With Single-Value Output: evaluate()

With Single-Value Output: `evaluate()`

The evaluate() method evaluates one server-side expression over the output of a query, expecting exactly one result, and it returns that result. Inevitably the expression will be a call to an aggregate function.

For any query q, you can call q.evaluate(exprn), provided that exprn is an exprn_mapper<T> for some mapped type T. q's value mapper will be visible to exprn.

q.evaluate(exprn) returns a T (i.e. exprn's result type).

If q produces no records, then q.evaluate(exprn) throws no_row_exception.
If exprn produces a single value, when evaluated over the totality of q's output, then q.evaluate(exprn) returns that value.
If exprn produces multiple values, when evaluated over the totality of q's output, then q.evaluate(exprn) throws multi_row_exception.

So, to print the sum of the lengths of all movie titles:

std::cout << movies.evaluate(sum(length(movies->title))) << std::endl;

Standard Evaluations: `size()`, `empty()`

The functions size() and empty() perform two of the most common uses of evaluate().

For any query q:

q.size() returns a uint64_t, equal to the number of records that q would produce.
q.empty() returns a bool, which is true if and only if q would produce no records.