support required fields

[qicosmos]

If defined required fields, but the xml string lack of required fileds, iguana will throw exception.

struct some_book {
  std::string_view title;
  std::string_view author;
};
REFLECTION(some_book, title, author);
REQUIRED(some_book, title, author);

TEST_CASE("test required filed") {
  some_book book{"book", "tom"};
  std::string xml_str;
  iguana::to_xml(book, xml_str);

  std::cout << xml_str << "\n";

  std::string s1 = R"(<book_t><title>book</title></book_t>)";
  some_book b;
  CHECK_THROWS_AS(iguana::from_xml(b, s1), std::invalid_argument);

  std::string s2 = R"(<book_t><author>tom</author></book_t>)";
  CHECK_THROWS_AS(iguana::from_xml(b, s2), std::invalid_argument);
}

[codecov-commenter]

Codecov Report

Patch coverage: 90.00% and project coverage change: +0.01 🎉

Comparison is base (37a5426) 42.49% compared to head (c12da6f) 42.50%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   42.49%   42.50%   +0.01%     
==========================================
  Files          44       44              
  Lines        5210     5220      +10     
==========================================
+ Hits         2214     2219       +5     
- Misses       2996     3001       +5     

Impacted Files	Coverage Δ
iguana/reflection.hpp	100.00% <ø> (ø)
iguana/xml_reader.hpp	33.33% <50.00%> (+33.33%)	⬆️
test/test_xml.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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