Multiple Contexts shutdown leak or not clear

[zlnimda]

Hello, I am using imgui and my app has multiple windows.
The version used is one year old but the weird problem is still present in last version.
I used as recommended since there the use of multiple context, one by windows.

I hooked our memory management system on Alloc & Free callbacks and it seems there was some leaks.
At first my code was looking like that:

auto defaultContext = ImGui::GetCurrentContext(); 
[...]
ImGui::CreateContext(...); // multiple times 
[...]
ImGui::DestroyContext(...); // multiple times
[...]
ImGui::SetCurrentContext(defaultContext);
ImGui::Shutdown();

I found that each allocation by context was not freed, so I changed my code in this :

auto defaultContext = ImGui::GetCurrentContext(); 
[...]
wContext = ImGui::CreateContext(...); // multiple times
[...]
ImGui::SetCurrentContext(wContext);
ImGui::Shutdown();
ImGui::DestroyContext(...);
[...]
ImGui::SetCurrentContext(defaultContext);
ImGui::Shutdown();

But it crashed because It seems that I use a single Font which is shared between context.

// imgui.cpp line:716
static ImFontAtlas      GImDefaultFontAtlas;
// imgui.cpp line:796
ImGuiIO::ImGuiIO()
{
    Fonts = &GImDefaultFontAtlas;
}

So now everything is working (no leak anymore) with this change:

ImGui::SetCurrentContext(wContext);
+ ImGui::GetIO().Fonts = null;
ImGui::Shutdown();

But this looks so unexpected and not documented in comments. I'm not sure this is the way it should be uninitialized, so I prefer to refer my code here to know what it is the real purpose of Shutdown() and DestroyContext(). The destructor ~ImGuiContext doesn't exist but is explicitly called in DestroyContext().

Please let me know what you think about this case.

[ocornut]

Thank you for the detailed post.

There are several issues related to this (#591, #1007, #586, #992).

I don't have the exact answer right now but those issues have been lingering and I will try to tackle some of them soon.

Also note that I am working on a new system to use a single imgui context for multiple windows (see #1542). If you are not looking for multi-threading or rendering at different frequency, using that new system will be much better :)

[ocornut]

@zlnimda With the changes included in #1565 (now merged) all the issues here should be solved :)