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# Creating a BSON Document

## The bson_t structure

BSON documents are created using the bson_t structure. This structure encapsulates the necessary logic for encoding using the BSON Specification. At the core, bson_t is a buffer manager and set of encoding routines.

Tip

BSON documents can live on the stack or the heap based on the performance needs or preference of the consumer.

Let’s start by creating a new BSON document on the stack. Whenever using libbson, make sure you `#include <bson/bson.h>`.

    bson_t b;

    bson_init (&b);

This creates an empty document. In JSON, this would be the same as `{}`.

We can now proceed to adding items to the BSON document. A variety of functions prefixed with `bson_append_` can be used based on the type of field you want to append. Let’s append a UTF-8 encoded string.

    bson_append_utf8 (&b, "key", -1, "value", -1);

Notice the two `-1` parameters. The first indicates that the length of `key` in bytes should be determined with `strlen()`. Alternatively, we could have passed the number `3`. The same goes for the second `-1`, but for `value`.

Libbson provides macros to make this less tedious when using string literals. The following two appends are identical.

    bson_append_utf8 (&b, "key", -1, "value", -1);
    BSON_APPEND_UTF8 (&b, "key", "value");

Now let’s take a look at an example that adds a few different field types to a BSON document.

    bson_t b = BSON_INITIALIZER;

    BSON_APPEND_INT32 (&b, "a", 1);
    BSON_APPEND_UTF8 (&b, "hello", "world");
    BSON_APPEND_BOOL (&b, "bool", true);

Notice that we omitted the call to bson_init(). By specifying `BSON_INITIALIZER` we can remove the need to initialize the structure to a base state.

## Sub-Documents and Sub-Arrays

To simplify the creation of sub-documents bson_append_document_begin() can be used to build a sub-document using the parent’s memory region as the destination buffer.

    bson_t parent = BSON_INITIALIZER;
    bson_t child;

    bson_append_document_begin (&parent, "foo", 3, &child);
    bson_append_int32 (&child, "baz", 3, 1);
    bson_append_document_end (&parent, &child);

    char *str = bson_as_relaxed_extended_json (&parent, NULL);
    printf ("%s\n", str); // Prints: { "foo" : { "baz" : 1 } }
    bson_free (str);

    bson_destroy (&parent);

To simplify the creation of sub-arrays bson_array_builder_t can be used to build a sub-array using the parent’s memory region as the destination buffer.

    bson_t parent = BSON_INITIALIZER;
    bson_array_builder_t *bab;

    bson_append_array_builder_begin (&parent, "foo", 3, &bab);
    bson_array_builder_append_int32 (bab, 9);
    bson_array_builder_append_int32 (bab, 8);
    bson_array_builder_append_int32 (bab, 7);
    bson_append_array_builder_end (&parent, bab);

    char *str = bson_as_relaxed_extended_json (&parent, NULL);
    printf ("%s\n", str); // Prints: { "foo" : [ 9, 8, 7 ] }
    bson_free (str);

    bson_destroy (&parent);

## Simplified BSON C Object Notation

Creating BSON documents by hand can be tedious and time consuming. BCON, or BSON C Object Notation, was added to allow for the creation of BSON documents in a format that looks closer to the destination format.

The following example shows the use of BCON. Notice that values for fields are wrapped in the `BCON_*` macros. These are required for the variadic processor to determine the parameter type.

    bson_t *doc;

    doc = BCON_NEW ("foo",
                    "{",
                    "int",
                    BCON_INT32 (1),
                    "array",
                    "[",
                    BCON_INT32 (100),
                    "{",
                    "sub",
                    BCON_UTF8 ("value"),
                    "}",
                    "]",
                    "}");

Creates the following document

    { "foo" : { "int" : 1, "array" : [ 100, { "sub" : "value" } ] } }