Backends: OpenGL3: Attempt to automatically detect default GL loader by using __has_include. Followup to 44cd8e3 (#2798)

docs/CHANGELOG.txt

@@ -74,6 +74,7 @@ Other Changes:
   usage has never been reported as a problem, so this is merely a touch of overzealous luxury. (#2636)
 - Backends: OpenGL3: Tweaked initialization code allow application calling ImGui_ImplOpenGL3_CreateFontsTexture()
   before ImGui_ImplOpenGL3_NewFrame() if for some reason they wanted.
+- Backends: OpenGL3: Attempt to automatically detect default GL loader by using __has_include. (#2798) [@o-micron]
 - Backends: DX11: Fixed GSGetShader() call not passing an initialized instance count,
   would generally make the debug layer complain (Added in 1.72).
 - Backends: Vulkan: Added support for specifying multisample count.

examples/example_glfw_opengl3/main.cpp

@@ -7,9 +7,10 @@
 #include "imgui_impl_opengl3.h"
 #include <stdio.h>
 
-// About OpenGL function loaders: modern OpenGL doesn't have a standard header file and requires individual function pointers to be loaded manually.
-// Helper libraries are often used for this purpose! Here we are supporting a few common ones: gl3w, glew, glad.
-// You may use another loader/header of your choice (glext, glLoadGen, etc.), or chose to manually implement your own.
+// About Desktop OpenGL function loaders:
+//  Modern desktop OpenGL doesn't have a standard portable header file to load OpenGL function pointers.
+//  Helper libraries are often used for this purpose! Here we are supporting a few common ones (gl3w, glew, glad).
+//  You may use another loader/header of your choice (glext, glLoadGen, etc.), or chose to manually implement your own.
 #if defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)
 #include <GL/gl3w.h>    // Initialize with gl3wInit()
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)

examples/example_sdl_opengl3/main.cpp

@@ -9,9 +9,10 @@
 #include <stdio.h>
 #include <SDL.h>
 
-// About OpenGL function loaders: modern OpenGL doesn't have a standard header file and requires individual function pointers to be loaded manually.
-// Helper libraries are often used for this purpose! Here we are supporting a few common ones: gl3w, glew, glad.
-// You may use another loader/header of your choice (glext, glLoadGen, etc.), or chose to manually implement your own.
+// About Desktop OpenGL function loaders:
+//  Modern desktop OpenGL doesn't have a standard portable header file to load OpenGL function pointers.
+//  Helper libraries are often used for this purpose! Here we are supporting a few common ones (gl3w, glew, glad).
+//  You may use another loader/header of your choice (glext, glLoadGen, etc.), or chose to manually implement your own.
 #if defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)
 #include <GL/gl3w.h>    // Initialize with gl3wInit()
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)

examples/imgui_impl_opengl3.cpp

@@ -1,5 +1,5 @@
 // dear imgui: Renderer for modern OpenGL with shaders / programmatic pipeline
-// - Desktop GL: 3.x 4.x
+// - Desktop GL: 2.x 3.x 4.x
 // - Embedded GL: ES 2.0 (WebGL 1.0), ES 3.0 (WebGL 2.0)
 // This needs to be used along with a Platform Binding (e.g. GLFW, SDL, Win32, custom..)
 
@@ -13,6 +13,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2019-09-22: OpenGL: Detect default GL loader using __has_include compiler facility.
 //  2019-09-16: OpenGL: Tweak initialization code to allow application calling ImGui_ImplOpenGL3_CreateFontsTexture() before the first NewFrame() call.
 //  2019-05-29: OpenGL: Desktop GL only: Added support for large mesh (64K+ vertices), enable ImGuiBackendFlags_RendererHasVtxOffset flag.
 //  2019-04-30: OpenGL: Added support for special ImDrawCallback_ResetRenderState callback to reset render state.
@@ -79,12 +80,21 @@
 // Auto-detect GL version
 #if !defined(IMGUI_IMPL_OPENGL_ES2) && !defined(IMGUI_IMPL_OPENGL_ES3)
 #if (defined(__APPLE__) && (TARGET_OS_IOS || TARGET_OS_TV)) || (defined(__ANDROID__))
-#define IMGUI_IMPL_OPENGL_ES3       // iOS, Android  -> GL ES 3, "#version 300 es"
+#define IMGUI_IMPL_OPENGL_ES3           // iOS, Android  -> GL ES 3, "#version 300 es"
+#undef IMGUI_IMPL_OPENGL_LOADER_GL3W
+#undef IMGUI_IMPL_OPENGL_LOADER_GLEW
+#undef IMGUI_IMPL_OPENGL_LOADER_GLAD
+#undef IMGUI_IMPL_OPENGL_LOADER_CUSTOM
 #elif defined(__EMSCRIPTEN__)
-#define IMGUI_IMPL_OPENGL_ES2       // Emscripten    -> GL ES 2, "#version 100"
+#define IMGUI_IMPL_OPENGL_ES2           // Emscripten    -> GL ES 2, "#version 100"
+#undef IMGUI_IMPL_OPENGL_LOADER_GL3W
+#undef IMGUI_IMPL_OPENGL_LOADER_GLEW
+#undef IMGUI_IMPL_OPENGL_LOADER_GLAD
+#undef IMGUI_IMPL_OPENGL_LOADER_CUSTOM
 #endif
 #endif
 
+// GL includes
 #if defined(IMGUI_IMPL_OPENGL_ES2)
 #include <GLES2/gl2.h>
 #elif defined(IMGUI_IMPL_OPENGL_ES3)
@@ -149,6 +159,23 @@ bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
     strcpy(g_GlslVersionString, glsl_version);
     strcat(g_GlslVersionString, "\n");
 
+    // Dummy construct to make it easily visible in the IDE and debugger which GL loader has been selected. 
+    // The code actually never uses the 'gl_loader' variable! It is only here so you can read it!
+    // If auto-detection fails or doesn't select the same GL loader file as used by your application, 
+    // you are likely to get a crash below.
+    // You can explicitly select a loader by using '#define IMGUI_IMPL_OPENGL_LOADER_XXX' in imconfig.h or compiler command-line.
+    const char* gl_loader = "Unknown";
+    IM_UNUSED(gl_loader);
+#if defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)
+    gl_loader = "GL3W";
+#elif defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)
+    gl_loader = "GLEW";
+#elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
+    gl_loader = "GLAD";
+#else IMGUI_IMPL_OPENGL_LOADER_CUSTOM
+    gl_loader = "Custom";
+#endif
+
     // Make a dummy GL call (we don't actually need the result)
     // IF YOU GET A CRASH HERE: it probably means that you haven't initialized the OpenGL function loader used by this code.
     // Desktop OpenGL 3/4 need a function loader. See the IMGUI_IMPL_OPENGL_LOADER_xxx explanation above.

examples/imgui_impl_opengl3.h

@@ -1,5 +1,5 @@
 // dear imgui: Renderer for modern OpenGL with shaders / programmatic pipeline
-// - Desktop GL: 3.x 4.x
+// - Desktop GL: 2.x 3.x 4.x
 // - Embedded GL: ES 2.0 (WebGL 1.0), ES 3.0 (WebGL 2.0)
 // This needs to be used along with a Platform Binding (e.g. GLFW, SDL, Win32, custom..)
 
@@ -23,37 +23,42 @@
 
 #pragma once
 
+// Backend API
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_Init(const char* glsl_version = NULL);
+IMGUI_IMPL_API void     ImGui_ImplOpenGL3_Shutdown();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL3_NewFrame();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data);
+
+// (Optional) Called by Init/NewFrame/Shutdown
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateFontsTexture();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyFontsTexture();
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateDeviceObjects();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyDeviceObjects();
+
 // Specific OpenGL versions
 //#define IMGUI_IMPL_OPENGL_ES2     // Auto-detected on Emscripten
 //#define IMGUI_IMPL_OPENGL_ES3     // Auto-detected on iOS/Android
 
-// Set default OpenGL3 loader to be gl3w
-#if !defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)     \
- && !defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)     \
- && !defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)     \
+// Desktop OpenGL: attempt to detect default GL loader based on available header files.
+// If auto-detection fails or doesn't select the same GL loader file as used by your application, 
+// you are likely to get a crash in ImGui_ImplOpenGL3_Init(). 
+// You can explicitly select a loader by using '#define IMGUI_IMPL_OPENGL_LOADER_XXX' in imconfig.h or compiler command-line.
+#if !defined(IMGUI_IMPL_OPENGL_LOADER_GL3W) \
+ && !defined(IMGUI_IMPL_OPENGL_LOADER_GLEW) \
+ && !defined(IMGUI_IMPL_OPENGL_LOADER_GLAD) \
  && !defined(IMGUI_IMPL_OPENGL_LOADER_CUSTOM)
-    // to avoid problem with non-clang compilers not having this macro.
     #if defined(__has_include)
-        // check if the header exists, then automatically define the macros ..
         #if __has_include(<GL/glew.h>)
             #define IMGUI_IMPL_OPENGL_LOADER_GLEW
         #elif __has_include(<glad/glad.h>)
             #define IMGUI_IMPL_OPENGL_LOADER_GLAD
-        #else
+        #elif __has_include(<GL/gl3w.h>)
             #define IMGUI_IMPL_OPENGL_LOADER_GL3W
+        #else
+            #error "Cannot detect OpenGL loader!"
         #endif
     #else
-        #define IMGUI_IMPL_OPENGL_LOADER_GL3W
+        #define IMGUI_IMPL_OPENGL_LOADER_GL3W       // Default to GL3W
     #endif
 #endif
 
-IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_Init(const char* glsl_version = NULL);
-IMGUI_IMPL_API void     ImGui_ImplOpenGL3_Shutdown();
-IMGUI_IMPL_API void     ImGui_ImplOpenGL3_NewFrame();
-IMGUI_IMPL_API void     ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data);
-
-// Called by Init/NewFrame/Shutdown
-IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateFontsTexture();
-IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyFontsTexture();
-IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateDeviceObjects();
-IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyDeviceObjects();