As we saw with the Calculator example from the Introduction, the simplest way to get a DSEL up and running is simply to define some terminals, as follows.

// Define a literal integer Proto expression.
proto::terminal<int>::type i = {0};

// This creates an expression template.
i + 1;

With some terminals and Proto's operator overloads, you can immediately start creating expression templates.

Defining terminals -- with aggregate initialization -- can be a little awkward at times. Proto provides an easier-to-use wrapper for literals that can be used to construct Protofied terminal expressions. It's called proto::literal<>.

// Define a literal integer Proto expression.
proto::literal<int> i = 0;

// Proto literals are really just Proto terminal expressions.
// For example, this builds a Proto expression template:
i + 1;

There is also a proto::lit() function for constructing a proto::literal<> in-place. The above expression can simply be written as:

// proto::lit(0) creates an integer terminal expression
proto::lit(0) + 1;