GdaHolder: GNOME Data Access 5 manual

## GdaHolder GdaHolder — Container for a single GValue |

## Stability Level

Stable, unless otherwise indicated

## Functions

GdaHolder * | gda_holder_new ()
#define | gda_holder_new_string()
#define | gda_holder_new_boolean()
#define | gda_holder_new_int()
GdaHolder * | gda_holder_new_inline ()
GdaHolder * | gda_holder_copy ()
GType | gda_holder_get_g_type ()
const gchar * | gda_holder_get_id ()
const GValue * | gda_holder_get_value ()
gchar * | gda_holder_get_value_str ()
gboolean | gda_holder_set_value ()
gboolean | gda_holder_set_value_str ()
gboolean | gda_holder_take_value ()
GValue * | gda_holder_take_static_value ()
const GValue * | gda_holder_get_default_value ()
void | gda_holder_set_default_value ()
gboolean | gda_holder_set_value_to_default ()
gboolean | gda_holder_value_is_default ()
void | gda_holder_force_invalid ()
void | gda_holder_force_invalid_e ()
gboolean | gda_holder_is_valid ()
gboolean | gda_holder_is_valid_e ()
void | gda_holder_set_not_null ()
gboolean | gda_holder_get_not_null ()
gboolean | gda_holder_set_source_model ()
GdaDataModel * | gda_holder_get_source_model ()
gboolean | gda_holder_set_bind ()
GdaHolder * | gda_holder_get_bind ()
const GValue * | gda_holder_get_attribute ()
#define | gda_holder_set_attribute_static()
void | gda_holder_set_attribute ()

## Properties

gchar * | description | Read / Write
GdaHolder * | full-bind | Read / Write
GType * | g-type | Read / Write / Construct
gchar * | id | Read / Write
gchar * | name | Read / Write
gboolean | not-null | Read / Write
GdaHolder * | simple-bind | Read / Write
gint | source-column | Read / Write
GdaDataModel * | source-model | Read / Write
gboolean | validate-changes | Read / Write

## Signals

void | attribute-changed | Run First
void | changed | Run First
void | source-changed | Run First
GError* | validate-change | Run Last

## Types and Values

| GdaHolder

## Object Hierarchy

        GObject
        ╰── GdaHolder

## Implemented Interfaces

GdaHolder implements GdaLockable.

## Includes

    #include <libgda-report/gda-report-engine.h>

## Description

The GdaHolder is a container for a single GValue value. It also specifies various attributes of the contained value (default value, ...)

The type of a GdaHolder has to be set and cannot be modified, except if it's initialized with a GDA_TYPE_NULL GType (representing NULL values) where it can be changed once to a real GType.

Each GdaHolder object is thread safe.

## Functions

### gda_holder_new ()

    GdaHolder *
    gda_holder_new (GType type);

Creates a new holder of type `type`

#### Parameters

type | the GType requested |

#### Returns

a new GdaHolder object

### gda_holder_new_string()

    #define             gda_holder_new_string(id,str)

Creates a new boolean GdaHolder object with an ID set to `id` , and a value initialized to `str` .

#### Parameters

id | a string |
str | a string |

#### Returns

a new GdaHolder

### gda_holder_new_boolean()

    #define             gda_holder_new_boolean(id,abool)

Creates a new boolean GdaHolder object with an ID set to `id` , and a value initialized to `abool` .

#### Parameters

id | a string |
abool | a boolean value |

#### Returns

a new GdaHolder

### gda_holder_new_int()

    #define             gda_holder_new_int(id,anint)

Creates a new boolean GdaHolder object with an ID set to `id` , and a value initialized to `anint` .

#### Parameters

id | a string |
anint | an int value |

#### Returns

a new GdaHolder

### gda_holder_new_inline ()

    GdaHolder *
    gda_holder_new_inline (GType type,
                           const gchar *id,
                           ...);

Creates a new GdaHolder object with an ID set to `id` , of type `type` , and containing the value passed as the last argument.

Note that this function is a utility function and that only a limited set of types are supported. Trying to use an unsupported type will result in a warning, and the returned value holder holding a safe default value.

#### Parameters

type | a valid GLib type |
id | the id of the holder to create, or `NULL`. | [allow-none]
... | value to set |

#### Returns

a new GdaHolder object

### gda_holder_copy ()

    GdaHolder *
    gda_holder_copy (GdaHolder *orig);

Copy constructor.

Note1: if `orig` is set with a static value (see `gda_holder_take_static_value()`) its copy will have a fresh new allocated GValue, so that user should free it when done.

#### Parameters

orig | a GdaHolder object to copy |

#### Returns

a new GdaHolder object.

[transfer full]

### gda_holder_get_g_type ()

    GType
    gda_holder_get_g_type (GdaHolder *holder);

### gda_holder_get_id ()

    const gchar *
    gda_holder_get_id (GdaHolder *holder);

Get the ID of `holder` . The ID can be set using `holder` 's "id" property

#### Parameters

holder | a GdaHolder object |

#### Returns

the ID (don't modify the string).

### gda_holder_get_value ()

    const GValue *
    gda_holder_get_value (GdaHolder *holder);

Get the value held into the holder. If `holder` is set to use its default value and that default value is not of the same type as `holder` , then `NULL` is returned.

If `holder` is set to NULL, then the returned value is a GDA_TYPE_NULL GValue.

If `holder` is invalid, then the returned value is `NULL`.

#### Parameters

holder | a GdaHolder object |

#### Returns

the value, or `NULL`.

[allow-none][transfer none]

### gda_holder_get_value_str ()

    gchar *
    gda_holder_get_value_str (GdaHolder *holder,
                              GdaDataHandler *dh);

Same functionality as `gda_holder_get_value()` except that it returns the value as a string (the conversion is done using `dh` if not `NULL`, or the default data handler otherwise).

#### Parameters

holder | a GdaHolder object |
dh | a GdaDataHandler to use, or `NULL`. | [allow-none]

#### Returns

the value, or `NULL`.

[transfer full]

### gda_holder_set_value ()

    gboolean
    gda_holder_set_value (GdaHolder *holder,
                          const GValue *value,
                          GError **error);

Sets the value within the holder. If `holder` is an alias for another holder, then the value is also set for that other holder.

On success, the action of any call to `gda_holder_force_invalid()` is cancelled as soon as this method is called (even if `holder` 's value does not actually change)

If the value is not different from the one already contained within `holder` , then `holder` is not changed and no signal is emitted.

Note1: the `value` argument is treated the same way if it is `NULL` or if it is a GDA_TYPE_NULL value

Note2: if `holder` can't accept the `value` value, then this method returns FALSE, and `holder` will be left in an invalid state.

Note3: before the change is accepted by `holder` , the "validate-change" signal will be emitted (the value of which can prevent the change from happening) which can be connected to to have a greater control of which values `holder` can have, or implement some business rules.

#### Parameters

holder | a GdaHolder object |
value | a value to set the holder to, or `NULL`. | [allow-none]
error | a place to store errors, or `NULL` |

#### Returns

TRUE if value has been set

### gda_holder_set_value_str ()

    gboolean
    gda_holder_set_value_str (GdaHolder *holder,
                              GdaDataHandler *dh,
                              const gchar *value,
                              GError **error);

Same functionality as `gda_holder_set_value()` except that it uses a string representation of the value to set, which will be converted into a GValue first (using default data handler if `dh` is `NULL`).

Note1: if `value` is `NULL` or is the "NULL" string, then `holder` 's value is set to `NULL`. Note2: if `holder` can't accept the `value` value, then this method returns FALSE, and `holder` will be left in an invalid state.

#### Parameters

holder | a GdaHolder object |
dh | a GdaDataHandler to use, or `NULL` |
value | a value to set the holder to, as a string |
error | a place to store errors, or `NULL` |

#### Returns

TRUE if value has been set

### gda_holder_take_value ()

    gboolean
    gda_holder_take_value (GdaHolder *holder,
                           GValue *value,
                           GError **error);

Sets the value within the holder. If `holder` is an alias for another holder, then the value is also set for that other holder.

On success, the action of any call to `gda_holder_force_invalid()` is cancelled as soon as this method is called (even if `holder` 's value does not actually change).

If the value is not different from the one already contained within `holder` , then `holder` is not changed and no signal is emitted.

Note1: if `holder` can't accept the `value` value, then this method returns FALSE, and `holder` will be left in an invalid state.

Note2: before the change is accepted by `holder` , the "validate-change" signal will be emitted (the value of which can prevent the change from happening) which can be connected to to have a greater control of which values `holder` can have, or implement some business rules.

Note3: if user previously set this holder with `gda_holder_take_static_value()` the GValue stored internally will be forgiven and replaced by the `value` . User should then take care of the 'old' static GValue.

#### Parameters

holder | a GdaHolder object |
value | a value to set the holder to. | [transfer full]
error | a place to store errors, or `NULL` |

#### Returns

TRUE if value has been set

### gda_holder_take_static_value ()

    GValue *
    gda_holder_take_static_value (GdaHolder *holder,
                                  const GValue *value,
                                  gboolean *value_changed,
                                  GError **error);

Sets the const value within the holder. If `holder` is an alias for another holder, then the value is also set for that other holder.

The value will not be freed, and user should take care of it, either for its freeing or for its correct value at the moment of query.

If the value is not different from the one already contained within `holder` , then `holder` is not changed and no signal is emitted.

Note1: if `holder` can't accept the `value` value, then this method returns NULL, and `holder` will be left in an invalid state.

Note2: before the change is accepted by `holder` , the "validate-change" signal will be emitted (the value of which can prevent the change from happening) which can be connected to to have a greater control of which values `holder` can have, or implement some business rules.

#### Parameters

holder | a GdaHolder object |
value | a const value to set the holder to |
value_changed | a boolean set with TRUE if the value changes, FALSE elsewhere. |
error | a place to store errors, or `NULL` |

#### Returns

NULL if an error occurred or if the previous GValue was NULL itself. It returns the static GValue user set previously, so that he can free it.

### gda_holder_get_default_value ()

    const GValue *
    gda_holder_get_default_value (GdaHolder *holder);

Get the default value held into the holder. WARNING: the default value does not need to be of the same type as the one required by `holder` .

#### Parameters

holder | a GdaHolder object |

#### Returns

the default value

### gda_holder_set_default_value ()

    void
    gda_holder_set_default_value (GdaHolder *holder,
                                  const GValue *value);

Sets the default value within the holder. If `value` is `NULL` then `holder` won't have a default value anymore. To set a default value to `NULL`, then pass a GValue created using `gda_value_new_null()`.

NOTE: the default value does not need to be of the same type as the one required by `holder` .

#### Parameters

holder | a GdaHolder object |
value | a value to set the holder's default value, or `NULL` |

### gda_holder_set_value_to_default ()

    gboolean
    gda_holder_set_value_to_default (GdaHolder *holder);

Set `holder` 's value to its default value.

#### Parameters

holder | a GdaHolder object |

#### Returns

TRUE if `holder` has got a default value

### gda_holder_value_is_default ()

    gboolean
    gda_holder_value_is_default (GdaHolder *holder);

Tells if `holder` 's current value is the default one.

#### Parameters

holder | a GdaHolder object |

#### Returns

TRUE if `holder` `holder` 's current value is the default one

### gda_holder_force_invalid ()

    void
    gda_holder_force_invalid (GdaHolder *holder);

Forces a holder to be invalid; to set it valid again, a new value must be assigned to it using `gda_holder_set_value()` or `gda_holder_take_value()`.

`holder` 's value is set to `NULL`.

#### Parameters

holder | a GdaHolder object |

### gda_holder_force_invalid_e ()

    void
    gda_holder_force_invalid_e (GdaHolder *holder,
                                GError *error);

Forces a holder to be invalid; to set it valid again, a new value must be assigned to it using `gda_holder_set_value()` or `gda_holder_take_value()`.

`holder` 's value is set to `NULL`.

#### Parameters

holder | a GdaHolder object |
error | a GError explaining why `holder` is declared invalid, or `NULL`. | [allow-none][transfer full]

Since: 4.2.10

### gda_holder_is_valid ()

    gboolean
    gda_holder_is_valid (GdaHolder *holder);

Get the validity of `holder` (that is, of the value held by `holder` )

#### Parameters

holder | a GdaHolder object |

#### Returns

TRUE if `holder` 's value can safely be used

### gda_holder_is_valid_e ()

    gboolean
    gda_holder_is_valid_e (GdaHolder *holder,
                           GError **error);

Get the validity of `holder` (that is, of the value held by `holder` )

#### Parameters

holder | a GdaHolder object |
error | a place to store invalid error, or `NULL`. | [allow-none]

#### Returns

TRUE if `holder` 's value can safely be used

Since: 4.2.10

### gda_holder_set_not_null ()

    void
    gda_holder_set_not_null (GdaHolder *holder,
                             gboolean not_null);

Sets if the holder can have a NULL value. If `not_null` is TRUE, then that won't be allowed

#### Parameters

holder | a GdaHolder object |
not_null | TRUE if `holder` should not accept `NULL` values |

### gda_holder_get_not_null ()

    gboolean
    gda_holder_get_not_null (GdaHolder *holder);

Get wether the holder can be NULL or not

#### Parameters

holder | a GdaHolder object |

#### Returns

TRUE if the holder cannot be NULL

### gda_holder_set_source_model ()

    gboolean
    gda_holder_set_source_model (GdaHolder *holder,
                                 GdaDataModel *model,
                                 gint col,
                                 GError **error);

Sets an hint that `holder` 's values should be restricted among the values contained in the `col` column of the `model` data model. Note that this is just a hint, meaning this policy is not enforced by `holder` 's implementation.

If `model` is `NULL`, then the effect is to cancel ant previous call to `gda_holder_set_source_model()` where `model` was not `NULL`.

#### Parameters

holder | a GdaHolder object |
model | a GdaDataModel object or `NULL` |
col | the reference column in `model` |
error | location to store error, or `NULL` |

#### Returns

TRUE if no error occurred

### gda_holder_get_source_model ()

    GdaDataModel *
    gda_holder_get_source_model (GdaHolder *holder,
                                 gint *col);

If `gda_holder_set_source_model()` has been used to provide a hint that `holder` 's value should be among the values contained in a column of a data model, then this method returns which data model, and if `col` is not `NULL`, then it is set to the restricting column as well.

Otherwise, this method returns `NULL`, and if `col` is not `NULL`, then it is set to 0.

#### Parameters

holder | a GdaHolder |
col | a place to store the column in the model sourcing the holder, or `NULL` |

#### Returns

a pointer to a GdaDataModel, or `NULL`.

[transfer none]

### gda_holder_set_bind ()

    gboolean
    gda_holder_set_bind (GdaHolder *holder,
                         GdaHolder *bind_to,
                         GError **error);

Sets `holder` to change when `bind_to` changes (and does not make `bind_to` change when `holder` changes). For the operation to succeed, the GType of `holder` and `bind_to` must be the same, with the exception that any of them can have a `GDA_TYPE_NULL` type (in this situation, the GType of the two GdaHolder objects involved is set to match the other when any of them sets its type to something different than GDA_TYPE_NULL).

If `bind_to` is `NULL`, then `holder` will not be bound anymore.

#### Parameters

holder | a GdaHolder |
bind_to | a GdaHolder or `NULL` |
error | a place to store errors, or `NULL` |

#### Returns

TRUE if no error occurred

### gda_holder_get_bind ()

    GdaHolder *
    gda_holder_get_bind (GdaHolder *holder);

Get the holder which makes `holder` change its value when the holder's value is changed.

#### Parameters

holder | a GdaHolder |

#### Returns

the GdaHolder or `NULL`.

[transfer none]

### gda_holder_get_attribute ()

    const GValue *
    gda_holder_get_attribute (GdaHolder *holder,
                              const gchar *attribute);

Get the value associated to a named attribute.

Attributes can have any name, but Libgda proposes some default names, see this section.

#### Parameters

holder | a GdaHolder |
attribute | attribute name as a string |

#### Returns

a read-only GValue, or `NULL` if not attribute named `attribute` has been set for `holder`

### gda_holder_set_attribute_static()

    #define             gda_holder_set_attribute_static(holder,attribute,value)

This function is similar to `gda_holder_set_attribute()` but for static strings

#### Parameters

holder | a GdaHolder |
attribute | attribute's name |
value | a GValue, or `NULL`. | [allow-none]

### gda_holder_set_attribute ()

    void
    gda_holder_set_attribute (GdaHolder *holder,
                              const gchar *attribute,
                              const GValue *value,
                              GDestroyNotify destroy);

Set the value associated to a named attribute. The `attribute` string is 'stolen' by this method, and the memory it uses will be freed using the `destroy` function when no longer needed (if `destroy` is `NULL`, then the string will not be freed at all).

Attributes can have any name, but Libgda proposes some default names, see this section.

For example one would use it as:

`gda_holder_set_attribute (holder, g_strdup (my_attribute), my_value, g_free);``gda_holder_set_attribute (holder, GDA_ATTRIBUTE_NAME, my_value, NULL);`

If there is already an attribute named `attribute` set, then its value is replaced with the new value (`value` is copied), except if `value` is `NULL`, in which case the attribute is removed.

#### Parameters

holder | a GdaHolder |
attribute | attribute name |
value | a GValue, or `NULL` |
destroy | a function to be called when `attribute` is not needed anymore, or `NULL` |

## Types and Values

### GdaHolder

    typedef struct _GdaHolder GdaHolder;

## Property Details

### The `“description”` property

      “description”              gchar *

Holder's description.

Flags: Read / Write

Default value: NULL

### The `“full-bind”` property

      “full-bind”                GdaHolder *

Make value holder follow other GdaHolder's changes and the other way around.

Flags: Read / Write

### The `“g-type”` property

      “g-type”                   GType *

Holder's GType.

Flags: Read / Write / Construct

Allowed values: void

### The `“id”` property

      “id”                       gchar *

Holder's ID.

Flags: Read / Write

Default value: NULL

### The `“name”` property

      “name”                     gchar *

Holder's name.

Flags: Read / Write

Default value: NULL

### The `“not-null”` property

      “not-null”                 gboolean

Can the value holder be NULL?.

Flags: Read / Write

Default value: FALSE

### The `“simple-bind”` property

      “simple-bind”              GdaHolder *

Make value holder follow other GdaHolder's changes.

Flags: Read / Write

### The `“source-column”` property

      “source-column”            gint

Column number to use in coordination with the source-model property.

Flags: Read / Write

Allowed values: >= 0

Default value: 0

### The `“source-model”` property

      “source-model”             GdaDataModel *

Data model among which the holder's value should be.

Flags: Read / Write

### The `“validate-changes”` property

      “validate-changes”         gboolean

Defines if the "validate-change" signal gets emitted when the holder's value changes.

Flags: Read / Write

Default value: TRUE

Since: 5.2.0

## Signal Details

### The `“attribute-changed”` signal

    void
    user_function (GdaHolder *holder,
                   gchar     *att_name,
                   GValue    *att_value,
                   gpointer   user_data)

Gets emitted when any `holder` 's attribute has changed

#### Parameters

holder | the GdaHolder |
att_name | attribute's name |
att_value | attribute's value |
user_data | user data set when the signal handler was connected. |

Flags: Run First

### The `“changed”` signal

    void
    user_function (GdaHolder *holder,
                   gpointer   user_data)

Gets emitted when `holder` 's value has changed

#### Parameters

holder | the GdaHolder |
user_data | user data set when the signal handler was connected. |

Flags: Run First

### The `“source-changed”` signal

    void
    user_function (GdaHolder *holder,
                   gpointer   user_data)

Gets emitted when the data model in which `holder` 's values should be has changed

#### Parameters

holder | the GdaHolder |
user_data | user data set when the signal handler was connected. |

Flags: Run First

### The `“validate-change”` signal

    GError*
    user_function (GdaHolder *holder,
                   GValue    *new_value,
                   gpointer   user_data)

Gets emitted when `holder` is going to change its value. One can connect to this signal to control which values `holder` can have (for example to implement some business rules)

#### Parameters

holder | the object which received the signal |
new_value | the proposed new value for `holder` |
user_data | user data set when the signal handler was connected. |

#### Returns

NULL if `holder` is allowed to change its value to `new_value` , or a GError otherwise.

Flags: Run Last

## See Also

The GdaSet object which "groups" several GdaHolder objects