Draw array of points with OpenGL - c++

Question

Why in this program four points drawn by glVertex3f rendered in pointed positions, but instead of four array points rendered only one in the center of the window?
#include <GL/glew.h>
#include <GL/glut.h>
#include <iostream>
#include <vector>
using namespace std;
struct point3 {
GLfloat x, y, z;
};
vector<point3> Vertices(4);
void display(void)
{
glClear (GL_COLOR_BUFFER_BIT);
GLuint abuffer;
glGenVertexArrays(1, &abuffer);
glBindVertexArray(abuffer);
GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
Vertices = {{0.3f, 0.3f, 0.5f}, {0.6f, 0.6f, 0.5f}, {0.6f, 0.3f, 0.5f}, {0.3f, 0.6f, 0.5f}};
glBufferData(GL_ARRAY_BUFFER, Vertices.size() * sizeof(Vertices[0]), &Vertices[0], GL_STATIC_DRAW);
glVertexPointer(3, GL_FLOAT, 0, &Vertices[0]);
glDrawArrays(GL_POINTS, 0, Vertices.size());
glFlush();
glBegin(GL_POINTS);
glVertex3f(0.25f, 0.25f, 0.5f);
glVertex3f(0.25f, 0.75f, 0.5f);
glVertex3f(0.75f, 0.75f, 0.5f);
glVertex3f(0.75f, 0.25f, 0.5f);
glEnd();
glFlush();
}
void init (void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0, 0);
glutCreateWindow("simple OpenGL example");
glutDisplayFunc(display);
GLenum err = glewInit();
if (GLEW_OK != err) {
cerr << "Error: " << glewGetErrorString(err) << endl;
}
cout << "Status: Using GLEW" << glewGetString(GLEW_VERSION) << endl;
init();
glutMainLoop();
}

Answer 1

You haven't enabled the attribute to actually use the array. You do that by:
(OpenGL 2) glEnableClientState(GL_VERTEX_ARRAY) (or GL_COLOR_ARRAY, etc)
(OpenGL 3) glEnableVertexArray(attributeId)
Without that, what you point to with glVertexPointer (GL2) or glVertexAttribPointer (GL3) won't be actually used.