c++ - Lua C API: what's the difference between lua_gettop() and -1? -

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i don't understand stack exactly. 

lua_gettop()

returns index of top element in stack. because indices start @ 1, result equal number of elements in stack (and 0 means empty stack).

so what's difference between , -1?

lua_getglobal(l,"foo"); if( lua_isfunction(l,lua_gettop(l)) ) {  lua_getglobal(l,"foo"); if( lua_isfunction(l,-1) ) {

you can imagine stack growing bottom, bottom (i.e., first pushed) element having index 1, push element (index 2), 1 (index 3), etc.. have situation:

+-----------------------+ | element index 6  | <-- top ("relative" index -1) +-----------------------+ | element index 5  | <-- -2 +-----------------------+ | element index 4  | <-- -3 +-----------------------+ | element index 3  | <-- -4 +-----------------------+ | element index 2  | <-- -5 +-----------------------+ | element index 1  | <-- bottom ("relative" index -6 ) +-----------------------+

you "normal index" (the 1 indexing bottom) absolute index of element (like of array in c, besides starting 1). instead negative index "relative" top of stack.lua_gettop gives absolute index of stack top (which has relative index -1).

why there 2 ways of indexing stack, then? because useful access elements array (using absolute index) , need access last pushed elements (so indexing top).

btw, visualize lua stack reversed: starting above , growing downwards (i.e. stack top @ bottom of mental representation). find mental model more useful because interpret index -1 "step in code (upwards, therefore) until find the first push". in fashion, index -2 "step in code until find second push", etc.. helps me identify pushed what.

however, avoid confusion, here used more classical representation, stack top drawn @ top!


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