Partial fix of v2.2.0 alpha2 Unhandled exception - extrusion width #3482

Exceptions thrown by Flow calculation were made explicit classes derived
from std::invalid_argument.

The PresetHints::recommended_thin_wall_thickness() newly catches these
exceptions and it shows the reason of why the hint is invalid.

src/libslic3r/Flow.cpp

@@ -11,6 +11,12 @@
 
 namespace Slic3r {
 
+FlowErrorNegativeSpacing::FlowErrorNegativeSpacing() : 
+	FlowError("Flow::spacing() produced negative spacing. Did you set some extrusion width too small?") {}
+
+FlowErrorNegativeFlow::FlowErrorNegativeFlow() :
+    FlowError("Flow::mm3_per_mm() produced negative flow. Did you set some extrusion width too small?") {}
+
 // This static method returns a sane extrusion width default.
 float Flow::auto_extrusion_width(FlowRole role, float nozzle_diameter)
 {
@@ -52,7 +58,7 @@ static inline FlowRole opt_key_to_flow_role(const std::string &opt_key)
 
 static inline void throw_on_missing_variable(const std::string &opt_key, const char *dependent_opt_key) 
 {
-	throw std::runtime_error((boost::format(L("Cannot calculate extrusion width for %1%: Variable \"%2%\" not accessible.")) % opt_key % dependent_opt_key).str());
+	throw FlowErrorMissingVariable((boost::format(L("Cannot calculate extrusion width for %1%: Variable \"%2%\" not accessible.")) % opt_key % dependent_opt_key).str());
 }
 
 // Used to provide hints to the user on default extrusion width values, and to provide reasonable values to the PlaceholderParser.
@@ -174,7 +180,7 @@ float Flow::spacing() const
 #endif
 //    assert(res > 0.f);
 	if (res <= 0.f)
-		throw std::runtime_error("Flow::spacing() produced negative spacing. Did you set some extrusion width too small?");
+		throw FlowErrorNegativeSpacing();
 	return res;
 }
 
@@ -190,7 +196,7 @@ float Flow::spacing(const Flow &other) const
         0.5 * this->spacing() + 0.5 * other.spacing());
 //    assert(res > 0.f);
 	if (res <= 0.f)
-		throw std::runtime_error("Flow::spacing() produced negative spacing. Did you set some extrusion width too small?");
+		throw FlowErrorNegativeSpacing();
 	return res;
 }
 
@@ -204,7 +210,7 @@ double Flow::mm3_per_mm() const
         float(this->height * (this->width - this->height * (1. - 0.25 * PI)));
     //assert(res > 0.);
 	if (res <= 0.)
-		throw std::runtime_error("Flow::mm3_per_mm() produced negative flow. Did you set some extrusion width too small?");
+		throw FlowErrorNegativeFlow();
     return res;
 }
 

src/libslic3r/Flow.hpp

@@ -27,6 +27,31 @@ enum FlowRole {
     frSupportMaterialInterface,
 };
 
+class FlowError : public std::invalid_argument
+{
+public:
+	FlowError(const std::string& what_arg) : invalid_argument(what_arg) {}
+	FlowError(const char* what_arg) : invalid_argument(what_arg) {}
+};
+
+class FlowErrorNegativeSpacing : public FlowError
+{
+public:
+    FlowErrorNegativeSpacing();
+};
+
+class FlowErrorNegativeFlow : public FlowError
+{
+public:
+    FlowErrorNegativeFlow();
+};
+
+class FlowErrorMissingVariable : public FlowError
+{
+public:
+    FlowErrorMissingVariable(const std::string& what_arg) : FlowError(what_arg) {}
+};
+
 class Flow
 {
 public:

src/slic3r/GUI/PresetHints.cpp

@@ -262,12 +262,16 @@ std::string PresetHints::recommended_thin_wall_thickness(const PresetBundle &pre
         int num_lines = std::min(num_perimeters * 2, 10);
         out += (boost::format(_utf8(L("Recommended object thin wall thickness for layer height %.2f and"))) % layer_height).str() + " ";
         // Start with the width of two closely spaced 
-        double width = external_perimeter_flow.width + external_perimeter_flow.spacing();
-        for (int i = 2; i <= num_lines; thin_walls ? ++ i : i += 2) {
-            if (i > 2)
-                out += ", ";
-            out += (boost::format(_utf8(L("%d lines: %.2f mm"))) % i %  width).str() + " ";
-            width += perimeter_flow.spacing() * (thin_walls ? 1.f : 2.f);
+        try {
+	        double width = external_perimeter_flow.width + external_perimeter_flow.spacing();
+	        for (int i = 2; i <= num_lines; thin_walls ? ++ i : i += 2) {
+	            if (i > 2)
+	                out += ", ";
+	            out += (boost::format(_utf8(L("%d lines: %.2f mm"))) % i %  width).str() + " ";
+	            width += perimeter_flow.spacing() * (thin_walls ? 1.f : 2.f);
+	        }
+	    } catch (const FlowErrorNegativeSpacing &) {
+            out = _utf8(L("Recommended object thin wall thickness: Not available due to excessively small extrusion width."));
         }
     }
     return out;