ImFormatString, ImFormatStringV(): clarifying specs so that passing a NULL buffer should return the desired length. (#1038)

imgui.cpp

@@ -1010,16 +1010,18 @@ static const char* ImAtoi(const char* src, int* output)
     return src;
 }
 
-// MSVC version appears to return -1 on overflow, whereas glibc appears to return total count (which may be >= buf_size). 
+// A) MSVC version appears to return -1 on overflow, whereas glibc appears to return total count (which may be >= buf_size). 
 // Ideally we would test for only one of those limits at runtime depending on the behavior the vsnprintf(), but trying to deduct it at compile time sounds like a pandora can of worm.
+// B) When buf==NULL vsnprintf() will return the output size.
 #ifndef IMGUI_DISABLE_FORMAT_STRING_FUNCTIONS
 int ImFormatString(char* buf, int buf_size, const char* fmt, ...)
 {
-    IM_ASSERT(buf_size > 0);
     va_list args;
     va_start(args, fmt);
     int w = vsnprintf(buf, buf_size, fmt, args);
     va_end(args);
+    if (buf == NULL)
+        return w;
     if (w == -1 || w >= buf_size)
         w = buf_size - 1;
     buf[w] = 0;
@@ -1028,8 +1030,9 @@ int ImFormatString(char* buf, int buf_size, const char* fmt, ...)
 
 int ImFormatStringV(char* buf, int buf_size, const char* fmt, va_list args)
 {
-    IM_ASSERT(buf_size > 0);
     int w = vsnprintf(buf, buf_size, fmt, args);
+    if (buf == NULL)
+        return w;
     if (w == -1 || w >= buf_size)
         w = buf_size - 1;
     buf[w] = 0;
@@ -1671,7 +1674,7 @@ void ImGuiTextBuffer::appendv(const char* fmt, va_list args)
     va_list args_copy;
     va_copy(args_copy, args);
 
-    int len = vsnprintf(NULL, 0, fmt, args);         // FIXME-OPT: could do a first pass write attempt, likely successful on first pass.
+    int len = ImFormatStringV(NULL, 0, fmt, args);         // FIXME-OPT: could do a first pass write attempt, likely successful on first pass.
     if (len <= 0)
         return;
 
@@ -1684,7 +1687,7 @@ void ImGuiTextBuffer::appendv(const char* fmt, va_list args)
     }
 
     Buf.resize(needed_sz);
-    ImFormatStringV(&Buf[write_off] - 1, len+1, fmt, args_copy);
+    ImFormatStringV(&Buf[write_off - 1], len + 1, fmt, args_copy);
 }
 
 void ImGuiTextBuffer::append(const char* fmt, ...)

imgui_demo.cpp

@@ -2359,6 +2359,7 @@ struct ExampleAppConsole
 
     void    AddLog(const char* fmt, ...) IM_FMTARGS(2)
     {
+        // FIXME-OPT
         char buf[1024];
         va_list args;
         va_start(args, fmt);