imgui: Unable to get ImGui to show in simple SDL program

Hi, I’m learning SDL and ImGui. I compiled and successfully ran the sdl_opengl3_example project. However, when I write my own sdl program using imgui_impl_sdl_gl3.cpp and imgui_impl_sdl_gl3.h, I cannot get ImGui to show.

The code is simple; it sets up the SDL window and renderer, displays two primitives, and handles the ESC keyboard input. I wrote in the ImGui code based on the main.cpp example:

#include "SDL.h"
#include "imgui.h"
#include "imgui_impl_sdl_gl3.h"
#include "GL/gl3w.h"

int main(int, char**)
{
    // SDL initialization
    SDL_Window* window = SDL_CreateWindow("test",  SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_SHOWN);
    SDL_Event* event = new SDL_Event();
    SDL_Renderer* render = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);

    bool quit = false;

    // ImGui Initialization
    gl3wInit();
    ImGui_ImplSdlGL3_Init(window);

    // Main loop
    while(!quit && event->type != SDL_QUIT) {
        SDL_PollEvent(event);
        SDL_RenderClear(render);

        // ImGui keyboard events
        ImGui_ImplSdlGL3_ProcessEvent(event);

        if (event->type == SDL_KEYDOWN) {
            switch(event->key.keysym.sym) {
                case SDLK_ESCAPE:
                    quit = true;
                    ImGui_ImplSdlGL3_Shutdown();
                    break;
                default:
                    break;
            }
        }

        // Draw some primitives
        SDL_Rect fillRect = { 640 / 4, 480 / 4, 640 / 2, 480 / 2 }; 
        SDL_SetRenderDrawColor( render, 0xFF, 0x00, 0x00, 0xFF ); 
        SDL_RenderFillRect( render, &fillRect ); 
        SDL_Rect outlineRect = { 640 / 6, 480 / 6, 640 * 2 / 3, 480 * 2 / 3 };
        SDL_SetRenderDrawColor( render, 0x00, 0xFF, 0x00, 0xFF );     
        SDL_RenderDrawRect( render, &outlineRect );

        // Update SDL screen
        SDL_RenderPresent(render);

        // ImGui
        ImGui_ImplSdlGL3_NewFrame(window);
        ImGui::Begin("hello world");
        ImGui::Text("Hello, I am here");
        ImGui::End();

        // Render ImGui
        ImGui::Render();
    }

    return 0;
}

This only shows the two primitives on the screen. I have drawn ImGui last, so it should be overlayed on top of the primitives. I suspect it’s not because ImGui is using opengl, and I’m using SDL Renderer, but I’m not sure since I think SDL2 uses opengl under the hood anyway. Is it possible to get ImGui to use SDL’s Renderer instead?

Or is the problem something different?