packed_record_buffer.hpp

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#pragma once
 
/**
 * PackedRecordBuffer — byte buffer for a Fortran unformatted record whose
 * fields are mixed-width scalars.
 *
 * A Fortran WRITE of a list of scalars lays them end to end with no
 * alignment padding; a C struct with the same members does not. Appending
 * each field's bytes here reproduces the packed layout exactly, independent
 * of compiler alignment rules, and never exposes uninitialized bytes.
 */
 
#include <cstddef>
#include <cstring>
#include <type_traits>
#include <vector>
 
namespace q3 {
 
class PackedRecordBuffer {
  public:
    template <typename Scalar> void append(Scalar value) {
        static_assert(std::is_trivially_copyable<Scalar>::value,
                      "PackedRecordBuffer appends raw bytes of trivially copyable scalars");
        const std::size_t offset = bytes_.size();
        bytes_.resize(offset + sizeof(Scalar));
        std::memcpy(bytes_.data() + offset, &value, sizeof(Scalar));
    }
 
    [[nodiscard]] const char* data() const {
        return bytes_.data();
    }
    [[nodiscard]] std::size_t size() const {
        return bytes_.size();
    }
 
  private:
    std::vector<char> bytes_;
};
 
} // namespace q3