...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Property tree resembles (almost is) a standard container with value type of
pair<string, ptree>
.
It has the usual member functions, such as insert
,
push_back
,
find
,
erase
,
etc. These can of course be used to populate and access the tree. For example
the following code adds key "pi"
with data (almost) equal to mathematical pi value:
ptree
pt; pt.push_back
(ptree
::value_type
("pi",ptree
("3.14159")));
To find the value of pi
we
might do the following:
ptree
::const_iterator
it = pt.find
("pi"); double pi = boost::lexical_cast<double>(it->second.data
());
This looks quite cumbersome, and would be even more so if pi
value was not stored so near the top of the tree, and we cared just a little
bit more about errors. Fortunately, there is another, correct way of doing
it:
ptree pt; pt.put
("pi", 3.14159); // put double double pi = pt.get
<double>("pi"); // get double
It doesn't get simpler than that. Basically, there are 2 families of member
functions, get
and put
,
which allow intuitive access to data stored in the tree (direct children or
not).
There are three versions of get: get, get (default-value version), and get_optional, which differ by failure handling strategy. All versions take path specifier, which determines in which key to search for a value. It can be a single key, or a path to key, where path elements are separated with a special character (a '.' if not specified differently). For example debug.logging.errorlevel might be a valid path with dot as a separator.
get
):
This call locates the proper node in the tree and tries to translate its data string to a float value. If that fails, exception is thrown. If path does not exist, it will beptree
pt; /* ... */ float v = pt.get
<float>("a.path.to.float.value");
ptree_bad_path
exception. If value could not be translated, it will be ptree_bad_data
.
Both of them derive from ptree_error
to make common handling possible.
get
):
It will do the same as above, but if it fails, it will return the default value specified by second parameter (here -1.f) instead of throwing. This is very useful in common situations where one wants to allow omitting of some keys. Note that type specification needed in throwing version is normally not necessary here, because type is determined by the default value parameter.ptree
pt; /* ... */ float v = pt.get
("a.path.to.float.value", -1.f);
get_optional
):
This version uses boost::optional class to handle extraction failure. On successful extraction, it will return boost::optional initialized with extracted value. Otherwise, it will return uninitialized boost::optional.ptree
pt; /* ... */ boost::optional<float> v = pt.get_optional
<float>("a.path.to.float.value");
To retrieve a value from this tree (not some subkey), use get_value
,
get_value
(default-value version), and get_value_optional
.
They have identical semantics to get
functions, except they don't take the path
parameter. Don't call get
with and empty path
to do this as it will try to extract contents of subkey with empty name.
To use a separator character other than default '.
', you
need to construct a path object explicitly. The path type for a ptree
is a string_path instantiation,
so the easiest way to refer to it is ptree
::path_type.
This way you can use trees that have dots in their keys:
typedef ptree::path_type path; pt.get<float>(path("p.a.t.h/t.o/v.a.l.u.e", '/')); pt.get(path("p.a.t.h/t.o/v.a.l.u.e", '/'), 0, NULL); pt.get_optional<std::string>(path("p.a.t.h/t.o/v.a.l.u.e", '/'));
Note: the special overloads of get
and get_optional
taking a separator character that existed in pre-release versions of PropertyTree
have been removed. This is because the overloads conflicted with using per-call
data translators.
To complement get
,
there are put
and add
.
Contrary to get
,
they have only one variant each. This is because there is no need to deal with
missing values when adding data. If the supplied value cannot be converted
to the tree's data type, the functions will throw ptree_bad_data
.
ptree
pt; pt.put
("a.path.to.float.value", 3.14f); // Overwrites the value pt.put
("a.path.to.float.value", 2.72f); // Adds a second node with the new value. pt.add
("a.path.to.float.value", 3.14f);
Calling put
will insert a new value at specified path, so that a call to get
specifying the same path will retrieve it. Further, put
will insert any missing path elements during path traversal. For example, calling
put("key1.key2.key3", 3.14f)
on an empty tree will insert three new children: key1
,
key1.key2
and key1.key2.key3
. The last one will receive a string
"3.14"
as data, while
the two former ones will have empty data strings. put
always inserts new keys at the back of the existing sequences. The difference
between put
and add
is that put will overwrite existing values if there are any, while add will
create a new node to hold the value even if the specified path references an
existing node.
Similar to get_value
,
there is also a put_value
function. It does the same for this property tree what put
does for its children. Thus, it does not receive a path
:
ptree
pt; pt.put_value
(3.14f);
There is no add_value function.