Boost.Pfr

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C++ 14 Added in Boost 1.75.0

Categories: Data , Metaprogramming

Basic reflection for user defined types.

Documentation www.stage.boost.cppalliance.org/doc/libs/1_86_0/libs/pfr/index.html

GitHub Issues github.com/boostorg/pfr/issues

Source Code github.com/boostorg/pfr/tree/boost-1.86.0

This Release

Antony Polukhin

Maintainer

Boost.PFR

This is a C++14 library for very basic reflection that gives you access to structure elements by index and provides other std::tuple like methods for user defined types without any macro or boilerplate code.

Boost.PFR is a part of the Boost C++ Libraries. However, Boost.PFR is a header only library that does not depend on Boost. You can just copy the content of the "include" folder from the github into your project, and the library will work fine.

For a version of the library without boost:: namespace see PFR.

Test results

Branches	Build	Tests coverage	More info
Develop:			details...
Master:			details...

Latest developer documentation

Motivating Example #0

#include <iostream>
#include <fstream>
#include <string>

#include "boost/pfr.hpp"

struct some_person {
  std::string name;
  unsigned birth_year;
};

int main(int argc, const char* argv[]) {
  some_person val{"Edgar Allan Poe", 1809};

  std::cout << boost::pfr::get<0>(val)                // No macro!
      << " was born in " << boost::pfr::get<1>(val);  // Works with any aggregate initializables!

  if (argc > 1) {
    std::ofstream ofs(argv[1]);
    ofs << boost::pfr::io(val);                       // File now contains: {"Edgar Allan Poe", 1809}
  }
}

Outputs:

Edgar Allan Poe was born in 1809

Run the above sample

Motivating Example #1

#include <iostream>
#include "boost/pfr.hpp"

struct my_struct { // no ostream operator defined!
    int i;
    char c;
    double d;
};

int main() {
    my_struct s{100, 'H', 3.141593};
    std::cout << "my_struct has " << boost::pfr::tuple_size<my_struct>::value
        << " fields: " << boost::pfr::io(s) << "\n";
}

Outputs:

my_struct has 3 fields: {100, H, 3.14159}

Motivating Example #2

#include <iostream>
#include "boost/pfr.hpp"

struct my_struct { // no ostream operator defined!
    std::string s;
    int i;
};

int main() {
    my_struct s{{"Das ist fantastisch!"}, 100};
    std::cout << "my_struct has " << boost::pfr::tuple_size<my_struct>::value
        << " fields: " << boost::pfr::io(s) << "\n";
}

Outputs:

my_struct has 2 fields: {"Das ist fantastisch!", 100}

Motivating Example #3

#include <iostream>
#include <string>

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/adapt_boost_pfr.hpp>

#include "boost/pfr/io.hpp"

namespace x3 = boost::spirit::x3;

struct ast_employee { // No BOOST_FUSION_ADAPT_STRUCT defined
    int age;
    std::string forename;
    std::string surname;
    double salary;
};

auto const quoted_string = x3::lexeme['"' >> +(x3::ascii::char_ - '"') >> '"'];

x3::rule<class employee, ast_employee> const employee = "employee";
auto const employee_def =
    x3::lit("employee")
    >> '{'
    >>  x3::int_ >> ','
    >>  quoted_string >> ','
    >>  quoted_string >> ','
    >>  x3::double_
    >>  '}'
    ;
BOOST_SPIRIT_DEFINE(employee);

int main() {
    std::string str = R"(employee{34, "Chip", "Douglas", 2500.00})";
    ast_employee emp;
    x3::phrase_parse(str.begin(),
                     str.end(),
                     employee,
                     x3::ascii::space,
                     emp);
    std::cout << boost::pfr::io(emp) << std::endl;
}