Library Version 18.1.40

Working with primitive types
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## Working with primitive types

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.

### Storing strings

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:

1. 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);

2. 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.

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