While parsing input, it is often desirable to combine some constant elements with variable parts. For instance, let us look at the example of parsing or formatting a complex number, which is written as (real, imag), where real and imag are the variables representing the real and imaginary parts of our complex number. This can be achieved by writing:

'(' >> double_ >> ", " >> double_ >> ')'

Literals (such as '(' and ", ") do not expose any attribute (well actually, they do expose the special type unused_type, but in this context unused_type is interpreted as if the component does not expose any attribute at all). It is very important to understand that the literals don't consume any of the elements of a fusion sequence passed to this component sequence. As said, they just don't expose any attribute and don't produce (consume) any data. The following example shows this:

// the following parses "(1.0, 2.0)" into a pair of double
std::string input("(1.0, 2.0)");
std::string::iterator strbegin = input.begin();
std::pair<double, double> p;
x3::parse(strbegin, input.end(),
    '(' >> x3::double_ >> ", " >> x3::double_ >> ')', // parser grammar
    p);                                               // attribute to fill while parsing

where the first element of the pair passed in as the data to generate is still associated with the first double_, and the second element is associated with the second double_ parser.

This behavior should be familiar as it conforms to the way other input and output formatting libraries such as scanf, printf or boost::format are handling their variable parts. In this context you can think about spirit_x3's primitive components (such as the double_ above) as of being type safe placeholders for the attribute values.

	Tip
	Similarly to the tip provided above, this example could be rewritten using Spirit's multi-attribute API function: double d1 = 0.0, d2 = 0.0; x3::parse(begin, end, '(' >> x3::double_ >> ", " >> x3::double_ >> ')', d1, d2); which provides a clear and comfortable syntax, more similar to the placeholder based syntax as exposed by printf or boost::format.

Let's take a look at this from a more formal perspective:

a: A, b: Unused --> (a >> b): A

which reads as:

This rule is the key to the understanding of the attribute handling in sequences as soon as literals are involved. It is as if elements with unused_type attributes 'disappeared' during attribute propagation. Notably, this is not only true for sequences but for any compound components. For instance, for alternative components the corresponding rule is:

a: A, b: Unused --> (a | b): A

again, allowing to simplify the overall attribute type of an expression.