Empty triangle list check and clarification in the example in README.md

Author: JoseAntFer

README.md

@@ -1,14 +1,16 @@
 # TriangleMeshDistance
 Header only, single file, simple and efficient C++ library to compute the signed distance function to a triangle mesh.
 
-The distance computation to the triangle collection is accelerated with a sphere bounding volume hierarchy. The sign of the distance is resolved with the method presented in *"Generating Signed Distance Fields From Triangle Meshes"* by Bærentzen, Andreas & Aanæs, Henrik. (2002).
+The distance computation to the triangle collection is accelerated with a sphere bounding volume hierarchy. The signed of the distance is resolved with the method presented in *"Generating Signed Distance Fields From Triangle Meshes"* by Bærentzen, Andreas & Aanæs, Henrik. (2002).
 
 ## Example
 ```cpp
 // Declare mesh vertices and triangles
 std::vector<std::array<double, 3>> vertices;
 std::vector<std::array<int, 3>> triangles;
 
+// (... fill the `vertices` and `triangles` with the mesh data ...)
+
 // Initialize TriangleMeshDistance
 tmd::TriangleMeshDistance mesh_distance(vertices, triangles);
 
@@ -24,6 +26,6 @@ std::cout << "Nearest triangle index: " << result.triangle_id << std::endl;
 
 
 ## What you need to know about TriangleMeshDistance
-- `TriangleMeshDistance` keeps an internal copy of the vertex and triangle data.
+- `TriangleMeshDistance` keeps a copy of the vertex and triangle data passed as input.
 - Additionally, the pseudonormals required to compute signed distances are calculated and stored at building time.
-- `TriangleMeshDistance` can be declared empty and constructed multiple times with different meshes. This can potentially reuse memory allocations.
+- `TriangleMeshDistance` can be declared empty and constructed multiple times with different meshes. This can potentially reuse memory allocations.

TriangleMeshDistance/include/tmd/TriangleMeshDistance.h

@@ -270,6 +270,11 @@ inline tmd::Result tmd::TriangleMeshDistance::unsigned_distance(const Eigen::Vec
 
 inline void tmd::TriangleMeshDistance::_construct()
 {
+	if (this->triangles.size() == 0) {
+		std::cout << "DistanceTriangleMesh error: Empty triangle list." << std::endl;
+		exit(-1);
+	}
+	
 	// Build the tree containing the triangles
 	std::vector<Triangle> triangles;