ImGui is not being displayed - c++

Question

I'm trying to make an OpenGL program on Windows. Since the main exe file was getting bigger and bigger, I decided to split it into DLLs. And this is how my problem is started.
For ImGui functions, I created a class. Here is render() function of my class:
cout << "render" << endl;
imgui_newFrame();
{
ImGui::SetNextWindowSize(ImVec2(30, 30), ImGuiSetCond_FirstUseEver);
ImGui::Begin("Test", &show_another_window);
ImGui::Button("Test Window");
ImGui::End();
}
glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
ImGui::Render();
Before calling render() function in my class, I initiate ImGui in another function in my class with this:
if (!imgui_init(glfw_window)) {
return false;
}
And here is my main glfw loop:
while (!glfwWindowShouldClose(window)) {
glClear(GL_COLOR_BUFFER_BIT);
glfwPollEvents();
MyMyGuiClass.render(); //here i ask my class to render imgui
glfwSwapBuffers(window);
}
With this code I am able to make ImGui clear window color (glClearColor function works and my console prints "render")
But it doesn't show anything else.
By the way, here is the command working perfectly when I run it.
while (!glfwWindowShouldClose(window)) {
glClear(GL_COLOR_BUFFER_BIT);
glfwPollEvents();
imgui_newFrame();
{
ImGui::SetNextWindowSize(ImVec2(30, 30), ImGuiSetCond_FirstUseEver);
ImGui::Begin("", &show_another_window);
ImGui::Button("Test Window");
ImGui::End();
}
glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
ImGui::Render();
glfwSwapBuffers(window);
}
I'm using VS2017 and my compiler doesn't show any warnings or errors when i compile this. Also I tried to make my class' functions static, and I got nothing.
So simply, is ImGui not able to render when called from inside a class?

Answer 1

The problem you're running into is that ImGui maintains a global state and that this state has to be kept somewhere. ImGui keeps it around in a module-local global symbol.
Note the "module-local" here! It means that every DLL (and the main EXE) gets its very own copy of that state. So doing things with ImGui in DLL "A" (or EXE "1" for that matter) will operate on its very own instance of ImGui state.
There is a solution to this, by making that pesky ImGui global state shared across DLLs. How to share data between DLLs is described in MSDN here https://msdn.microsoft.com/en-us/library/h90dkhs0(v=vs.90).aspx – as for the details in ImGui itself. It mostly boils down to the ImGuiContext that's being used. For now this is a module-local global variable, but the ImGui devs plan on making it explicit-per call and user managed eventually.
Comment from the ImGui code:
// Default context storage + current context pointer. Implicitely used by all
// ImGui functions. Always assumed to be != NULL. Change to a different context
// by calling ImGui::SetCurrentContext() ImGui is currently not thread-safe
// because of this variable. If you want thread-safety to allow N threads to
// access N different contexts, you might work around it by: - Having multiple
// instances of the ImGui code compiled inside different namespace
// (easiest/safest, if you have a finite number of contexts) - or: Changing
// this variable to be TLS. You may #define GImGui in imconfig.h for further
// custom hackery. Future development aim to make this context pointer explicit
// to all calls. Also read https://github.com/ocornut/imgui/issues/586