Why does ImGui attempt to free memory that I (the user) have created?

[johannesugb]

Version/Branch of Dear ImGui:

Version: v1.89.9
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_glfw.cpp + imgui_impl_vulkan.cpp
Compiler: VS version v17.7.4, MSVC version 19.37.32824
Operating System: Windows 10, Windows 11

My Issue/Question:

I am using AddFontFromMemoryTTF to successfully load a TTF font from memory. What I don't understand is that I am not the one responsible for deleting the allocated memory, as it appears. This looks like a violation of responsibility principles.

Here's how I load the TTF font:

ImGuiIO& io = ImGui::GetIO();
io.Fonts->Clear();
auto  font_cfg = ImFontConfig();
ImFormatString(font_cfg.Name, IM_ARRAYSIZE(font_cfg.Name), "%s, %.0fpx", "MyFont.ttf", 13.f);
uint8_t* data= new uint8_t[] { 0x0, 0x1, 0x0, 0x0, 0x0, 0x11, 0x1, 0x0, 0x0, 0x4, ...  };
size_t data_size = 273900;
io.Fonts->AddFontFromMemoryTTF(data, (int)data_size, fontSize, &font_cfg, nullptr);

The crucial part here is the line where I allocate the font data: uint8_t* data= new uint8_t[] { ... };

Now, I would expect that I (the user of ImGui) has to delete data after uploading the fonts (which in my case means after ImGui_ImplVulkan_CreateFontsTexture and ImGui_ImplVulkan_DestroyFontUploadObjects and using a fence for these operations to have completed) and execute a delete [] data; afterwards.

But if I execute that delete [] data, then my compiler points out that that ImGui attempts to free that memory again on its own, namely here:

static void    FreeWrapper(void* ptr, void* user_data)        { IM_UNUSED(user_data); free(ptr); }

which is in imgui.cpp, line 1146.

The comments indicate that I could "SetAllocatorFunctions() to change them"... but why should I actually? I only want to free that data on my own and not have ImGui free my data.

What are your thoughts on this?
What is the intended way of handling this?

[ocornut]

See docs: Loading Font Data from Memory

ImFont* font = io.Fonts->AddFontFromMemoryTTF(data, data_size, size_pixels, ...);

IMPORTANT: AddFontFromMemoryTTF() by default transfer ownership of the data buffer to the font atlas, which will attempt to free it on destruction. This was to avoid an unnecessary copy, and is perhaps not a good API (a future version will redesign it). If you want to keep ownership of the data and free it yourself, you need to clear the FontDataOwnedByAtlas field:

ImFontConfig font_cfg;
font_cfg.FontDataOwnedByAtlas = false;
ImFont* font = io.Fonts->AddFontFromMemoryTTF(data, data_size, size_pixels, &font_cfg);

[johannesugb]

Sorry for not reading the docs better & thank you. 🙏