writing struct to .txt file

Question

I'm trying to store my struct into txt file using boost but unable to do it. I'm using boost library. example structure

struct Frame
{
    uint32_t address{ 0 };
    uint16_t marks{ 0 };
    uint16_t age{ 0 };
    char gender{ 'M' };
    std::string userName;
};

for binary there is simple code

boost::archive::binary_oarchive ar(ofs, boost::archive::no_header);
ar << boost::serialization::make_binary_object(&f, sizeof(Frame));

assume file is open with fstream object ofs and 'f' is object of 'Frame'

I want to know if there is similar way to write structure to txt file and I do not want to write data types one by one. assume we don't know the types/number of datatypes inside structure.

Answer 1

As others have commented you will have to provide serialization helpers that tell Boost how to member-wise serialize.

If you have only aggregates like this, you can automate the generation of this function to a degree with Boost PFR :

pfr::for_each_field(s, [&](auto&& f) { ar & f; });               

Here's an example:

namespace MyLib {
    struct Frame {
        uint32_t    address{0};
        uint16_t    marks{0};
        uint16_t    age{0};
        char        gender{'M'};
        std::string userName;
    };

    struct Other {
        std::string userName;
        std::map<uint32_t, std::string> properties;
    };

} // namespace MyLib

Note I stick them in a namespace for good style and also so we can highlight that ADL is used to find a serialize overload. Now let's define the overloads:

namespace MyLib {
    #define SERIALIZER(Aggregate)                                                  \
        template <typename Archive>                                                \
        void serialize(Archive& ar, Aggregate& s, unsigned version)                \
        {                                                                          \
            pfr::for_each_field(s, [&](auto&& f) { ar & f; });               \
        }


    SERIALIZER(Frame)
    SERIALIZER(Other)
} // namespace MyLib

Using the macro we avoid repeated code. Of course you could do this without a macro as well.

Now you can serialize both. Let's say we have:

MyLib::Frame const diablo{178, 42, 37, 'F', "Diablo"};
MyLib::Other const other{"diablo", {{1, "one"}, {2, "two"}, {3, "three"},}};

Then serializing to a text stream:

boost::archive::text_oarchive oa(ss);
oa & diablo;
oa & other;

Already results in the stream containing eg

22 serialization::archive 19 0 0 178 42 37 70 6 Diablo 0 0 6 diablo 0 0 3 0 0
 0 1 3 one 2 3 two 3 5 three

Full Demo

This demo checks that the result of deserializing is actually identical to the original structs:

Live On Coliru

#include <string>
#include <map>

namespace MyLib {
    struct Frame {
        uint32_t    address{0};
        uint16_t    marks{0};
        uint16_t    age{0};
        char        gender{'M'};
        std::string userName;
    };

    struct Other {
        std::string userName;
        std::map<uint32_t, std::string> properties;
    };

} // namespace MyLib

#include <boost/pfr.hpp>
namespace pfr = boost::pfr;

namespace MyLib {
#define SERIALIZER(Aggregate)                                                  \
    template <typename Archive>                                                \
    void serialize(Archive& ar, Aggregate& s, unsigned version)                \
    {                                                                          \
        pfr::for_each_field(s, [&](auto&& f) { ar & f; });               \
    }

    SERIALIZER(Frame)
    SERIALIZER(Other)
} // namespace MyLib

#include <iostream>
#include <boost/archive/binary_oarchive.hpp>
#include <boost/archive/binary_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/map.hpp>

int main() {
    MyLib::Frame const diablo{178, 42, 37, 'F', "Diablo"};
    MyLib::Other const other{"diablo", {{1, "one"}, {2, "two"}, {3, "three"},}};

    std::stringstream ss;
    {
        boost::archive::text_oarchive oa(ss);
        oa << diablo;
        oa << other;
    }

    std::cout << ss.str() << "\n";

    {
        boost::archive::text_iarchive ia(ss);
        MyLib::Frame f;
        MyLib::Other o;

        ia >> f >> o;

        std::cout << std::boolalpha;
        std::cout << "Frame identical: " << pfr::eq(diablo, f) << "\n";
        std::cout << "Other identical: " << pfr::eq(other, o) << "\n";
    }
}

Prints

g++ -std=c++2a -O2 -Wall -pedantic -pthread main.cpp -lboost_serialization && ./a.out
22 serialization::archive 19 0 0 178 42 37 70 6 Diablo 0 0 6 diablo 0 0 3 0 0 0 1 3 one 2 3 two 3 5 three

Frame identical: true
Other identical: true