Structured or opaque — one bit decides (L2/L3 codec)¶

opt.PL is the whole of TLV composition. Clear, and the body is payload bytes — an opaque TLV, the leaf. Set, and the body is a packed run of sub-TLVs — a structured TLV, the composite, whose type code says what the children mean (CONTEXT.md §Two compositions). There is no LIST type and no array flag; nesting is this bit plus a purpose type byte.

The example makes the point by encoding POINT{VALUE, VALUE} once, clearing PL on a copy, and decoding the same bytes as one opaque payload whose length is exactly the children region’s.

What to notice¶

Meaning composes at L3; storage composes at L1, and they do not constrain each other. A structured TLV’s children are a meaning tree. The rope that physically holds those bytes is a storage tree, and a link boundary may fall anywhere, including mid-header — see the lazy view. That decoupling is the zero-copy story.
A decoded TLV is one or the other, never both. children is populated and payload empty for a structured TLV; the reverse for an opaque one. Reading the wrong field gets you a legitimately empty container, not an error.
The children region is contiguous bytes with no framing of its own. That is why the same body reads as one opaque payload of frame.size() - 4: sub-TLVs are self-describing, so no count field and no separators are needed.
Nothing here is conditional — the target builds and runs under every CI leg.

Source¶

/*
 * SPDX-License-Identifier: Apache-2.0
 * SPDX-FileCopyrightText: Copyright 2026 avatarsd LLC
 */

/**
 * @file
 * @brief ONE CONCEPT — `opt.PL`: an opaque TLV holds bytes, a structured TLV holds children.
 *
 * TLV composition (L3) composes MEANING: the leaf is an opaque TLV (`opt.PL = 0`, its body
 * is payload bytes), the composite is a structured TLV (`opt.PL = 1`, its body is a packed
 * run of sub-TLVs) whose type code says what the children mean (`CONTEXT.md` §Two
 * compositions). One bit decides which, and it decides it for the SAME body bytes: this
 * example encodes `POINT{VALUE, VALUE}`, clears `PL` on the copy, and decodes the very same
 * bytes as one opaque payload of exactly the children's length.
 *
 * That is the decoupling from memory composition (L1): meaning is `opt.PL`, storage is the
 * view/rope chain, and neither constrains the other.
 *
 * Runs under ctest as `example_wire_structured_vs_opaque`; returns non-zero on any failure.
 */

#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <span>
#include <vector>

#include "libtracer/tracer.hpp"

namespace {

using tr::wire::opt_t;
using tr::wire::tlv_t;
using tr::wire::type_t;

/** @brief Report expectation @p what and record a failure on @p ok. */
void check(bool& ok, bool cond, const char* what) {
    std::printf("  [%s] %s\n", cond ? "ok" : "FAIL", what);
    ok = ok && cond;
}

}  // namespace

int main() {
    bool ok = true;
    const std::vector<std::byte> x{std::byte{0x11}, std::byte{0x22}};
    const std::vector<std::byte> y{std::byte{0x33}, std::byte{0x44}};

    tlv_t point;
    point.type = type_t::POINT;
    point.opt.pl = true;  // structured: the body is children, not bytes
    point.children.push_back(tlv_t{.type = type_t::VALUE, .payload = std::span(x)});
    point.children.push_back(tlv_t{.type = type_t::VALUE, .payload = std::span(y)});

    std::vector<std::byte> frame = tr::wire::encode(point);
    std::printf("POINT{VALUE,VALUE} is %zu bytes: 4 header + 2 x (4 header + 2 body)\n",
                frame.size());

    const auto structured = tr::wire::decode(frame);
    check(ok, structured.has_value(), "the structured frame decodes");
    check(ok, structured && structured->children.size() == 2, "PL = 1 gives two children");
    check(ok, structured && structured->payload.empty(), "and no opaque payload at all");

    // The SAME body bytes, read with PL cleared. Byte 1 is the root's opt (see
    // wire_tlv_header): clearing bit 6 tells the decoder "this body is payload".
    std::vector<std::byte> as_opaque = frame;
    as_opaque[1] = static_cast<std::byte>(static_cast<std::uint8_t>(as_opaque[1]) & 0xBFu);

    const auto opaque = tr::wire::decode(as_opaque);
    check(ok, opaque.has_value(), "the same bytes with PL = 0 also decode");
    check(ok, opaque && opaque->children.empty(), "PL = 0 gives no children");
    check(ok, opaque && opaque->payload.size() == frame.size() - 4,
          "the payload IS the children region, byte for byte");
    return ok ? 0 : 1;
}