Explicitly deleting a shared_ptr

Question

Simple question here: are you allowed to explicitly delete a boost::shared_ptr yourself? Should you ever?

Clarifying, I don\'t mean delete the pointer held

Answer 1

Expliticly deleting comes in handy in some (very?) rare cases.

In addition to explicitly deleting, sometimes you HAVE to explicitly destruct a shared pointer when you are 'deleting' it!

Things can get weird when interfacing with C code, passing a shared_ptr as an opaque value.

For example I have the following for passing objects to and from the Lua scripting language which is written in C. (www.lua.org)

static void push( lua_State *L, std::shared_ptr sp )
{
    if( sp == nullptr ) {
        lua_pushnil( L );
        return;
    }

    // This is basically malloc from C++ point of view.
    void *ud = lua_newuserdata( L, sizeof(std::shared_ptr));

    // Copy constructor, bumps ref count.
    new(ud) std::shared_ptr( sp );

    luaL_setmetatable( L, B::class_name );
}

So thats a shared_ptr in some malloc'd memory. The reverse is this... (setup to be called just before Lua garbage collects an object and 'free's it).

static int destroy( lua_State *L )
{
    // Grab opaque pointer
    void* ud = luaL_checkudata( L, 1, B::class_name );

    std::shared_ptr *sp = static_cast*>(ud);

    // Explicitly called, as this was 'placement new'd
    // Decrements the ref count
    sp->~shared_ptr();

    return 0;
}