我正在尝试实现一个函数，以将AST从表达式语法传递给字符串，然后测试将其与正确的字符串进行比较是否正确。为此，我实现了一个访问者来访问Boost :: Variant变量，并重载了一个名为“ getString”的函数。
关于错误的定义，我会收到很多错误，并且我一直在通过StackOverflow寻找类似的问题，但是一切正常。
错误（针对访问者上每个重载的运算符重复出现）：
In file included from /usr/include/boost/spirit/home/support/info.hpp:14,
from /usr/include/boost/spirit/home/qi/domain.hpp:16,
from /usr/include/boost/spirit/home/qi/meta_compiler.hpp:15,
from /usr/include/boost/spirit/home/qi/action/action.hpp:14,
from /usr/include/boost/spirit/home/qi/action.hpp:14,
from /usr/include/boost/spirit/home/qi.hpp:14,
from /usr/include/boost/spirit/include/qi.hpp:16,
from src/./Expression.cpp:2,
from src/main.cpp:1:
/usr/include/boost/variant/variant.hpp: In instantiation of 'boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = std::basic_string<char>; Visitor = const testing::visitor; boost::detail::variant::invoke_visitor<Visitor>::result_type = std::basic_string<char>]':
/usr/include/boost/variant/detail/visitation_impl.hpp:114:9: required from 'typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::true_) [with Visitor = boost::detail::variant::invoke_visitor<const testing::visitor>; VoidPtrCV = void*; T = std::basic_string<char>; typename Visitor::result_type = std::basic_string<char>; mpl_::true_ = mpl_::bool_<true>]'
/usr/include/boost/variant/detail/visitation_impl.hpp:154:41: required from 'typename Visitor::result_type boost::detail::variant::visitation_impl_invoke(int, Visitor&, VoidPtrCV, T*, NoBackupFlag, int) [with Visitor = boost::detail::variant::invoke_visitor<const testing::visitor>; VoidPtrCV = void*; T = std::basic_string<char>; NoBackupFlag = boost::variant<std::basic_string<char>, double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression> >::has_fallback_type_; typename Visitor::result_type = std::basic_string<char>]'
/usr/include/boost/variant/detail/visitation_impl.hpp:238:5: required from 'typename Visitor::result_type boost::detail::variant::visitation_impl(int, int, Visitor&, VoidPtrCV, mpl_::false_, NoBackupFlag, Which*, step0*) [with Which = mpl_::int_<0>; step0 = boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<5>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<4>, double, boost::mpl::l_item<mpl_::long_<3>, bool, boost::mpl::l_item<mpl_::long_<2>, boost::recursive_wrapper<ast::unary>, boost::mpl::l_item<mpl_::long_<1>, boost::recursive_wrapper<ast::expression>, boost::mpl::l_end> > > > > >, boost::mpl::l_iter<boost::mpl::l_end> >; Visitor = boost::detail::variant::invoke_visitor<const testing::visitor>; VoidPtrCV = void*; NoBackupFlag = boost::variant<std::basic_string<char>, double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression> >::has_fallback_type_; typename Visitor::result_type = std::basic_string<char>; mpl_::false_ = mpl_::bool_<false>]'
/usr/include/boost/variant/variant.hpp:2384:48: required from 'static typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<const testing::visitor>; VoidPtrCV = void*; T0_ = std::basic_string<char>; TN = {double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression>}; typename Visitor::result_type = std::basic_string<char>]'
/usr/include/boost/variant/variant.hpp:2398:43: required from 'typename Visitor::result_type boost::variant<T0, TN>::internal_apply_visitor(Visitor&) [with Visitor = boost::detail::variant::invoke_visitor<const testing::visitor>; T0_ = std::basic_string<char>; TN = {double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression>}; typename Visitor::result_type = std::basic_string<char>]'
/usr/include/boost/variant/variant.hpp:2423:52: required from 'typename Visitor::result_type boost::variant<T0, TN>::apply_visitor(Visitor&) [with Visitor = const testing::visitor; T0_ = std::basic_string<char>; TN = {double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression>}; typename Visitor::result_type = std::basic_string<char>]'
/usr/include/boost/variant/detail/apply_visitor_unary.hpp:84:43: required from 'typename Visitor::result_type boost::apply_visitor(const Visitor&, Visitable&) [with Visitor = testing::visitor; Visitable = boost::variant<std::basic_string<char>, double, bool, boost::recursive_wrapper<ast::unary>, boost::recursive_wrapper<ast::expression> >; typename Visitor::result_type = std::basic_string<char>]'
src/./Printer.cpp:29:64: required from here
/usr/include/boost/variant/variant.hpp:1046:24: error: no match for call to '(const testing::visitor) (std::basic_string<char>&)'
return visitor_(operand);
~~~~~~~~^~~~~~~~~
In file included from src/main.cpp:2:
src/./Printer.cpp:20:21: note: candidate: 'std::string testing::visitor::operator()(std::string&)' <near match>
std::string operator()(std::string& v) { return v; }
^~~~~~~~
src/./Printer.cpp:20:21: note: passing 'const testing::visitor*' as 'this' argument discards qualifiers
src/./Printer.cpp:21:21: note: candidate: 'std::string testing::visitor::operator()(double&)'
std::string operator()(double& v) { return boost::lexical_cast<std::string>(v); }
^~~~~~~~
src/./Printer.cpp:21:21: note: no known conversion for argument 1 from 'std::basic_string<char>' to 'double&'
src/./Printer.cpp:22:21: note: candidate: 'std::string testing::visitor::operator()(bool&)'
std::string operator()(bool& b) { return b ? "True" : "False"; }
^~~~~~~~
src/./Printer.cpp:22:21: note: no known conversion for argument 1 from 'std::basic_string<char>' to 'bool&'
src/./Printer.cpp:24:21: note: candidate: 'std::string testing::visitor::operator()(ast::expression&)'
std::string operator()(ast::expression& e) { return getString(e); }
^~~~~~~~
src/./Printer.cpp:24:21: note: no known conversion for argument 1 from 'std::basic_string<char>' to 'ast::expression&'
src/./Printer.cpp:25:21: note: candidate: 'std::string testing::visitor::operator()(ast::unary&)'
std::string operator()(ast::unary& e) { return getString(e); }
^~~~~~~~
src/./Printer.cpp:25:21: note: no known conversion for argument 1 from 'std::basic_string<char>' to 'ast::unary&'
Printer.cpp：
#include <boost/lexical_cast.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/variant/recursive_wrapper.hpp>
#include <string>
#include "AST.hpp"
namespace testing {
std::string getString(ast::expression& e);
std::string getString(ast::operation& op);
std::string getString(ast::unary& op);
std::string getString(ast::optoken& opt);
struct visitor : boost::static_visitor<std::string>
{
std::string operator()(std::string& v) { return v; }
std::string operator()(double& v) { return boost::lexical_cast<std::string>(v); }
std::string operator()(bool& b) { return b ? "True" : "False"; }
std::string operator()(ast::expression& e) { return getString(e); }
std::string operator()(ast::unary& e) { return getString(e); }
};
std::string getString(ast::expression& e) {
std::string s = boost::apply_visitor(visitor(), e.first);
std::string result = "( " + s;
for (auto it = e.rest.begin(); it != e.rest.end(); ++it) {
result += getString(*it);
}
result += " )";
return result;
}
std::string getString(ast::unary& op) {
std::string s = boost::apply_visitor(visitor(), op.operand_);
return " " + getString(op.operator_) + " " + s;
}
std::string getString(ast::operation& op) {
std::string s = boost::apply_visitor(visitor(), op.operand_);
return " " + getString(op.operator_) + " " + s;
}
std::string getString(ast::optoken& opt) {
switch(opt) {
case ast::op_plus: return "+";
case ast::op_minus: return "-";
case ast::op_times: return "*";
case ast::op_divide: return "/";
case ast::op_not: return "!";
case ast::op_equal: return "==";
case ast::op_not_equal: return "!=";
case ast::op_less: return "<";
case ast::op_less_equal: return "<=";
case ast::op_greater: return ">";
case ast::op_greater_equal: return ">=";
case ast::op_and: return "&&";
case ast::op_or: return "||";
}
return "undf";
}
}
AST.hpp
namespace ast
{
struct unary;
struct expression;
typedef boost::variant<
std::string,
double,
bool,
boost::recursive_wrapper<unary>,
boost::recursive_wrapper<expression>
> operand;
enum optoken
{
op_plus,
op_minus,
op_times,
op_divide,
op_positive,
op_negative,
op_not,
op_equal,
op_not_equal,
op_less,
op_less_equal,
op_greater,
op_greater_equal,
op_and,
op_or
};
struct operation {
optoken operator_;
operand operand_;
};
struct unary {
optoken operator_;
operand operand_;
};
struct expression
{
operand first;
std::list<operation> rest;
};
}
答案 0 :(得分：1)
您的访问者方法应为import Foundation
import UIKit
class MainNavigationController: UINavigationController {
@IBOutlet weak var IfFromDate: UITextField!
@IBOutlet weak var IfToDate: UITextField!
let datePicker = UIDatePicker()
override func viewDidLoad() {
super.viewDidLoad()
showDatePicker()
}
func showDatePicker(){
//Formate Date
datePicker.datePickerMode = .date
IfFromDate.inputView = datePicker //accruing error on this line
}
@IBAction func btnSignIn(_ sender: Any) {
}
}
：
const