enough to quote and escape data only for those data types that actually require it.

Because this example modifies the sample database, you may want to run resetdb after running this program.

It almost as easy to modify data with SSQLS as to add it. This is examples/ssqls3.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    const char* db = 0, *server = 0, *user = 0, *pass = "";
    if (!parse_command_line(argc, argv, &db, &server, &user, &pass)) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(db, server, user, pass);

        // Build a query to retrieve the stock item that has Unicode
        // characters encoded in UTF-8 form.
        mysqlpp::Query query = con.query(
                "select * from stock where item = \"Nürnberger Brats\"");

        // Retrieve the row, throwing an exception if it fails.
        mysqlpp::StoreQueryResult res = query.store();
        if (res.empty()) {
            throw mysqlpp::BadQuery("UTF-8 bratwurst item not found in "
                    "table, run resetdb");
        }

        // Because there should only be one row in the result set,
        // there's no point in storing the result in an STL container.
        // We can store the first row directly into a stock structure
        // because one of an SSQLS's constructors takes a Row object.
        stock row = res[0];

        // Create a copy so that the replace query knows what the
        // original values are.
        stock orig_row = row;

        // Change the stock object's item to use only 7-bit ASCII, and
        // to deliberately be wider than normal column widths printed
        // by print_stock_table().
        row.item = "Nuerenberger Bratwurst";

        // Form the query to replace the row in the stock table.
        query.update(orig_row, row);

        // Show the query about to be executed.
        cout << "Query: " << query << endl;

        // Run the query with execute(), since UPDATE doesn't return a
        // result set.
        query.execute();

        // Retrieve and print out the new table contents.
        print_stock_table(query);
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

Don’t forget to run resetdb after running the example.

SSQLS structures can be sorted and stored in STL associative containers as demonstrated in the next example. This is examples/ssqls4.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    const char* db = 0, *server = 0, *user = 0, *pass = "";
    if (!parse_command_line(argc, argv, &db, &server, &user, &pass)) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(db, server, user, pass);

        // Retrieve all rows from the stock table and put them in an
        // STL set.  Notice that this works just as well as storing them
        // in a vector, which we did in ssqls1.cpp.  It works because
        // SSQLS objects are less-than comparable.
        mysqlpp::Query query = con.query("select * from stock");
        set<stock> res;
        query.storein(res);

        // Display the result set.  Since it is an STL set and we set up
        // the SSQLS to compare based on the item column, the rows will
        // be sorted by item.
        print_stock_header(res.size());
        set<stock>::iterator it;
        cout.precision(3);
        for (it = res.begin(); it != res.end(); ++it) {
            print_stock_row(it->item.c_str(), it->num, it->weight,
                    it->price, it->sdate);
        }

        // Use set's find method to look up a stock item by item name.
        // This also uses the SSQLS comparison setup.
        it = res.find(stock("Hotdog Buns"));
        if (it != res.end()) {
            cout << endl << "Currently " << it->num <<
                    " hotdog buns in stock." << endl;
        }
        else {
            cout << endl << "Sorry, no hotdog buns in stock." << endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

The find() call works because of the way the SSQLS was declared. It’s properly covered elsewhere, but suffice it to say, the "1" in the declaration of stock above tells it that only the first field needs to be checked in comparing two SSQLSes. In database terms, this makes it the primary key. Therefore, when searching for a match, our exemplar only had to have its first field populated.

Up to this point, we haven’t been using all of the features in the SSQLS structures we’ve been generating. What else can we do with SSQLSes? Consider this:

query.insert(s);

This does exactly what you think it does: it builds an INSERT query to insert the contents of s into the database. You have only to call query::execute() to actually insert it. This is possible because SSQLSes have functions that Query can call to get the list of fields and such, which it uses to build the INSERT query. query::update() and query::replace() also rely on these features.

Another feature you might find a use for is changing the table name MySQL++ uses to build queries involving SSQLSes. By default, the database server table is assumed to have the same name as the SSQLS structure type. But if this is inconvenient, you can globally change the table name used in queries like this:

stock::table("MyStockData");

It’s also possible to change the name of a table on a per-instance basis:

stock s;
s.instance_table("AlternateTable");

This is useful when you have an SSQLS definition that is compatible with multiple tables, so the table name to use for each instance is different. The simplest way this can happen is if the tables all have identical definitions; it saves you from having to define a separate SSQLS for each table. It is also useful for mapping a class hierarchy onto a set of table definitions. The common SSQLS definition is the “superclass” for a given set of tables.

Strictly speaking, you only need to use this feature in multithreaded programs. Changing the static table name before using each instance is safe if all changes happen within a single thread. That said, it may still be convenient to change the name of the table for an SSQLS instance in a single-threaded program if it gets used for many operations over an extended span of code.

It’s convenient to define an SSQLS in a header file so you can use it in multiple modules. You run into a bit of a problem, though, because each SSQLS includes a few static data members to hold information common to all structures of that type. (The table name and the list of field names.) When you #include that header in more than one module, you get a multiply-defined symbol error at link time.

The way around this is to define the preprocessor macro MYSQLPP_SSQLS_NO_STATICS in all but one of the modules that use the header definining the SSQLS. When this macro is defined, it suppresses the static data members in any SSQLS defined thereafter.

Imagine we have a file my_ssqls.h which includes a sql_create_N macro call to define an SSQLS, and that that SSQLS is used in at least two modules. One we’ll call foo.cpp, and we’ll say it’s just a user of the SSQLS; it doesn’t “own” it. Another of the modules, my_ssqls.cpp uses the SSQLS more heavily, so we’ve called it the owner of the SSQLS. If there aren’t very many modules, this works nicely:

// File foo.cpp, which just uses the SSQLS, but doesn't "own" it:
#define MYSQLPP_SSQLS_NO_STATICS
#include "my_ssqls.h"

// File my_ssqls.cpp, which owns the SSQLS, so we just #include it directly
#include "my_ssqls.h"

If there are many modules that need the SSQLS, adding all those #defines can be a pain. In that case, it’s easier if you flip the above pattern on its head:

// File my_ssqls.h:
#if !defined(EXPAND_MY_SSQLS_STATICS)
#   define MYSQLPP_SSQLS_NO_STATICS
#endif
sql_create_X(Y, Z....) // the SSQLS definition

// File foo.cpp, a mere user of the SSQLS:
#include "my_ssqls.h"

// File my_ssqls.cpp, which owns the SSQLS:
#define EXPAND_MY_SSQLS_STATICS
#include "my_ssqls.h"

The sql_create macros define several methods for each SSQLS. These methods are mostly for use within the library, but some of them are useful enough that you might want to harness them for your own ends. Here is some pseudocode showing how the most useful of these methods would be defined for the stock structure used in all the ssqls*.cpp examples:

// Basic form
template <class Manip>   
stock_value_list<Manip> value_list(cchar *d = ",",
  Manip m = mysqlpp::quote) const;  

template <class Manip>   
stock_field_list<Manip> field_list(cchar *d = ",",   
  Manip m = mysqlpp::do_nothing) const;  

template <class Manip>   
stock_equal_list<Manip> equal_list(cchar *d = ",",
  cchar *e = " = ", Manip m = mysqlpp::quote) const;  


// Boolean argument form
template <class Manip>
stock_cus_value_list<Manip> value_list([cchar *d, [Manip m,] ]   
  bool i1, bool i2 = false, ... , bool i5 = false) const;  

// List form  
template <class Manip>
stock_cus_value_list<Manip> value_list([cchar *d, [Manip m,] ]  
  stock_enum i1, stock_enum i2 = stock_NULL, ...,
  stock_enum i5 = stock_NULL) const;  

// Vector form  
template <class Manip>
stock_cus_value_list<Manip> value_list([cchar *d, [Manip m,] ]  
  vector<bool> *i) const;  

...Plus the obvious equivalents for field_list() and equal_list()

Rather than try to learn what all of these methods do at once, let’s ease into the subject. Consider this code:

stock s("Dinner Rolls", 75, 0.95, 0.97, sql_date("1998-05-25"));   
cout << "Value list: " << s.value_list() << endl;  
cout << "Field list: " << s.field_list() << endl;  
cout << "Equal list: " << s.equal_list() << endl;

That would produce something like:

Value list: 'Dinner Rolls',75,0.95,0.97,'1998-05-25'   
Field list: item,num,weight,price,sdate  
Equal list: item = 'Dinner Rolls',num = 75,weight = 0.95, price = 0.97,sdate = '1998-05-25'

That is, a “value list” is a list of data member values within a particular SSQLS instance, a “field list” is a list of the fields (columns) within that SSQLS, and an “equal list” is a list in the form of an SQL equals clause.

Just knowing that much, it shouldn’t surprise you to learn that Query::insert() is implemented more or less like this:

*this << "INSERT INTO " << v.table() << " (" << v.field_list() <<
    ") VALUES (" << v.value_list() << ")";

where ‘v’ is the SSQLS you’re asking the Query object to insert into the database.

Now let’s look at a complete example, which uses one of the more complicated forms of equal_list(). This example builds a query with fewer hard-coded strings than the most obvious technique requires, which makes it more robust in the face of change. Here is examples/ssqls5.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>
#include <vector>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    const char* db = 0, *server = 0, *user = 0, *pass = "";
    if (!parse_command_line(argc, argv, &db, &server, &user, &pass)) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(db, server, user, pass);

        // Get all the rows in the stock table.
        mysqlpp::Query query = con.query("select * from stock");
        vector<stock> res;
        query.storein(res);

        if (res.size() > 0) {
            // Build a select query using the data from the first row
            // returned by our previous query.
            query << "select * from stock where " <<
                    res[0].equal_list(" and ", stock_weight, stock_price);

            // Display the finished query.
            cout << "Custom query:\n" << query << endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

This example uses the list form of equal_list(). The arguments stock_weight and stock_price are enum values equal to the position of these columns within the stock table. sql_create_# generates this enum for you automatically.

The boolean argument form of that equal_list() call would look like this:

query << "select * from stock where " <<
    res[0].equal_list(" and ", false, false, true, true, false);

It’s a little more verbose, as you can see. And if you want to get really complicated, use the vector form:

vector<bool> v(5, false);
v[stock_weight] = true;
v[stock_price] = true;
query << "select * from stock where " <<
    res[0].equal_list(" and ", v);

This form makes the most sense if you are building many other queries, and so can re-use that vector object.

Many of these methods accept manipulators and custom delimiters. The defaults are suitable for building SQL queries, but if you’re using these methods in a different context, you may need to override these defaults. For instance, you could use these methods to dump data to a text file using different delimiters and quoting rules than SQL.

At this point, we’ve seen all the major aspects of the SSQLS feature. The final sections of this chapter look at some of the peripheral aspects.

There’s a more advanced SSQLS creation macro, which all the others are built on top of. Currently, the only feature it adds over what’s described above is that it lets you name your SSQLS fields differently from the names used by the database server. Perhaps you want to use Hungarian notation in your C++ program without changing the SQL database schema:

sql_create_complete_5(stock, 1, 5,   
    mysqlpp::sql_char, m_sItem, "item",
    mysqlpp::sql_bigint, m_nNum, "num",
    mysqlpp::sql_double, m_fWeight, "weight",
    mysqlpp::sql_decimal, m_fPrice, "price",
    mysqlpp::sql_date, m_Date, "sdate")

If you ever need to see the code that a given SSQLS declaration expands out to, use the utility doc/ssqls-pretty, like so:

doc/ssqls-pretty < myprog.cpp |less

This Perl script locates the first SSQLS declaration in that file, then uses the C++ preprocessor to expand that macro. (The script assumes that your system’s preprocessor is called cpp, and that its command line interface follows Unix conventions.)

If you run it from the top MySQL++ directory, as shown above, it will use the header files in the distribution’s lib subdirectory. Otherwise, it assumes the MySQL++ headers are in their default location, /usr/include/mysql++. If you want to use headers in some other location, you’ll need to change the directory name in the -I flag at the top of the script.

The SSQLS header ssqls.h is automatically generated by the Perl script ssqls.pl. Although it is possible to change this script to get additional functionality, most of the time it’s better to just derive a custom class from the generated SSQLS to add functionality to it. (See the next section to see how to do this correctly.)

That said, ssqls.pl does have a few configurables you might want to tweak.

The first configurable value sets the maximum number of data members allowed in an SSQLS. It’s 25 out of the box, set by the $max_data_members variable at the top of ssqls.pl. Beware, making this value larger increases the size of ssqls.h exponentially; this will increase compile time, and can even make the file uncompilable due to compiler limits. Before you increase this value, take a good, hard look at your database schema and ask if it’s really the best design. Almost always, having so many columns is a sign that you need to refactor the table.

The second configurable is the default floating point precision used for comparison. As described above (Section 5.2, “SSQLS Comparison and Initialization”) SSQLSes can be compared for equality. The only place this is tricky is with floating-point numbers, since rounding errors can make two “equal” values compare as distinct. This property of floating-point numbers means we almost never want to do exact comparison. MySQL++ lets you specify the precision you want it to use. If the difference between two values is under a given threshold, MySQL++ considers the values equal. The default threshold is 0.00001. This threshold works well for “human” scale values, but because of the way floating-point numbers work, it can be wildly inappropriate for very large or very small quantities like those used in scientific applications.

There are actually two ways to change this threshold. If you need a different system-wide default, edit ssqls.pl and change the $fp_min_delta variable at the top of the file, then rebuild ssqls.h as described below. If you need different thresholds per file or per project, it’s better to set the C macro MYSQLPP_FP_MIN_DELTA instead. The Perl variable sets this macro’s default; if you give a different value before #including ssqls.h, it will use that instead.

To rebuild ssqls.h after changing ssqls.pl, you’ll need a Perl interpreter. The only modern Unixy system I’m aware of where Perl isn’t installed by default is Cygwin, and it’s just a setup.exe choice away there. You’ll probably only have to download and install a Perl interpreter if you’re on Windows and don’t want to use Cygwin.

If you’re on a system that uses autoconf, building MySQL++ automatically updates ssqls.h any time ssqls.pl changes. Otherwise, you’ll need to run the Perl interpreter by hand:

c:\mysql++> cd lib
c:\lib> perl ssqls.pl

Specialized SQL Structures make good base classes. They’re simple, and have few requirements on any class that derives from them. There are some gotchas to look out for, however.

Consider this:

sql_create_2(
  Base, 1, 2,
  mysqlpp::sql_varchar, a,
  mysqlpp::sql_int, b
);

class Derived : public Base
{
public:
  // constructor
  Derived(mysqlpp::sql_varchar _a, mysqlpp::sql_int _b) :
  Base(_a, _b)
  {
  }

  // functionality added to the SSQLS through inheritance
  bool do_something_interesting(int data);
};

We’ve derived a class from an SSQLS in order to add a method to it. Easy, right?

Sadly, too easy. The code has a rather large flaw which makes our derived class unusable as an SSQLS. In C++, if a derived class has a function of the same name as one in the base class, the base class versions of that function are all hidden by those in the derived class. This applies to constructors, too: an SSQLS defines several constructors, but our derived class defines only one, causing that one to hide all of the ones in the base class. Many of the MySQL++ mechanisms that use SSQLSes rely on having these contructors, so our Derived above is-not-a Base, and so it isn’t an SSQLS. If you try to use Derived as an SSQLS, you’ll get compiler errors wherever MySQL++ tries to access one of these other constructors.

There’s another minor flaw, as well. Our lone constructor above takes its parameters by value, but the corresponding constructor in the SSQLS takes them by const reference. Our derived class has technically hidden a fourth base class constructor this way, but this particular case is more a matter of efficiency than correctness. Code that needs the full-creation constructor will still work with our code above, but passing stringish types like sql_varchar by value instead of by const reference is inefficient.

This is the corrected version of the above code:

sql_create_2(
  Base, 1, 2,
  mysqlpp::sql_varchar, a,
  mysqlpp::sql_int, b
);

class Derived : public Base
{
public:
  // default constructor[]
  Derived() :
  Base()
  {
  }

  // for-comparison constructor[]
  Derived(const mysqlpp::sql_varchar& _a) :
  Base(_a)
  {
  }

  // full creation constructor
  Derived(const mysqlpp::sql_varchar& _a, const mysqlpp::sql_int& _b) :
  Base(_a, _b)
  {
  }

  // population constructor[]
  Derived(const mysqlpp::Row& row) :
  Base(row)
  {
  }

  // functionality added to the SSQLS through inheritance
  bool do_something_interesting(int data);
};

Now Derived is-an SSQLS.

You might wonder if you can use protected inheritance above to redefine the SSQLS’s public interface. For instance, OO purists might object to the public data members in an SSQLS. You could encapsulate these public data members in the derived class by using protected inheritance, exposing access to the base class’s data members with public accessor methods. The problem with this is that each SSQLS has dozens of public member functions. These are needed by MySQL++ internals, so unless you re-exposed all of them as we did with the constructors above, you’d again have an SSQLS derivative that is-not-an SSQLS. Simply put, only public inheritance is practical with SSQLSes.

It takes special care to use SSQLS with BLOB columns. It’s safest to declare the SSQLS field as of type mysqlpp::sql_blob. This is currently a typedef alias for String, which is the form the data is in just before the SSQLS mechanism populates the structure. Thus, when the data is copied from the internal MySQL++ data structures into your SSQLS, you get a direct copy of the String object’s contents, without interference.

Because C++ strings handle binary data just fine, you might think you can use std::string instead of sql_blob, but the current design of String converts to std::string via a C string. As a result, the BLOB data is truncated at the first embedded null character during population of the SSQLS. There’s no way to fix that without completely redesigning either String or the SSQLS mechanism.

The sql_blob typedef may be changed to alias a different type in the future, so using it instead of String ensures that your code tracks these library changes automatically. Besides, String is only intended to be an internal mechanism within MySQL++. The only reason the layering is so thin here is because it’s the only way to prevent BLOB data from being corrupted while avoiding that looming redesign effort.

You can see this technique in action in the cgi_jpeg example:

#include <mysql++.h>
#include <ssqls.h>

#define IMG_DATABASE    "mysql_cpp_data"
#define IMG_HOST        "localhost"
#define IMG_USER        "root"
#define IMG_PASSWORD    "nunyabinness"

sql_create_2(images,
    1, 2,
    mysqlpp::sql_int_unsigned, id,
    mysqlpp::sql_blob, data)

int main()
{
    unsigned int img_id = 0;
    char* cgi_query = getenv("QUERY_STRING");
    if (cgi_query) {
        if ((strlen(cgi_query) < 4) || memcmp(cgi_query, "id=", 3)) {
            std::cout << "Content-type: text/plain" << std::endl << std::endl;
            std::cout << "ERROR: Bad query string" << std::endl;
            return 1;
        }
        else {
            img_id = atoi(cgi_query + 3);
        }
    }
    else {
        std::cerr << "Put this program into a web server's cgi-bin "
                "directory, then" << std::endl;
        std::cerr << "invoke it with a URL like this:" << std::endl;
        std::cerr << std::endl;
        std::cerr << "    http://server.name.com/cgi-bin/cgi_jpeg?id=2" <<
                std::endl;
        std::cerr << std::endl;
        std::cerr << "This will retrieve the image with ID 2." << std::endl;
        std::cerr << std::endl;
        std::cerr << "You will probably have to change some of the #defines "
                "at the top of" << std::endl;
        std::cerr << "examples/cgi_jpeg.cpp to allow the lookup to work." <<
                std::endl;
        return 1;
    }

    try {
        mysqlpp::Connection con(IMG_DATABASE, IMG_HOST, IMG_USER,
                IMG_PASSWORD);
        mysqlpp::Query query = con.query();
        query << "SELECT * FROM images WHERE id = " << img_id;
        mysqlpp::UseQueryResult res = query.use();
        if (res) {
            images img = res.fetch_row();
            std::cout << "Content-type: image/jpeg" << std::endl;
            std::cout << "Content-length: " << img.data.length() << "\n\n";
            std::cout << img.data;
        }
        else {
            std::cout << "Content-type: text/plain" << std::endl << std::endl;
            std::cout << "ERROR: No such image with ID " << img_id << std::endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        std::cout << "Content-type: text/plain" << std::endl << std::endl;
        std::cout << "QUERY ERROR: " << er.what() << std::endl;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cout << "Content-type: text/plain" << std::endl << std::endl;
        std::cout << "GENERAL ERROR: " << er.what() << std::endl;
        return 1;
    }

    return 0;
}

SSQLS works on all platforms supported by MySQL++ except for Visual C++ 2003. (Because the rest of MySQL++ works just fine with Visual C++ 2003, we haven’t removed this platform from the supported list entirely.)

If you do need SSQLS and are currently on Visual C++ 2003, you have these options: