title: I cannot access some openGL functions
body: <p>When I attempt to generate a buffer by executing the <code>glGenBuffer()</code> function - no function like that is found.</p>
  <p>Some functions are still working, and from what I see most do work, for instance the following code works perfectly:</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;iostream&gt;
  #include &lt;GLFW/glfw3.h&gt;
  using namespace std;
  
  class Window_Manager
  {
  public:
      GLFWwindow* window;
  
      int Create_Window(int width, int height, const char* title) {
          if (!glfwInit()) {
              return -1;
          }
  
          glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
          glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
          glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
          glfwWindowHint(GLFW_RESIZABLE, GL_TRUE);
          glfwWindowHint(GLFW_FOCUSED, GL_TRUE);
          
          window = glfwCreateWindow(width, height, title, NULL, NULL);
          if (window == NULL) {
              cout &lt;&lt; &quot;Failed to create a window! Aborting early...&quot; &lt;&lt; endl;
              Terminate_Window(window);
              return -1;
          }
          glfwMakeContextCurrent(window);
          
          glViewport(0, 0, width, height); 
          
          return 1;
      }
      
      void Terminate_Window(GLFWwindow* window) {
          glfwSetWindowShouldClose(window, GL_TRUE);
          glfwTerminate();
      }
  };
  
  </code></pre>
  <p>but this code does not:</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;GLFW/glfw3.h&gt;
  #include &lt;Engine_Manager.h&gt;
  #include &lt;iostream&gt;
  using namespace std;
  class Shape2D
  {
      int VBO;
      int Setting_Up(){
              VBO = glGenBuffer();
              return 1;
      }
  };
  </code></pre>