I cannot access some openGL functions - c++

Question

When I attempt to generate a buffer by executing the glGenBuffer() function - no function like that is found.
Some functions are still working, and from what I see most do work, for instance the following code works perfectly:
#include <iostream>
#include <GLFW/glfw3.h>
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 << "Failed to create a window! Aborting early..." << 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();
}
};
but this code does not:
#include <GLFW/glfw3.h>
#include <Engine_Manager.h>
#include <iostream>
using namespace std;
class Shape2D
{
int VBO;
int Setting_Up(){
VBO = glGenBuffer();
return 1;
}
};

Answer 1

In addition to replacing glGenBuffer with glGenBuffers(1, &VBO) as rpress mentioned in the comments, OpenGL is a weird library in that you have to load most of the functions dynamically.
The exact details differ from platform to platform. On Windows, for example, you can use wglGetProcAddress to get pointers to desired functions.
Rather than do it manually, GLEW is an excellent library for getting OpenGL function pointers easily. Other options are documented on the OpenGL wiki.