是的。更好的方法是制作语法，为其编写解析器并解析为 AST（抽象语法树，或只是数据包的强类型表示）。
这样的 Spirit 语法如下所示：
-
我总是从 AST 类型开始：
namespace ast {
struct nil {
friend std::ostream& operator<<(std::ostream& os, nil) { return os << "<nil>"; }
};
using value = boost::variant<nil, double, std::string>;
struct parameter {
std::string _key;
value _val;
};
enum class action {
get,
set,
};
using parameters = std::vector<parameter>;
struct packet {
action _action;
parameters _params;
};
using packets = std::vector<packet>;
}
为了简单起见
- 假定的参数（模式/延迟）将具有数字或字符串值。
- 对
GET 和SET 请求使用相同的数据包定义（GET 请求将只使用我们列出的参数的nil 值）
-
接下来我们使用Boost Spirit Qi定义一个语法：
template <typename It, typename Skipper=qi::space_type>
struct grammar : qi::grammar<It, ast::packets(), Skipper> {
grammar():grammar::base_type(start) {
using qi::raw;
using qi::no_case;
param_key_.add
("delay")
("mode");
start = *(packet_ >> ';');
packet_ =
(no_case["get"] >> qi::attr(ast::action::get) >> *(',' >> get_param_))
| (no_case["set"] >> qi::attr(ast::action::set) >> *(',' >> set_param_))
;
get_param_ = raw[no_case[param_key_]] >> qi::attr(ast::nil());
set_param_ = raw[no_case[param_key_]] >> "," >> value_;
value_ = qi::double_ | string_;
string_ = '"' >> *~qi::char_('"') >> '"';
BOOST_SPIRIT_DEBUG_NODES((start)(packet_)(get_param_)(set_param_)(value_)(string_))
}
// ... field declarations
};
这里有一点学习曲线，但要注意的关键点是可以创建 also debuggable (see here for BOOST_SPIRIT_DEBUG enabled output) 的可维护代码。
-
最后，由于 AST 很简单，我们可以制作一个假请求处理器，它使用请求上下文（在本例中是包含参数当前值的映射）来实际处理请求：
struct request_context {
std::map<std::string, ast::value> properties;
request_context()
: properties { { "MODE", 2 }, { "DELAY", 300 } } // defaults
{
}
boost::optional<ast::packet> process_request(ast::packet packet) {
switch (packet._action) {
case ast::action::get:
for(auto& param : packet._params) {
param._val = properties[param._key];
}
return packet;
case ast::action::set:
for(auto& param : packet._params) {
std::cout << "DEBUG: setting property '" << param._key << "' to value '" << param._val << "'\n";
properties[param._key] = param._val;
}
return boost::none;
default:
throw std::runtime_error("bad packet"); // TODO proper exception type
};
}
};
想象一下，如果您将它与解析代码或其他所有内容混合在一起，会有多混乱的情况stringly typed
Live On Coliru
//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>
#include <map>
namespace qi = boost::spirit::qi;
namespace ast {
struct nil {
friend std::ostream& operator<<(std::ostream& os, nil) { return os << "<nil>"; }
};
using value = boost::variant<nil, double, std::string>;
struct parameter {
std::string _key;
value _val;
};
enum class action {
get,
set,
};
using parameters = std::vector<parameter>;
struct packet {
action _action;
parameters _params;
};
using packets = std::vector<packet>;
static std::ostream& operator<<(std::ostream& os, action a) {
switch(a) {
case action::get: return os << "GET";
case action::set: return os << "SET";
}
return os << "(other)";
}
}
BOOST_FUSION_ADAPT_STRUCT(ast::parameter,(std::string,_key)(ast::value,_val))
BOOST_FUSION_ADAPT_STRUCT(ast::packet,(ast::action,_action)(ast::parameters,_params))
template <typename It, typename Skipper=qi::space_type>
struct grammar : qi::grammar<It, ast::packets(), Skipper> {
grammar():grammar::base_type(start) {
using qi::raw;
using qi::no_case;
param_key_.add
("delay")
("mode");
start = *(packet_ >> ';');
packet_ =
(no_case["get"] >> qi::attr(ast::action::get) >> *(',' >> get_param_))
| (no_case["set"] >> qi::attr(ast::action::set) >> *(',' >> set_param_))
;
get_param_ = raw[no_case[param_key_]] >> qi::attr(ast::nil());
set_param_ = raw[no_case[param_key_]] >> "," >> value_;
value_ = qi::double_ | string_;
string_ = '"' >> *~qi::char_('"') >> '"';
BOOST_SPIRIT_DEBUG_NODES((start)(packet_)(get_param_)(set_param_)(value_)(string_))
}
private:
qi::symbols<char, std::string> param_key_;
qi::rule<It, ast::parameter(), Skipper> set_param_, get_param_;
qi::rule<It, ast::packets(), Skipper> start;
qi::rule<It, ast::packet(), Skipper> packet_;
qi::rule<It, ast::value(), Skipper> value_;
qi::rule<It, std::string()> string_;
};
struct request_context {
std::map<std::string, ast::value> properties;
request_context()
: properties { { "MODE", 2 }, { "DELAY", 300 } } // defaults
{
}
boost::optional<ast::packet> process_request(ast::packet packet) {
switch (packet._action) {
case ast::action::get:
for(auto& param : packet._params) {
param._val = properties[param._key];
}
return packet;
case ast::action::set:
for(auto& param : packet._params) {
std::cout << "DEBUG: setting property '" << param._key << "' to value '" << param._val << "'\n";
properties[param._key] = param._val;
}
return boost::none;
default:
throw std::runtime_error("bad packet"); // TODO proper exception type
};
}
};
int main()
{
std::string const input =
"GET,DELAY,MODE;"
"SET,DELAY,0,MODE,\"we can have string values too\";GET,MODE;SET,MODE,42;GET,MODE,DELAY;";
using It = std::string::const_iterator;
It f(input.begin()), l(input.end());
grammar<It> p;
ast::packets parsed;
bool ok = qi::phrase_parse(f,l,p,qi::space,parsed);
if (ok) {
std::cout << parsed.size() << " packets successfully parsed\n";
request_context ctx;
for(auto& packet : parsed)
{
auto response = ctx.process_request(packet);
if (response) {
std::cout << "response: " << response->_action;
for(auto& kv : packet._params) {
std::cout << "," << kv._key << "," << kv._val;
}
std::cout << ";\n";
}
}
} else {
std::cout << "Parse error\n";
}
if (f!=l)
std::cout << "Remaining unparsed input: '" << std::string(f,l) << "'\n";
}
打印：
5 packets successfully parsed
response: GET,DELAY,300,MODE,2;
DEBUG: setting property 'DELAY' to value '0'
DEBUG: setting property 'MODE' to value 'we can have string values too'
response: GET,MODE,we can have string values too;
DEBUG: setting property 'MODE' to value '42'
response: GET,MODE,42,DELAY,0;