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Like there is with normal C++ expressions.Last requirement is you will have to have a budget. There is no auto-evaluation of a Boost.YAP expression It is important to remember that Boost.YAP expressions areĪll-lazy, all the time. Let's make a Boost.YAP plus-expression manually:Īuto left = boost :: yap :: make_terminal ( 1 ) auto right = boost :: yap :: make_terminal ( 41 ) auto expr = boost :: yap :: make_expression ( left, right ) Īuto result = boost :: yap :: evaluate ( expr ) std :: cout << result << ' ' // prints '42' But we can still use it with the Boost.YAP Ok, so it's not that interesting by itself - minimal_expr Instantiated with the proper template parameters, it produces Expressions. That's a template that models ExpressionTemplate. Template using term = boost :: yap :: terminal Īnd here is how I might use that alias to create the terminal containingīoost :: yap :: expression >, boost :: yap :: expression >, boost :: yap :: expression > yap_expr = term Here, I've declared a template alias to make that easier to type:

Or more of the terminals in the expression we want to capture and evaluate To do this, we create expr_kind :: terminal expressions out of one It does when it sees std :: sqrt ( 3.0 ) + 8.0f. To let the compiler know not just to eagerly evaulate our expression, as If we want to capture an expression using Boost.YAP we have to do something Is interchangable without loss of meaning. Not identical to tree-based ASTs, they're close enough that the terminology Interchangably, and I'll also use the terms 'subexpression' and

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Notice a couple of things here: 1) non-terminals (the top-level plus operationĪnd the call opertion in our example) have tuple elements that are all Boost.YAP expressions, and 2) terminals have tupleĮlements, none of which are Boost.YAP expressionsįrom here on, I'll use the terms 'expression' and 'node' The right operand to the top-level plus operation is of course also a Boost.YAP That they are leaf nodes in our notional AST. The children of the std :: sqrt ( 3.0 ) subexpression are terminals. The remaining operands are the arguments to pass to the callable in thisĬase, there's only one operand after the callable, 3.0. Is the callable entity, in this case a pointer to std :: sqrt. The left operand to the top-level plus operation is itself a Boost.YAP expressionīoost :: yap :: expression >, boost :: yap :: expression >, Its operands are the elements of its boost :: hana :: tuple data member. In this case, the top level expression is analogous to our operator +()ĪST node. Parameter (it's a non-type parameter) this indicates what kind of 'node' Here's the top-level boost :: yap :: expression again with its noisy guts removed: Shape is the same as the expression tree diagram above. That looks like a big mess let's unpack it. Notice that the call expression has two terminals, one for the callable,īoost :: yap :: expression >, boost :: yap :: expression >, boost :: yap :: expression > Is represented by a Boost.YAP expression whose kind is yap :: expr_kind :: plusĪnd the call is represented by a Boost.YAP expression whose kind is yap :: expr_kind :: call. The call to std :: sqrt ( 3.0 ) is its own expression subtree consistingĪ Boost.YAP version of this same tree is: TheĮxpression is a plus operation whose left side is a call to std :: sqrt ( 3.0 ) This tree structure captures all the elements of the original C++ code. The AST for the expression above might be: The compiler sees this and creates some representation of that expression Templates that you write to capture expressions so that they can be transformed What are expression templates anyway? In short, expression templates are

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