Class FieldContext

src/sys/expressionparser.cxx:101–134

Define a new context to evaluate an expression in This is essentially a dynamic scope mechanism

/// Define a new context to evaluate an expression in
/// This is essentially a dynamic scope mechanism
class FieldContext : public FieldGenerator {
public:
  using variable_list = std::vector<std::pair<string, FieldGeneratorPtr>>;

  /// Create with a list of context variables to modify
  /// and an expression to evaluate in that new context
  FieldContext(variable_list variables, FieldGeneratorPtr expr)
      : variables(std::move(variables)), expr(std::move(expr)) {}

  double generate(const Context& ctx) override {
    // Create a new context
    Context new_context{ctx};

    // Set values in the context by evaluating the generators
    for (auto const& var : variables) {
      new_context.set(var.first, var.second->generate(ctx));
    }

    // Evaluate the expression in the new context
    return expr->generate(new_context);
  }
  std::string str() const override {
    auto result = "["s;
    for (auto const& var : variables) {
      result += var.first + "="s + var.second->str() + ","s;
    }
    result += "]("s + expr->str() + ")"s;
    return result;
  }

private:
  variable_list variables; ///< A list of context variables to modify
  FieldGeneratorPtr expr;  ///< The expression to evaluate in the new context
};

/// Sum expressions in a loop, given symbol, count and expression
class FieldSum : public FieldGenerator {