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Rici Lake wrote:

On 16-May-05, at 10:37 PM, Chris Marrin wrote:

The other question is still outstanding, though. If I give a string to Lua, will it always give me back the same address in every call to lua_tostring() for that same sequence of characters? Does this work across GC cycles, for instance?

The short answer is yes, providing the string doesn't get garbage collected (that is, as long as you keep a reference to it somewhere).

The longer answer is that the reference manual clearly states that the string's address will remain constant provided the string is on the stack. This presumably gives flexibility to language designers, but the "reference implementation" of Lua does not currently take advantage of that.

Ok, RTFM makes it a bit more clear. When it says "stack" it really means as long as the string exists as a valid object inside the Lua VM, right. It implies this where it talks about the registry (3.18). So that means I should be able to do this:

    lua_pushstring(L, "foo");
    const char * myFoo = lua_tostring(L, -1);
    lua_pushlightuserdata(L, s);
    lua_insert(L, -2);
    lua_rawset(L, L_REGISTRYINDEX);

Now I can put 's' in some global place and my C++ code can use this to do equality comparisons against strings in the stack, right? That would be very cool since I use strings a lot. If this is all true, then I can also push a string onto the stack like this:

    lua_pushlightuserdata(L, myFoo);
    lua_rawget(L, L_REGISTRYINDEX);

Any idea about whether this is faster that just calling lua_pushstring()? Has anyone tested the performance of this? If not, I can do it and let you know...

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"As a general rule,don't solve puzzles that open portals to Hell"'