ImGuiListClipper behaviour with garbage collected instance (when not calling End)

[sonoro1234]

Version: 1.86
Branch:docking

with LuaJIT-ImGui

running this program

local igwin = require"imgui.window"
local win = igwin:GLFW(800,400, "list clipper")

function win:draw(ig)
    local cols = 10
    local clipper = ig.ImGuiListClipper()
    clipper:Begin(1000)
    while (clipper:Step()) do
        for line = clipper.DisplayStart,clipper.DisplayEnd-1 do
            for N=line*cols+1,line*cols+cols do
                if ig.Button(string.format("%d",N)) then
                    print(N)
                end
                if not ((N)%cols == 0) then ig.SameLine() end
            end
        end
    end
end

win:start()

I am getting this assertion:

Assertion failed!

Program: c:\anima\luajit.exe
File: C:\LuaGL\gitsources\anima\LuaJIT-ImGui\cimgui\imgui\imgui.cpp, Line 2443

Expression: data->ListClipper == this

I dont know if it is due to bug or misuse (Althought it used to work before)

[sonoro1234]

This does not happen with main.cpp modified for having ImGuiListClipper.
It does not happen in LuaJIT-ImGui if code is modified for making sure ImGuiListClipper is destroyed before creating a new one with code:

local igwin = require"imgui.window"
local win = igwin:SDL(800,400, "list clipper")

function win:draw(ig)
    local cols = 10
    if ig.Begin("testt") then
        local clipper = ig.ImGuiListClipper()
        clipper:Begin(1000)
        while (clipper:Step()) do
            for line = clipper.DisplayStart,clipper.DisplayEnd-1 do
                for N=line*cols+1,line*cols+cols do
                    if ig.Button(string.format("%d",N)) then
                        print(N)
                    end
                    if not ((N)%cols == 0) then ig.SameLine() end
                end
            end
        end
    end
    ig.End()
    collectgarbage()
end

win:start()

But if collectgarbage is not used (it calls __gc on clipper) so that there are several ImGuiListClipper creations before destroy is called the problem reappears.
It seems dificult to mimic this behaviour in C++: We should store ImGuiListClippers in an array and call destructor when the array exceds some given size.
Perhaps the cause is in imgui.cpp:2424 where TempData is set?

[ocornut]

What happen if you simply call clipper.End() at the end of your loop ?

[sonoro1234]

What happen if you simply call clipper.End() at the end of your loop ?

if collectgarbage is active there is a crash without assertion in the first iteration

gdb posts SIGSEGV in memcpy

[sonoro1234]

correction: with clipper.End() at the end of my loop problem is gone

[ocornut]

I will try to rework things to have the final step automatically do the End() before waiting for the destructor. I recall there was an issue preventing to do this easily but i’ll work it out.

[sonoro1234]

Having to call clipper.End at the end of Step loop seems to me a clearer pattern for the casual ImGui user

[ocornut]

seems to me a clearer pattern for the casual ImGui user

Yes but technically we have a regression now, and Rust users are also affected. I'll look into it now.

[ocornut]

Pushed a fix now d9e60d2

Thanks for reporting!