2011-09-13 17:44:24 +00:00
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< p > Library Version 11.2.5.2< / p >
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2011-12-20 00:07:10 +00:00
< a href = "stl_misc.html#id3946759" > Special notes about trivial methods< / a >
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2011-12-20 00:07:10 +00:00
< a href = "stl_misc.html#id3946773" > Using correct container and iterator public types< / a >
2011-09-13 17:44:24 +00:00
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2011-12-20 00:07:10 +00:00
< h3 class = "title" > < a id = "id3946759" > < / a > Special notes about trivial methods< / h3 >
2011-09-13 17:44:24 +00:00
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There are some standard STL methods which are meaningless in dbstl, but they are kept in
dbstl as no-ops so as to stay consistent with the standard. These are:
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< code class = "methodname" > db_vecter::reserve();< / code >
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< code class = "methodname" > db_vector::max_size();< / code >
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< code class = "methodname" > db_vector::capacity();< / code >
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< code class = "methodname" > db_map::reserve();< / code >
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< code class = "methodname" > db_map::max_size();< / code >
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< code class = "methodname" > db_vector< > ::max_size()< / code > and
< code class = "methodname" > db_map< > ::max_size()< / code > both return 2^30. This does not mean
that Berkeley DB can only hold that much data. This value is returned to conform to some
compilers' overflow rules — if we set bigger numbers like 2^32 or 2^31, some compilers
complain that the number has overflowed.
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See the Berkeley DB documentation for information about limitations on how much data a
database can store.
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There are also some read-only functions. You set the configuration
for these using the Berkeley DB API. You access them using the container's methods. Again,
this is to keep consistent with C++ standard STL containers, such as:
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< code class = "methodname" > db_map::key_comp();< / code >
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< code class = "methodname" > db_map::value_comp();< / code >
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< code class = "methodname" > db_map::hash_funct();< / code >
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< code class = "methodname" > db_map::key_eq();< / code >
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2011-12-20 00:07:10 +00:00
< h3 class = "title" > < a id = "id3946773" > < / a > Using correct container and iterator public types< / h3 >
2011-09-13 17:44:24 +00:00
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All public types defined by the C++ STL specification are present in dbstl. One thing to
note is the < span class = "bold" > < strong > value_type< / strong > < / span > . dbstl defines the < span class = "bold" > < strong > value_type< / strong > < / span > for each iterator and container class to be the raw
type without the < code class = "classname" > ElementRef< / code > /< code class = "classname" > ElementHolder< / code >
wrapper, so this type of variable can not be used to store data in a database. There is a
< span class = "bold" > < strong > value_type_wrap< / strong > < / span > type for each container and iterator type,
with the raw type wrapped by the
< code class = "classname" > ElementRef< / code > /< code class = "classname" > ElementHolder< / code > .
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For example, when type < code class = "literal" > int_vector_t< / code > is defined as
< / p >
< pre class = "programlisting" > db_vector< int, ElementHolder< int> > < / pre >
< p >
its < span class = "bold" > < strong > value_type< / strong > < / span > is < code class = "literal" > int< / code > , its
< span class = "bold" > < strong > value_type_wrap< / strong > < / span > is < code class = "literal" > ElementHolder< int> < / code > ,
and its reference and pointer types are < code class = "literal" > ElementHolder< int> & < / code > and
< code class = "literal" > ElementHolder< int> *< / code > respectively. If you need to store data, use
< span class = "bold" > < strong > value_type_wrap< / strong > < / span > to make use of the wrapper to store data
into database.
< / p >
< p >
The reason we leave < span class = "bold" > < strong > value_type< / strong > < / span > as the raw type is that we
want the existing algorithms in the STL library to work with dbstl because we have seen that
without doing so, a few tests will fail.
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You need to use the same type as the return type of the data element retrieval functions to
hold a value in order to properly manipulate the data element. For example, when calling
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< pre class = "programlisting" > db_vector< T> ::operator[]< / pre >
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check that the return type for this function is
< / p >
< pre class = "programlisting" > db_vector< T> ::datatype_wrap< / pre >
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Then, hold the return value using an object of the same type:
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< pre class = "programlisting" > db_vector< T> ::datatype_wrap refelem = vctr[3];< / pre >
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