ImGuiTextBuffer: Avoid heap allocation when empty.

docs/CHANGELOG.txt

@@ -59,6 +59,7 @@ Other Changes:
 - BeginMenu(): Fixed menu popup horizontal offset being off the item in the menu bar when WindowPadding=0.0f.
 - ArrowButton(): Fixed arrow shape being horizontally misaligned by (FramePadding.y-FramePadding.x) if they are different.
 - Drag and Drop: Added GetDragDropPayload() to peek directly into the payload (if any) from anywhere. (#143)
+- ImGuiTextBuffer: Avoid heap allocation when empty.
 - ImDrawList: Fixed AddConvexPolyFilled() undefined behavior when passing points_count smaller than 3,
   in particular, points_count==0 could lead to a memory stomp if the draw list was previously empty.
 - Examples: DirectX10, DirectX11: Removed seemingly unnecessary calls to invalidate and recreate device objects

imgui.cpp

@@ -1956,6 +1956,8 @@ bool ImGuiTextFilter::PassFilter(const char* text, const char* text_end) const
 #endif
 #endif
 
+char ImGuiTextBuffer::EmptyString[1] = { 0 };
+
 // Helper: Text buffer for logging/accumulating text
 void ImGuiTextBuffer::appendfv(const char* fmt, va_list args)
 {
@@ -1969,7 +1971,8 @@ void ImGuiTextBuffer::appendfv(const char* fmt, va_list args)
         return;
     }
 
-    const int write_off = Buf.Size;
+    // Add zero-terminator the first time
+    const int write_off = (Buf.Size != 0) ? Buf.Size : 1;
     const int needed_sz = write_off + len;
     if (write_off + len >= Buf.Capacity)
     {

imgui.h

@@ -1366,25 +1366,27 @@ struct ImGuiTextFilter
     int                 CountGrep;
 };
 
-// Helper: Text buffer for logging/accumulating text
+// Helper: Growable text buffer for logging/accumulating text
+// (this could be called 'ImGuiTextBuilder' / 'ImGuiStringBuilder')
 struct ImGuiTextBuffer
 {
     ImVector<char>      Buf;
+    static char         EmptyString[1];
 
-    ImGuiTextBuffer()   { Buf.push_back(0); }
+    ImGuiTextBuffer()   { }
-    inline char         operator[](int i) { return Buf.Data[i]; }
+    inline char         operator[](int i)       { IM_ASSERT(Buf.Data != NULL); return Buf.Data[i]; }
-    const char*         begin() const { return &Buf.front(); }
+    const char*         begin() const           { return Buf.Data ? &Buf.front() : EmptyString; }
-    const char*         end() const { return &Buf.back(); }      // Buf is zero-terminated, so end() will point on the zero-terminator
+    const char*         end() const             { return Buf.Data ? &Buf.back() : EmptyString; }   // Buf is zero-terminated, so end() will point on the zero-terminator
-    int                 size() const { return Buf.Size - 1; }
+    int                 size() const            { return Buf.Data ? Buf.Size - 1 : 0; }
-    bool                empty() { return Buf.Size <= 1; }
+    bool                empty()                 { return Buf.Size <= 1; }
-    void                clear() { Buf.clear(); Buf.push_back(0); }
+    void                clear()                 { Buf.clear(); }
-    void                reserve(int capacity) { Buf.reserve(capacity); }
+    void                reserve(int capacity)   { Buf.reserve(capacity); }
-    const char*         c_str() const { return Buf.Data; }
+    const char*         c_str() const           { return Buf.Data ? Buf.Data : EmptyString; }
     IMGUI_API void      appendf(const char* fmt, ...) IM_FMTARGS(2);
     IMGUI_API void      appendfv(const char* fmt, va_list args) IM_FMTLIST(2);
 };
 
-// Helper: key->value storage
+// Helper: Key->Value storage
 // Typically you don't have to worry about this since a storage is held within each Window.
 // We use it to e.g. store collapse state for a tree (Int 0/1)
 // This is optimized for efficient lookup (dichotomy into a contiguous buffer) and rare insertion (typically tied to user interactions aka max once a frame)