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Databases, continued.

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Emmanuel Deloget


The main problem about databases lies in how one can fetch the data from the db. Simply put, storing the result of a SELECT statement is a pain.

Consider this simple table:

CREATE TABLE user (id INTEGER, login TEXT, password TEXT);

Now, a very simple select statement:


The C++ typical code to deal with the result of this query is something along the line of:

if (teh_sql_api_exec_function("SELECT * FROM user;")) {
int rowcount = teh_sql_api_getrowcount();
int colcount = teh_sql_api_getcolcount();
for (int r=0; r for (int c=0; c std::string s = teh_sql_api_getstringvalue(c, r);
my_clever_query_object.setdata(c, r, s);

We just have lost the type of our different fields - our id is not more an integer, it is a string. You can avoid this by doing:

teh_sql_type t = teh_sql_api_getdatatype(c);
if (t == teh_sql_integer) {
int i = teh_sql_api_getintvalue(c, r);
my_clever_query_object.setintdata(c, r, i);
} else if (t == teh_sql_string) {
std:string s = teh_sql_api_getstringvalue(c, r);
my_clever_query_object.setstringdata(c, r, s);
} else {
// ... other cases

We now know the type of our objects but we can't say anymore that the code is generic. We need to have one method per SQL data type in my_clever_query_object (MySQL has many many different types). We also have to store the values in different places, because a string is not a date and an integer is not a float - C++ is bad at mixing oranges and apples. Thus, you'll need one container per type. It means that you'll have to cleverly store your informations in my_clever_query_object to be able to fetch them back when needed (you'll be able to rename it to my_uber_clever_query_object if you succeed).

There is obviously a better solution to this problem - and you already know its name: variants.

You can implement variant in numerous ways. Qt variant is a basic one - everything is done without using templates (at least, it was true in Qt3. I haven't checked this in Qt4). The Qt variant is very easy to use - and believe it or not but they use it to store database results from their SQL drivers, as shown here.

Another solution is to use typelists[1]. Both the Tiny Template Library variant and the boost.variant use this clever technique.

With variants, we are able to store any kind of data in a single container. Our code becomes:

teh_sql_type t = teh_sql_api_getdatatype(c);
my_cOOl_variant v;
if (t == teh_sql_integer) {
v = teh_sql_api_getintvalue(c, r);
} else if (t == teh_sql_string) {
v = teh_sql_api_getstringvalue(c, r);
} else {
// ... other cases
my_clever_query_object.setdata(c, r, v);

That's still a pita, but it's probably the best we can do in C++ :)

[1] typelists are described by A. Alexandrescu in his Modern C++ Design book.
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