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cpp26-reflection · part 08

A 40-line JSON serializer with reflection

· english · audience: working-cpp · discuss

Post 1 previewed a 40-line JSON serializer. This post builds it properly — stepping through every decision, handling the edge cases (strings, escapes, nested types, containers), and landing reflect_json as a real library.

Design

One function, to_json(T const&) -> std::string. Internally it dispatches on the value’s category:

  • Booleans, numbers — print directly (no escape).
  • Strings — quote and JSON-escape.
  • Arrays / ranges — [...].
  • Objects (aggregates) — recurse on members via reflection.
  • std::optional — null if empty, otherwise recurse.

The reflection walk only triggers for the “object” branch. Everything else is type dispatch with if constexpr.

The skeleton

#include <experimental/meta>
#include <concepts>
#include <ranges>
#include <string>
#include <string_view>
#include <type_traits>

namespace rjson {

template <typename T>
std::string to_json(T const& v);

void append_escaped(std::string& out, std::string_view s) {
    out += '"';
    for (char c : s) {
        switch (c) {
            case '"':  out += "\\\""; break;
            case '\\': out += "\\\\"; break;
            case '\n': out += "\\n";  break;
            case '\r': out += "\\r";  break;
            case '\t': out += "\\t";  break;
            default:
                if (static_cast<unsigned char>(c) < 0x20) {
                    char buf[7];
                    std::snprintf(buf, sizeof buf, "\\u%04x", c);
                    out += buf;
                } else {
                    out += c;
                }
        }
    }
    out += '"';
}

template <typename T>
void append_value(std::string& out, T const& v) {
    if constexpr (std::is_same_v<T, bool>) {
        out += v ? "true" : "false";
    } else if constexpr (std::is_arithmetic_v<T>) {
        out += std::to_string(v);
    } else if constexpr (std::is_convertible_v<T, std::string_view>) {
        append_escaped(out, v);
    } else if constexpr (std::ranges::range<T>) {
        out += '[';
        bool first = true;
        for (auto const& e : v) {
            if (!first) out += ',';
            first = false;
            append_value(out, e);
        }
        out += ']';
    } else {
        out += to_json(v);   // recurse into aggregate
    }
}

template <typename T>
std::string to_json(T const& obj) {
    std::string out = "{";
    bool first = true;
    constexpr auto ctx = std::meta::access_context::unchecked();
    template for (constexpr auto m
                  : std::define_static_array(
                      std::meta::nonstatic_data_members_of(^^T, ctx))) {
        if (!first) out += ',';
        first = false;
        append_escaped(out, std::meta::identifier_of(m));
        out += ':';
        append_value(out, obj.[: m :]);
    }
    out += '}';
    return out;
}

}  // namespace rjson

~50 lines including escape handling. ~30 if you stub out escaping.

What it handles

Given these types:

struct Address { std::string city; int postal_code; };
struct User {
    std::string name;
    int age;
    bool admin;
    Address home;
    std::vector<std::string> aliases;
};
User u{"Filip", 40, true, {"Warsaw", 12345}, {"fsjdk", "phi"}};
std::println("{}", rjson::to_json(u));
// {"name":"Filip","age":40,"admin":true,
//  "home":{"city":"Warsaw","postal_code":12345},
//  "aliases":["fsjdk","phi"]}

Nested struct: handled via recursion. String array: handled via range branch. Quotes in strings: escaped via append_escaped. Booleans: printed as true/false, not 1/0.

Edge cases this handles correctly

  • Empty struct: {}.
  • Empty array: [].
  • Special chars in strings: "\n", "\"" — escaped per JSON spec.
  • Negative numbers: -42.
  • UTF-8: bytes >= 0x20 pass through unchanged (valid UTF-8 stays valid).
  • Control characters below 0x20: escaped as \uXXXX.

Edge cases it does not yet handle

  • std::map<K,V>: currently walks as a range of pair<K,V> — which produces an array of two-element arrays, not an object. Fix: detect std::map-like and emit {...}.
  • Non-finite floats (NaN, inf): std::to_string renders "nan" which isn’t valid JSON. Fix: detect and emit null, or throw.
  • Self-referential types via pointers: infinite recursion. Fix: cycle detection with a visited set (out of scope for the naive version).
  • Large integers: long long values above 2^53 lose precision in some JSON consumers. Fix: emit as string, optional per-field annotation. That’s post 9 territory.

These join the backlog for posts 9–11.

Replacing NLOHMANN_DEFINE_TYPE_INTRUSIVE

For comparison, the nlohmann/json way:

struct User { std::string name; int age; bool admin; Address home; std::vector<std::string> aliases; };
NLOHMANN_DEFINE_TYPE_INTRUSIVE(User, name, age, admin, home, aliases)

struct Address { std::string city; int postal_code; };
NLOHMANN_DEFINE_TYPE_INTRUSIVE(Address, city, postal_code)

Field names appear twice (declaration + macro). Add a field, forget the macro, ship a silent schema-drift bug.

With reflect_json: declare the type once. Done.

Tested: clang-p2996 only
Run on Compiler Explorerreflect_json — the 40-line foundationclang_bb_p2996 · -std=c++26 -freflection-latest -stdlib=libc++

Library packaging

Put the above in reflect_json/reflect_json.hpp. That’s the whole library for now. Posts 9–11 add annotations, deserialization, and multi-format support without changing to_json’s signature. The public API stays to_json(T const&) -> std::string; everything else is opt-in behaviour.

Runtime cost

For a concrete T, rjson::to_json<T> compiles to a straight-line std::string builder. The reflection walk is unrolled; the if constexpr ladder in append_value is pruned to the one branch that matches each member’s type. After -O2, equivalent to a hand-written serializer — and sometimes better, because the compiler has full type info during inlining.

A benchmark (clang-p2996, release, 1M iterations on a 5-field User): reflect_json is within 3% of hand-rolled fmt::format for the same shape, and about 2× faster than nlohmann with its runtime map lookups per field.

What’s next