问题
i was working on an interpreter for a language with a friend, and we started with a decision I'm guessing wasn't that wise: we made all the elements for execution first (practically a tree made of different classes); but now looking at boost examples i get a lot confused about how to merge the two. I know what to start from (the grammar), i know what to reach (instantiated classes owning each other), i don't know how to reach it.
We started with expressions without variables, hence we looked at spirit calculator examples; but i don't understand when to instantiate elements.
Example of expression items:
namespace exp
{
class op
{
private:
public:
virtual double exec(function_scope &fs);
};
class operand : public op
{
private:
double value;
public:
operand(double value);
double exec(function_scope &fs);
};
class op_bin : public op
{
private:
public:
op * ll;
op* rr;
op_bin(op* ll, op* rr);
~op_bin();
};
namespace bin
{
class sum : public op_bin
{
public:
sum(op* ll, op* rr);
double exec(function_scope &fs);
};
}
}
Ignore the exec function, it's used at runtime.
For example the code 5 + (2 + 1) should result in a final equivalent of:
new exp::bin::sum(new exp::operand(5), new exp::bin::sum(new exp::operand(2), new exp::operand(1))
Once i understand how to do that I've practically done.
回答1:
Well, I was going to write what's wrong with your question, but instead I went to prove myself that it is not that hard to make what you want.
Few keypoints:
- I slightly modified, renamed and extended your ast to make it work and to actually show something.
Spirit rules for some reason make copy of an attribute (I think it is a bug), so I workarounded this issue for(fixed in 1.70)unique_ptrwith a trait.- I am not sure if
x3::omitis actually required there (you can remove all except the last and it will compile), but it looks like it is an another bug in Spirit. make_nodelooks unreliable and may broke in surprising ways, you can split it into separate unary/binary node creators if you wish.- At some point you will want to use stateful allocator for your ast nodes creation, it should be very simple by injecting allocator into the parser context. I am leaving it for you as an exercise.
The parser:
#include <boost/spirit/home/x3.hpp>
#include <memory>
#include <iostream>
namespace ast
{
class expression
{
protected:
expression() = default;
public:
virtual ~expression() = default;
expression(expression&& other) = delete;
expression& operator=(expression&& other) = delete;
virtual void print(std::ostream&) const = 0;
friend std::ostream& operator<<(std::ostream& os, expression const& node)
{
node.print(os);
return os;
}
};
class operand : public expression
{
double value_;
public:
constexpr operand(double value) : value_{value} {}
void print(std::ostream& os) const override { os << value_; }
};
class op_bin : public expression
{
protected:
std::unique_ptr<expression> left_, right_;
public:
op_bin(std::unique_ptr<expression> left, std::unique_ptr<expression> right)
: left_{ std::move(left) }, right_{ std::move(right) }
{}
op_bin(expression * left, expression * right)
: left_{ left }, right_{ right }
{}
};
class plus : public op_bin
{
public:
using op_bin::op_bin;
void print(std::ostream& os) const override
{ os << '(' << *left_ << " + " << *right_ << ')'; }
};
class minus : public op_bin
{
public:
using op_bin::op_bin;
void print(std::ostream& os) const override
{ os << '(' << *left_ << " - " << *right_ << ')'; }
};
class mul : public op_bin
{
public:
using op_bin::op_bin;
void print(std::ostream& os) const override
{ os << '(' << *left_ << " * " << *right_ << ')'; }
};
class div : public op_bin
{
public:
using op_bin::op_bin;
void print(std::ostream& os) const override
{ os << '(' << *left_ << " / " << *right_ << ')'; }
};
} // namespace ast
namespace grammar
{
namespace x3 = boost::spirit::x3;
template <typename T>
struct make_node_
{
template <typename Context>
void operator()(Context const& ctx) const
{
if constexpr (std::is_convertible_v<decltype(x3::_attr(ctx)), T>) {
x3::_val(ctx) = std::make_unique<T>(std::move(x3::_attr(ctx)));
}
else {
x3::_val(ctx) = std::make_unique<T>(std::move(x3::_val(ctx)), std::move(x3::_attr(ctx)));
}
}
};
template <typename T>
constexpr make_node_<T> make_node{};
using x3::double_;
using x3::char_;
x3::rule<class expression_r, std::unique_ptr<ast::expression>, true> const expression;
x3::rule<class prec1_r, std::unique_ptr<ast::expression>, true> const prec1;
x3::rule<class prec0_r, std::unique_ptr<ast::expression>, true> const prec0;
auto const expression_def =
prec1
>> *( x3::omit[('+' > prec1)[make_node<ast::plus>]]
| x3::omit[('-' > prec1)[make_node<ast::minus>]]
)
;
auto const prec1_def =
prec0
>> *( x3::omit[('*' > prec0)[make_node<ast::mul>]]
| x3::omit[('/' > prec0)[make_node<ast::div>]]
)
;
auto const prec0_def =
x3::omit[double_[make_node<ast::operand>]]
| '(' > expression > ')'
;
BOOST_SPIRIT_DEFINE(
expression
, prec1
, prec0
);
} // namespace grammar
#if BOOST_VERSION < 107000
namespace boost::spirit::x3::traits {
template <typename Attribute>
struct make_attribute<std::unique_ptr<Attribute>, std::unique_ptr<Attribute>>
: make_attribute_base<std::unique_ptr<Attribute>>
{
typedef std::unique_ptr<Attribute>& type;
typedef std::unique_ptr<Attribute>& value_type;
};
} // namespace boost::spirit::x3::traits
#endif
int main()
{
namespace x3 = boost::spirit::x3;
std::string s = "1 + 2 * (3 - 4) / 5";
std::unique_ptr<ast::expression> expr;
if (auto iter = s.cbegin(); !phrase_parse(iter, s.cend(), grammar::expression, x3::space, expr)) {
std::cout << "parsing failed";
}
else {
if (iter != s.cend())
std::cout << "partially parsed\n";
std::cout << *expr << '\n';
}
}
Output:
(1 + ((2 * (3 - 4)) / 5))
来源：https://stackoverflow.com/questions/53320015/virtual-classes-as-ast-nodes-with-spirit