A library protocol for dynamically-sized objects

Mathias Gaunard <loufoque@gmail.com>
Wed, 21 Jan 2009 12:51:11 CST
The C++ construction model of objects doesn't allow dynamically-sized
objects, which restricts how optimal in terms of compactness, memory
usage, locality and efficiency certain objects can be.

I thus propose a fairly simple library protocol so that one can design
dynamically-sized objects. Any help to better the system would be

The idea is simple: if the type is dynamically-sized, then instead of
using sizeof to know how much memory to allocate and then calling the
constructor on it, a type-specific functor that returns the size of
the object and that takes the constructor arguments is called, then
memory is allocated, then the constructor is called.

To construct a dynamically-sized object T(arg1, ..., argN);, you would
write something like this:
void* p = ::operator new(dyn_sizeof<T>()(arg1, ..., argN)); // or
alloca or whatever you want
T* t = new(p) T(arg1, ..., argN);

And type T would specialize dyn_sizeof to be an appropriate functor.

Here is an example with a string type:

struct compact_string
    template<typename N>
    compact_string(const char (&s)[N]) : len(N);
        memcpy(str, s, N);

    compact_string(const char* s) : len(0)
        for(size_t i=0; s[i]; i++)
            str[i] = s[i];

    compact_string(const char* s, size_t n) : len(n)
        memcpy(str, s, n);

    compact_string(const compact_string& s) : len(s.len)
        memcpy(str, s.str, len);

    compact_string& operator=(const compact_string& s)
        if(s.len > len)
            throw whatever;

        len = s.len;
        memcpy(str, s.str, len);

    size_t size() const { return len; }
    char& operator[](size_t i) { return str[i]; }
    const char& operator[](size_t i) const { return str[i]; }

    size_t len;
    char str[1];

template<> dyn_sizeof<compact_string>
    template<typename N>
    size_t operator()(const char (&s)[N]) { return sizeof(size_t) +
N; }

    size_t operator()(const char* s) { return sizeof(size_t) + strlen
(s); }
    size_t operator()(const char* s, size_t n) { return sizeof(size_t)
+ n; }
    size_t operator()(const compact_string& s) { return sizeof(size_t)
+ s.size(); }

There are of course a few problems that need to be solved. One is to
not invoke undefined or unspecified behaviour, which this probably
The other would be that sizeof should not work with dynamically-sized
objects, because if it does the objects might be constructed without
the proper amount of memory.

A macro to allocate on the stack would be quite useful:

DECLARE(compact_string, s, ("foo"));

would expand to

compact_string& s = *new(alloca(dyn_sizeof<compact_string>()("foo")))
scope_destruct<compact_string> s_destruct(s);


template<typename T>
struct scope_destruct
    scope_destruct(T& t_) : t(t_) {}
    ~scope_destruct() { t->~T(); }
    T& t;

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      [ comp.lang.c++.moderated. First time posters: Do this! ]

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