我想了解使用Boost.Spirit封装BOOST_FUSION_ADAPT_STRUCT时需要struct的确切方案。
以下是两个例子。一个示例是具有（仅）struct数据成员的单成员variant。此版本不需要在Fusion容器中包装结构的BOOST_FUSION_ADAPT_STRUCT宏。构造函数足以让Spirit根据传入的rhs实例化/填充属性。
（请参阅代码中的注释，以了解由于属性折叠规则，我认为Boost.Spirit正在为规则定义的rhs生成的属性类型中的注释。）
第二个示例是单个成员struct，其中（仅）vector数据成员。即使定义了构造函数以允许Spirit基于rhs填充属性，它也无法在没有BOOST_FUSION_ADAPT_STRUCT的情况下进行编译。
为什么会有区别？我想了解为什么在第一种情况下，可以使用构造函数来填充属性（struct），而在第二种情况下，构造函数不够，必须使用BOOST_FUSION_ADAPT_STRUCT。
上面提到的例子如下。
示例1：变体
#include <string>
#include <vector>
#include <boost/variant.hpp>
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
typedef std::string::const_iterator It;
using intermediate = boost::variant<std::string, int>;
// Simple parser demonstrating successful build with 'works_great'
struct works_great // No need for BOOST_FUSION_ADAPT_STRUCT - whoopee!
// But why - even given the constructor??
{
intermediate i;
works_great() = default;
works_great(intermediate i) : i{i} {}
};
// Not required for 'works_great' - constructors work just fine
//BOOST_FUSION_ADAPT_STRUCT(works_great, v)
struct parser : qi::grammar<It, works_great()>
{
parser() : parser::base_type(works_great)
{
using namespace qi;
intermediate = qi::string("test") | qi::int_;
// rhs should have attribute of type 'variant',
// matching the constructor
works_great = '{' >> intermediate >> '}';
}
private:
qi::rule<It, intermediate()> intermediate;
qi::rule<It, works_great()> works_great;
};
int main()
{
// The following all compiles/builds just fine
// (I don't care about the actual runtime results).
static const parser p;
works_great wg;
std::string const data {"{test}"};
auto f(begin(data)), l(end(data));
qi::parse(f,l,p,wg);
}
示例2：矢量
#include <string>
#include <vector>
#include <boost/variant.hpp>
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
typedef std::string::const_iterator It;
// We need BOOST_FUSION_ADAPT_STRUCT for this one, but not for the above.
// Constructors don't help. Only difference seems to be
// the vector (rather than variant).
struct not_so_much // not so much - unless BOOST_FUSION_ADAPT_STRUCT is used
{
std::vector<int> s;
// Constructors do not help here
//not_so_much() = default;
//not_so_much(std::vector<int> s) : s{std::move(s)} {}
};
// Required for 'not_so_much' - constructors don't work
BOOST_FUSION_ADAPT_STRUCT(not_so_much, s)
// Simple parser demonstrating successful build with 'not_so_much' -
// but only when BOOST_FUSION_ADAPT_STRUCT is used.
struct parser : qi::grammar<It, not_so_much()>
{
parser() : parser::base_type(not_so_much)
{
using namespace qi;
// Note: I know that 'eps' is required, below, to compile the
// single-member struct successfully
// rhs should have attribute of type 'vector<int>',
// matching the constructor as well...
// but it doesn't work.
not_so_much = eps >> (qi::int_ % "|");
}
private:
qi::rule<It, not_so_much()> not_so_much;
};
int main()
{
// The following all compiles/builds just fine
static const parser p;
not_so_much nm;
std::string const data {"5|9|16"};
auto f(begin(data)), l(end(data));
qi::parse(f,l,p,nm);
}
答案 0 :(得分：3)
差异是双重的：
后者的区别，你注意到了。第一个：不是那么多。
真正有原则的答案是：
Qi属性传播是一种启发式机器。
可悲的是，很少有东西可以优化性能（X3做得更好）。其中一个关键区域是一个例外，它是对容器的增量解析（即使是跨多个规则）¹。
这很有意义（因为即使例如逐个字符构建字符串也会非常慢......）。但它确实会带来意外（例如。boost::spirit::qi duplicate parsing on the output，Understanding Boost.spirit's string parser）
¹（实际上也是非容器，但我离题了。我不认为它会在没有语义动作的情况下发挥作用）
你实际上可以改变属性传播的时间点，并且没有适应性，但是我建议反对它：只是调整更加一致和自我描述：
<强> Live On Coliru
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
namespace Ast {
using vec = std::vector<int>;
struct not_so_much {
vec s;
not_so_much() = default;
not_so_much(vec s) : s(std::move(s)) {}
};
}
typedef std::string::const_iterator It;
typedef qi::rule<It, Ast::not_so_much()> Parser;
template <typename Expr> void do_test(Expr const& expression) {
Parser const p = expression;
Ast::not_so_much nm;
std::string const data {"5|9|16"};
It f = begin(data), l = end(data);
if (qi::parse(f,l,p,nm)) {
std::cout << "Parsed " << nm.s.size() << " elements: ";
copy(nm.s.begin(), nm.s.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << "\n";
} else {
std::cout << "Parse failed\n";
}
if (f != l)
std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
}
int main() {
using namespace qi;
do_test(attr_cast<Ast::not_so_much, Ast::vec>(int_ % '|'));
do_test(attr_cast<Ast::not_so_much>(int_ % '|'));
do_test(as<Ast::vec>()[int_ % '|']);
}
打印
Parsed 3 elements: 5 9 16
Parsed 3 elements: 5 9 16
Parsed 3 elements: 5 9 16