# struct
> Fixed-width composite data type (a.k.a., a `struct`).
## Usage
```javascript
var struct = require( '@stdlib/dstructs/struct' );
```
#### struct( schema )
Returns a constructor for creating a fixed-width composite data type (a.k.a., a `struct`).
```javascript
var fields = [
{
'type': 'union',
'fields': [
{
'name': 'double',
'description': 'double-precision floating-point number',
'type': 'float64',
'enumerable': true,
'writable': true,
'castingMode': 'none'
},
{
'name': 'words',
'description': 'high and low words',
'type': 'uint32',
'length': 2,
'enumerable': true,
'writable': true,
'castingMode': 'none'
}
]
}
];
var Struct = struct( fields );
var data = {
'double': 3.14
};
var s = new Struct( data );
// returns
var v = s.double;
// returns 3.14
var w = s.words;
// e.g., [ 1374389535, 1074339512 ]
```
TODO: document schema
TODO: document constructor API
## Notes
- While struct instances aim to emulate C `struct` behavior, structs have the following differences:
- The struct schema supports default values. In C, uninitialized members are zero-filled.
- The struct schema supports "casting modes", which govern member assignment operations and support placing (or relaxing) strict requirements on what types of values may be assigned to struct members. In C, members have no such limitations, with the C compiler performing implicit casting (e.g., a signed integer will be implicitly cast to an unsigned integer when a member has an unsigned integer data type).
- Only fixed-width types are supported (e.g., `int8`, `float64`, etc). In C, members can have types which may vary across platforms (e.g., `int`, `enum`, `long double`, etc).
- Member types must be serializable to an ArrayBuffer (e.g., no functions, general objects, etc). In C, members have no such limitations (e.g., a member can be a function pointer).
- Union types must all have the same byte length. In C, members of union types can have different byte lengths.
- struct instances can have "hidden" (i.e., non-enumerable) fields. In C, all members are part of a `struct`'s public API.
- struct instances can have read-only fields. In C, one can use a `const` qualifier to prevent assignment after initialization; however, one can circumvent this restriction using pointer tricks.
- Member types can have an associated description, which is useful when wanting to inspect struct instances in interactive contexts, such as REPLs.
- struct instances support string and JSON serialization.
- A primary use case for emulating C `struct` behavior is to facilitate interoperation between JavaScript and C. For example, by creating a JavaScript struct instance which has the same alignment and layout as a C `struct`, one can pass a pointer to a struct instance's underlying byte buffer from JavaScript to C and then simply perform a cast to interpret as an equivalent C `struct`. This enables zero-copy interchange when, e.g., calling into Node.js native native add-ons and can greatly reduce overhead when working with heterogeneous composite data types.
## Examples
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var struct = require( '@stdlib/dstructs/struct' );
var fields = [
{
'name': 'rejected',
'description': 'boolean indicating whether the null hypothesis was rejected',
'type': 'bool',
'enumerable': true,
'writable': false,
'castingMode': 'none'
},
{
'name': 'alpha',
'description': 'significance level',
'type': 'float64',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'pValue',
'description': 'p-value',
'type': 'float64',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'statistic',
'description': 'test statistic',
'type': 'float64',
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'ci',
'description': 'confidence interval',
'type': 'float64',
'length': 2,
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'df',
'description': 'degrees of freedom',
'type': 'int32',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'nullValue',
'description': 'null value',
'type': 'float64',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'mean',
'description': 'computed mean',
'type': 'float64',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
},
{
'name': 'sd',
'description': 'standard error of the mean',
'type': 'float64',
'enumerable': true,
'writable': false,
'castingMode': 'mostly-safe'
}
];
var Struct = struct( fields );
var s = new Struct({
'rejected': true,
'alpha': 0.05,
'pValue': 0.01,
'statistic': 3.14,
'ci': new Float64Array( [ -5.0, 5.0 ] ),
'df': 10,
'nullValue': 1.0,
'mean': 1.01,
'sd': 0.025
});
// returns
var byteLength = Struct.byteLength;
console.log( 'Byte length: %d', byteLength );
var alignment = Struct.alignment;
console.log( 'Alignment: %d', alignment );
var names = Struct.fields;
console.log( 'Field names: %s', names.join( ', ' ) );
var str = s.toString({
'format': 'linear'
});
console.log( 'String:\n%s', str );
var o = s.toJSON();
console.log( o );
var offset = Struct.byteOffsetOf( 'alpha' );
console.log( 'Offset: %d', offset );
var desc = Struct.descriptionOf( 'alpha' );
console.log( 'Description: %s', desc );
```