throw exception if VBO is not bound when allocate is called, reset VBO offset on ID generation

src/GLObject.cpp

@@ -65,7 +65,7 @@ GLuint GLObject::id() const
     }
     else
     {
-        throw std::runtime_error("Cannot get ID for GL object that has no ID set yet");
+        throw std::runtime_error("Cannot get ID for GL object that has no ID generated yet");
     }
 }
 

src/VBO.cpp

@@ -9,14 +9,40 @@
 +-------------*/
 
 #include "VBO.hpp"
+
 using namespace sb;
 
 VBO::VBO() : Buffer(GL_ARRAY_BUFFER) {}
 
+void VBO::generate()
+{
+    Buffer::generate();
+    offset = 0;
+}
+
 void VBO::allocate(GLsizeiptr size, GLenum usage)
 {
-    /* Create data store */
-    glBufferData(target(), size, nullptr, usage);
+    /* Create data store, making sure the buffer currently bound matches this object's ID */
+    GLint bound_id;
+    glGetIntegerv(GL_ARRAY_BUFFER_BINDING, &bound_id);
+    if (static_cast<GLuint>(bound_id) == id())
+    {
+        glBufferData(target(), size, nullptr, usage);
+    }
+    else
+    {
+        std::ostringstream message;
+        message << "Error allocating vertex memory for " << *this << ": ";
+        if (bound_id == 0)
+        {
+            message << "no buffer is currently bound";
+        }
+        else
+        {
+            message << "currently bound buffer ID " << bound_id << " does not match " << *this;
+        }
+        throw std::runtime_error(message.str());
+    }
 
     /* Debug */
     std::ostringstream message;

src/VBO.hpp

@@ -54,6 +54,12 @@ namespace sb
          */
         VBO();
 
+        /*!
+         * Generate an ID for VBO using the base class's generate function. Reset the offset to 0 because any data previously stored will be
+         * automatically freed once a new ID is generated.
+         */
+        void generate();
+
         /*!
          * Allocate size bytes of unintialized vertex buffer memory on the GPU.
          *