Arenas and the almighty concatenation operator

I continue to streamline an arena-based paradigm, and stumbled upon a concise technique for dynamic growth — an efficient, generic “concatenate anything to anything” within an arena built atop a core of 9-ish lines of code. The key insight originated from a reader suggestion about dynamic arrays. The subject of concatenation can be a string, dynamic array, or even something else. The “system” is extensible, and especially useful for path handling.

Continuing from last time, the examples are in light, C-style C++. I chose it because templates and function overloading express the concepts succinctly. It uses no standard library functionality, so converting to C, or similar, should be straightforward. The core concatenation “operator”:

template<typename T>
T concat(arena *a, T head, T tail)
    if ((char *)( != a->beg) {
        head = T{a, head};
    head.len += T{a, tail}.len;
    return head;

This concatenates two objects of the same type in the arena, and does so in place if possible. That is, we can efficiently build a value piece by piece. The type T must have data and len members, and a “copy” constructor that makes a copy of the given object at the front of the arena. Size integer overflows and out-of-memory errors are, as usual, handled by the arena. In particular, note that the len addition happens after allocation.

Since the front-of-the-arena business implicit, consider asserting it if you’re worried. I’ve also considered declaring a clone “operator” where that behavior is an explicit part of its interface.

// Make a copy of the object at the front of the arena.
template<typename T> T clone(arena *, T);

// In concat, replace the T{} constructors with clone:
    head = clone(a, head);
    head.len += clone(a, tail).len;

Strings are perhaps them most interesting subject of concatenation. Here’s a compatible string, str, definition from my previous article:

struct str {
    union {
        uint8_t    *data = 0;
        char const *cdata;
    ptrdiff_t len = 0;

    str() = default;

    str(uint8_t *beg, uint8_t *end) : data{beg}, len{end-beg} {}

    template<ptrdiff_t N>
    constexpr str(char const (&s)[N]) : cdata{s}, len{N-1} {}

    str(arena *, str);  // TODO

    uint8_t &operator[](ptrdiff_t i) { return data[i]; }

This has data, len, and the necessary constructor declaration. Before showing the constructor definition, here’s an arena following the usual formula, which should be familiar to those who’ve been following along:

struct arena {
    char *beg;
    char *end;

template<typename T, typename ...A>
T *makefront(ptrdiff_t count, arena *a, A ...args)
    ptrdiff_t size  = sizeof(T);
    ptrdiff_t align = -(uintptr_t)a->beg & (alignof(T) - 1);
    assert(count < (a->end - a->beg - align)/size);  // OOM
    T *r = (T *)(a->beg + align);
    a->beg += align + size*count;
    for (ptrdiff_t i = 0; i < count; i++) {
        new (r+i) T(args...);
    return r;

Note how it bumps beg, not end, because it’s allocated at the front. That opens the end of the object for concatenation. When it returns, beg points just past the end of the new object, aligned to it. Later, concat inspects beg to see if it can extend in place. That will be true if nothing else has been allocated at the front in the meantime. That is, we can allocate objects at the end — such as hash map nodes — while efficiently growing an object at the front through concatenation. If it’s not true for whatever reason, concatenation still works, just with reduced efficiency.

With that out of the way, the “copy” constructor is simple:

str::str(arena *a, str s)
    data = makefront<uint8_t>(s.len, a);
    len = s.len;
    for (ptrdiff_t i = 0; i < len; i++) {
        data[i] = s[i];

That’s everything we need to put it into action. For example, a function that deletes a file at a path following a path template.

char *tocstr(arena *a, str s)
    return (char *)concat(a, s, str{"\0"}).data;

bool removeconfig(str home, str program, arena scratch)
    str path = {};
    path = concat(&scratch, path, home);
    path = concat(&scratch, path, str{"/.config/"});
    path = concat(&scratch, path, program);
    path = concat(&scratch, path, str{"/rc"});
    return !unlink(tocstr(&scratch, path));

First, concat does all the heavy lifting in a null-terminated “C string” conversion function that operates in place if possible. In removeconfig I construct a path from path components, starting from a zero-initialized null string. In the first concat, this null string is “copied” into the arena, laying a foundation for additional concatenations. Each path component is copied in place, so unlike a dumb strcat, it’s not quadratic.

Even more, notice it supports arbitrary path lengths. No PATH_MAX, MAX_PATH, etc., it grows into the arena as needed. No huge stack variables necessary, and the scratch arena automatically frees the path on return. Fancier yet, imagine a variadic function that glues path components together with the proper path delimiter, and it wouldn’t involve a single, error-prone size calculation.

The str{} business is unfortunate. The char array constructor normally kicks in in these situations, but compilers can’t resolve the template without an explicit str object. Perhaps there’s a workaround, but I’m not yet savvy enough with C++ to figure it out. In the C version you’d always need to wrap those literals in the string macro.

Extending concatenation

The “operator” can be extended by defining more overloads. For example, to concatenate 32-bit integers to a string:

str concat(arena *a, str s, int32_t x)
    uint8_t  buf[16];
    uint8_t *end = buf + countof(buf);
    uint8_t *beg = end;
    int32_t  neg = x<0 ? x : -x;
    do {
        *--beg = '0' - neg%10;
    } while (neg /= 10);
    if (x < 0) {
        *--beg = '-';
    return concat(a, s, {beg, end});

Now we can, say, construct a randomly-generated temporary path:

str path = {};
path = concat(&scratch, path, tempdir);
path = concat(&scratch, path, str{"/temp"});
int32_t id = rand32(&rng);
path = concat(&scratch, path, id);

Keep adding more definitions like this and you’ll have something like, or complementing, buffered output. It doesn’t stop there. Code points concatenated as UTF-8:

str concat(arena *a, str s, char32_t rune)
    enum { REPLACEMENT_CHARACTER = 0xfffd };
    if (rune>=0xd800 && rune<=0xdfff) {

    uint8_t  buf[4];
    uint8_t *end = 0;
    if (rune < 0x80) {
        buf[0] = rune;
        end = buf + 1;
    } else if (rune < 0x800) {
        buf[0] =  (rune >>  6)         | 0xc0;
        buf[1] = ((rune >>  0) & 0x3f) | 0x80;
        end = buf + 2;
    } else if (rune < 0x10000) {
        buf[0] =  (rune >> 12)         | 0xe0;
        buf[1] = ((rune >>  6) & 0x3f) | 0x80;
        buf[2] = ((rune >>  0) & 0x3f) | 0x80;
        end = buf + 3;
    } else {
        buf[0] =  (rune >> 18)         | 0xf0;
        buf[1] = ((rune >> 12) & 0x3f) | 0x80;
        buf[2] = ((rune >>  6) & 0x3f) | 0x80;
        buf[3] = ((rune >>  0) & 0x3f) | 0x80;
        end = buf + 4;
    return concat(a, s, {buf, end});

That composes well for general UTF-8 handling. For example, to ingest Win32 strings (arguments, paths, etc.):

str convert(arena *perm, char16_t *s)
    str r = {};
    while (*s) {
        char32_t rune = decode(&s);
        r = concat(perm, r, rune);
    return r;

Beyond strings

One of my most useful C++ templates has been a span structure:

template<typename T>
struct span {
    T        *data = 0;
    ptrdiff_t len  = 0;

    span() = default;

    span(T *beg, T *end) : data{beg}, len{end-beg} {}

    span(arena *, span);  // for concat

    T &operator[](ptrdiff_t i) { return data[i]; }

The span::span definition looks exactly like str::str. In fact, we could nearly define strings as uint8_t spans:

typedef span<uint8_t> str;  // hypothetical

Though I’ve found strings to be just special enough not to be worth it.

This span definition is now fleshed out sufficiently to use concat with no additional definitions! However, outside of strings, concatenating spans is unusual. More often we want to append individual elements. Again, we can build on that core concat template:

template<typename T>
span<T> concat(arena *a, span<T> s, T v)
    return concat(a, s, span{&v, &v+1});

Now span is ready for 99% of its use cases. For example:

    span<int32_t> squares;
    for (int32_t i = 1; i <= 1000; i++) {
        squares = concat(&scratch, squares, i*i);

It’s often good enough, but it’s not ideal as a general purpose dynamic array. Each append makes a trip through arena allocation, and this span cannot efficiently shrink and then grow again. Sometimes we’d like to track capacity, covering both those cases.

template<typename T>
struct list {
    T        *data = 0;
    ptrdiff_t len  = 0;
    ptrdiff_t cap  = 0;

    list() = default;

    list(arena *, list);  // for concat

    T &operator[](ptrdiff_t i) { return data[i]; }

Unfortunately cap is a curve ball that the core template can’t handle, requiring a slightly more complex definition. Since concatenating whole list objects is unusual, a definition for appending single elements:

template<typename T>
list<T> concat(arena *a, list<T> s, T v)
    if (s.len == s.cap) {
        if ((char *)( != a->beg) {
            s = list<T>{a, s};
        ptrdiff_t extend = s.cap ? s.cap : 4;
        makefront<T>(extend, a);
        s.cap += extend;
    s[s.len++] = v;
    return s;

Note how inside the if it’s basically the same core definition. As before, this definition extends in place if possible, but otherwise handles it correctly anyway. In addition the above concerns, this list is more suited to having multiple “open” dynamic arrays at once.

This concatenative concept has been a useful way to think about a variety of situations in order to solve them effectively with arena allocation.

Update: NRK sharply points out that “extend in place” as expressed in concat is incompatible with the alloc_size and malloc GCC function attributes, which I’ve suggested in the past. While considering how to mitigate this, we’ve also discovered that alloc_size has always been fundamentally broken in GCC. Correct use is impossible, and so it must not be used.

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null program

Chris Wellons (PGP)
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