To store simple primitive types such as int,
long, double, and so forth, an
additional type parameter for the container class
templates is needed. For example, to store an int
in a db_vector, use this container class:
db_vector<int, ElementHolder<int> >;
To map integers to doubles, use this:
db_map<int, double, ElementHolder<double> >;
To store a char* string with long keys,
use this:
db_map<long, char*, ElementHolder<char*> >;
Use this for const char* strings:
db_map<long, const char*, ElementHolder<const char*> >;
To map one const string to another, use this type:
db_map<const char*, const char*, ElementHolder<const char*> >;
The StlAdvancedFeaturesExample::primitive() method demonstrates more of these examples.
For char* and wchar_t* strings,
_DB_STL_StoreElement() must be called
following partial or total modifications before iterator movement,
container::operator[] or
iterator::operator*/-> calls. Without the
_DB_STL_StoreElement() call, the modified
change will be lost. If storing an new value like this:
*iterator = new_char_star_string;
the call to _DB_STL_StoreElement() is not needed.
Note that passing a NULL pointer to a container of
char* type or passing a
std::string with no contents at all will insert
an empty string of zero length into the database.
The string returned from a container will not live beyond the next
iterator movement call, container::operator[] or
iterator::operator*/-> call.
A db_map::value_type::second_type or
db_map::datatype_wrap should be used
to hold a reference to a container::operator[]
return value. Then the reference should be used for repeated
references to that value. The *iterator is of type
ElementHolder<char *>, which can be automatically converted to a
char * pointer using its type conversion operator.
Wherever an auto conversion is done by the compiler, the conversion
operator of ElementHolder<T> is called. This
avoids almost all explicit conversions, except for two use cases:
The *iterator is used as a "..." parameter like this:
printf("this is the special case %s", *iterator);
This compiles but causes errors. Instead, an explicit cast should be used:
printf("this is the special case %s", (char *)*iterator);
For some old compilers, such as gcc3.4.6, the *iterator cannot be
used with the ternary ? operator, like
this:
expr ? *iterator : var
Even when var is the same
type as the iterator's value_type, the
compiler fails to perform an auto conversion.
When using std::string or
std::wstring as the data type for dbstl
containers — that is, db_vector<string>,
and db_map<string, wstring> — the
string's content rather than the string object itself is stored in order
to maintain persistence.
You can find example code demonstrating string storage in the
StlAdvancedFeaturesExample::char_star_string_storage() and
StlAdvancedFeaturesExample::storing_std_strings() methods.