GL_POINTS test, help me understand this.

I am on OS X
Using Xcode

Problem : I can see 3 points on the screen even if there are only 2 vertices in the array.

math_3d.h

struct Vector3f {
    float x;
    float y;
    float z;
    
    Vector3f() {
    }
    
    Vector3f(float _x, float _y, float _z) {
        x = _x;
        y = _y;
        z = _z;
    }
};

main.cpp

#include <iostream>
#include <glew.h>
#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
#include <GLUT/glut.h>
#include "math_3d.h"

GLuint VBO;

static Vector3f Vertices[2];

static void RenderSceneCB()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    
    glDrawArrays(GL_POINTS, 0, sizeof(Vertices));
    
    glDisableVertexAttribArray(0);
    glutSwapBuffers();
}

static void InitializeGlutCallbacks()
{
    glutDisplayFunc(RenderSceneCB);
}

static void CreateVertexBuffer()
{
    
    Vertices[0] = Vector3f(0.4f, 0.4f, 0.0f);
    Vertices[1] = Vector3f(-0.5f, -0.5f, 0.0f);
    
     glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
}

int main(int argc, char * argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA);
    glutInitWindowSize(1024, 768);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Points");
    
    InitializeGlutCallbacks();
    GLenum glewRes = glewInit();
    if (glewRes != GLEW_OK) {
        fprintf(stderr, "Error: '%s'
", glewGetErrorString(glewRes));
        return 1;
    }
    
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    CreateVertexBuffer();
    glutMainLoop();
    
    return 0;
}

sizeof() returns a size in bytes. glDrawArrays OTOH takes the size in indices (vertices in this case) as the 3rd argument.

I believe you want to use ARRAY_SIZE( Vertices ) instead, where ARRAY_SIZE is defined by:

#define ARRAY_SIZE(a) (sizeof(a)/sizeof((a)[0]))

This will evaluate to 2 in your case, instead of the 24 you’re getting now.