Added commentary about DLL boundaries. Added GetAllocatorFunctions(). Tidying up. (#3836)

docs/CHANGELOG.txt

@@ -50,6 +50,7 @@ Other Changes:
   with ImGuiSliderFlags_AlwaysClamp + only one of either p_min or p_max set. (#3824) [@harry75369]
 - Window: Shrink close button hit-testing region when it covers an abnormally high portion of the window visible
   area (e.g. when window is collapsed + moved in a corner) to facilitate moving the window away. (#3825)
+- Added GetAllocatorFunctions() to facilitate sharing allocators accross DLL boundaries. (#3836)
 - ImDrawList: AddCircle, AddCircleFilled(): Tweaked default segment count calculation to honor MaxError
   with more accuracy. Made default segment count always even for better looking result. (#3808) [@thedmd]
 - ImDrawList: AddCircle, AddCircleFilled(): New default for style.

imconfig.h

@@ -20,7 +20,9 @@
 //#define IM_ASSERT(_EXPR)  ((void)(_EXPR))     // Disable asserts
 
 //---- Define attributes of all API symbols declarations, e.g. for DLL under Windows
-// Using dear imgui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
+// Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
+// DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
+// for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for more details.
 //#define IMGUI_API __declspec( dllexport )
 //#define IMGUI_API __declspec( dllimport )
 

imgui.cpp

@@ -911,27 +911,33 @@ static void             UpdateViewportsNewFrame();
 // [SECTION] CONTEXT AND MEMORY ALLOCATORS
 //-----------------------------------------------------------------------------
 
+// DLL users:
+// - Heaps and globals are not shared across DLL boundaries!
+// - You will need to call SetCurrentContext() + SetAllocatorFunctions() for each static/DLL boundary you are calling from.
+// - Same apply for hot-reloading mechanisms that are reliant on reloading DLL (note that many hot-reloading mechanism works without DLL).
+// - Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
+// - Confused? In a debugger: add GImGui to your watch window and notice how its value changes depending on your current location (which DLL boundary you are in).
+
 // Current context pointer. Implicitly used by all Dear ImGui functions. Always assumed to be != NULL.
-// ImGui::CreateContext() will automatically set this pointer if it is NULL. Change to a different context by calling ImGui::SetCurrentContext().
-// 1) Important: globals are not shared across DLL boundaries! If you use DLLs or any form of hot-reloading: you will need to call
-//    SetCurrentContext() (with the pointer you got from CreateContext) from each unique static/DLL boundary, and after each hot-reloading.
-//    In your debugger, add GImGui to your watch window and notice how its value changes depending on which location you are currently stepping into.
-// 2) Important: Dear ImGui functions are not thread-safe because of this pointer.
-//    If you want thread-safety to allow N threads to access N different contexts, you can:
-//    - Change this variable to use thread local storage so each thread can refer to a different context, in imconfig.h:
-//          struct ImGuiContext;
-//          extern thread_local ImGuiContext* MyImGuiTLS;
-//          #define GImGui MyImGuiTLS
-//      And then define MyImGuiTLS in one of your cpp file. Note that thread_local is a C++11 keyword, earlier C++ uses compiler-specific keyword.
-//    - Future development aim to make this context pointer explicit to all calls. Also read https://github.com/ocornut/imgui/issues/586
-//    - If you need a finite number of contexts, you may compile and use multiple instances of the ImGui code from different namespace.
+// - ImGui::CreateContext() will automatically set this pointer if it is NULL.
+//   Change to a different context by calling ImGui::SetCurrentContext().
+// - Important: Dear ImGui functions are not thread-safe because of this pointer.
+//   If you want thread-safety to allow N threads to access N different contexts:
+//   - Change this variable to use thread local storage so each thread can refer to a different context, in your imconfig.h:
+//         struct ImGuiContext;
+//         extern thread_local ImGuiContext* MyImGuiTLS;
+//         #define GImGui MyImGuiTLS
+//     And then define MyImGuiTLS in one of your cpp file. Note that thread_local is a C++11 keyword, earlier C++ uses compiler-specific keyword.
+//   - Future development aim to make this context pointer explicit to all calls. Also read https://github.com/ocornut/imgui/issues/586
+//   - If you need a finite number of contexts, you may compile and use multiple instances of the ImGui code from different namespace.
+// - DLL users: read comments above.
 #ifndef GImGui
 ImGuiContext*   GImGui = NULL;
 #endif
 
 // Memory Allocator functions. Use SetAllocatorFunctions() to change them.
-// If you use DLL hotreloading you might need to call SetAllocatorFunctions() after reloading code from this file.
-// Otherwise, you probably don't want to modify them mid-program, and if you use global/static e.g. ImVector<> instances you may need to keep them accessible during program destruction.
+// - You probably don't want to modify those mid-program, and if you use global/static e.g. ImVector<> instances you may need to keep them accessible during program destruction.
+// - DLL users: read comments above.
 #ifndef IMGUI_DISABLE_DEFAULT_ALLOCATORS
 static void*   MallocWrapper(size_t size, void* user_data)    { IM_UNUSED(user_data); return malloc(size); }
 static void    FreeWrapper(void* ptr, void* user_data)        { IM_UNUSED(user_data); free(ptr); }
@@ -939,10 +945,9 @@ static void    FreeWrapper(void* ptr, void* user_data)        { IM_UNUSED(user_d
 static void*   MallocWrapper(size_t size, void* user_data)    { IM_UNUSED(user_data); IM_UNUSED(size); IM_ASSERT(0); return NULL; }
 static void    FreeWrapper(void* ptr, void* user_data)        { IM_UNUSED(user_data); IM_UNUSED(ptr); IM_ASSERT(0); }
 #endif
-
-static void*  (*GImAllocatorAllocFunc)(size_t size, void* user_data) = MallocWrapper;
-static void   (*GImAllocatorFreeFunc)(void* ptr, void* user_data) = FreeWrapper;
-static void*    GImAllocatorUserData = NULL;
+static ImGuiMemAllocFunc*   GImAllocatorAllocFunc = MallocWrapper;
+static ImGuiMemFreeFunc*    GImAllocatorFreeFunc = FreeWrapper;
+static void*                GImAllocatorUserData = NULL;
 
 //-----------------------------------------------------------------------------
 // [SECTION] USER FACING STRUCTURES (ImGuiStyle, ImGuiIO)
@@ -3330,13 +3335,21 @@ void ImGui::SetCurrentContext(ImGuiContext* ctx)
 #endif
 }
 
-void ImGui::SetAllocatorFunctions(void* (*alloc_func)(size_t sz, void* user_data), void (*free_func)(void* ptr, void* user_data), void* user_data)
+void ImGui::SetAllocatorFunctions(ImGuiMemAllocFunc* alloc_func, ImGuiMemFreeFunc* free_func, void* user_data)
 {
     GImAllocatorAllocFunc = alloc_func;
     GImAllocatorFreeFunc = free_func;
     GImAllocatorUserData = user_data;
 }
 
+// This is provided to facilitate copying allocators from one static/DLL boundary to another (e.g. retrieve default allocator of your executable address space)
+void ImGui::GetAllocatorFunctions(ImGuiMemAllocFunc** p_alloc_func, ImGuiMemFreeFunc** p_free_func, void** p_user_data)
+{
+    *p_alloc_func = GImAllocatorAllocFunc;
+    *p_free_func = GImAllocatorFreeFunc;
+    *p_user_data = GImAllocatorUserData;
+}
+
 ImGuiContext* ImGui::CreateContext(ImFontAtlas* shared_font_atlas)
 {
     ImGuiContext* ctx = IM_NEW(ImGuiContext)(shared_font_atlas);

imgui.h

@@ -193,6 +193,8 @@ typedef void* ImTextureID;          // User data for rendering backend to identi
 typedef unsigned int ImGuiID;       // A unique ID used by widgets, typically hashed from a stack of string.
 typedef int (*ImGuiInputTextCallback)(ImGuiInputTextCallbackData* data);    // Callback function for ImGui::InputText()
 typedef void (*ImGuiSizeCallback)(ImGuiSizeCallbackData* data);             // Callback function for ImGui::SetNextWindowSizeConstraints()
+typedef void* (ImGuiMemAllocFunc)(size_t sz, void* user_data);              // Function signature for ImGui::SetAllocatorFunctions()
+typedef void (ImGuiMemFreeFunc)(void* ptr, void* user_data);                // Function signature for ImGui::SetAllocatorFunctions()
 
 // Character types
 // (we generally use UTF-8 encoded string in the API. This is storage specifically for a decoded character used for keyboard input and display)
@@ -255,8 +257,9 @@ struct ImVec4
 namespace ImGui
 {
     // Context creation and access
-    // Each context create its own ImFontAtlas by default. You may instance one yourself and pass it to CreateContext() to share a font atlas between imgui contexts.
-    // None of those functions is reliant on the current context.
+    // - Each context create its own ImFontAtlas by default. You may instance one yourself and pass it to CreateContext() to share a font atlas between contexts.
+    // - DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
+    //   for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for details.
     IMGUI_API ImGuiContext* CreateContext(ImFontAtlas* shared_font_atlas = NULL);
     IMGUI_API void          DestroyContext(ImGuiContext* ctx = NULL);   // NULL = destroy current context
     IMGUI_API ImGuiContext* GetCurrentContext();
@@ -868,9 +871,11 @@ namespace ImGui
     IMGUI_API bool          DebugCheckVersionAndDataLayout(const char* version_str, size_t sz_io, size_t sz_style, size_t sz_vec2, size_t sz_vec4, size_t sz_drawvert, size_t sz_drawidx); // This is called by IMGUI_CHECKVERSION() macro.
 
     // Memory Allocators
-    // - All those functions are not reliant on the current context.
-    // - If you reload the contents of imgui.cpp at runtime, you may need to call SetCurrentContext() + SetAllocatorFunctions() again because we use global storage for those.
-    IMGUI_API void          SetAllocatorFunctions(void* (*alloc_func)(size_t sz, void* user_data), void (*free_func)(void* ptr, void* user_data), void* user_data = NULL);
+    // - Those functions are not reliant on the current context.
+    // - DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
+    //   for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for more details.
+    IMGUI_API void          SetAllocatorFunctions(ImGuiMemAllocFunc* alloc_func, ImGuiMemFreeFunc* free_func, void* user_data = NULL);
+    IMGUI_API void          GetAllocatorFunctions(ImGuiMemAllocFunc** p_alloc_func, ImGuiMemFreeFunc** p_free_func, void** p_user_data);
     IMGUI_API void*         MemAlloc(size_t size);
     IMGUI_API void          MemFree(void* ptr);