This is the first code that ran correctly with correct shaders and mesh creation for SDL2 and modern OpenGL. Display class is taken from previous project.

SDL_plus_modern_OpenGL.cpp

#include<iostream>
#include"display.h"
#include<SDL2/SDL.h>
#include<GL/glew.h>
//#include "shader.h"
//#include "mesh.h"


using namespace ns_OCTAD::ns_display;

// we use int argc , char * argv[] to make the software to be run by cmd
int main(int argc, char * argv[])
{
  // Shaders
const GLchar* vertexShaderSource = "#version 330 core\n"
    "layout (location = 0) in vec3 position;\n"
    "void main()\n"
    "{\n"
    "gl_Position = vec4(position.x, position.y, position.z, 1.0);\n"
    "}\0";
const GLchar* fragmentShaderSource = "#version 330 core\n"
    "out vec4 color;\n"
    "void main()\n"
    "{\n"
    "color = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
    "}\n\0";

  std::cout<<"Hello World!"<<std::endl;

  Display d(640,480,"My main window");

 // Shader shader("./res/basicShader");

//  Vertex vertices[] = { Vertex(glm::vec3(-0.5,-0.5,0.0)),
//					  Vertex(glm::vec3(0.0,0.5,0.0)),
//					  Vertex(glm::vec3(0.5,-0.5,0.0))
//					};

  //Mesh mesh(vertices,sizeof(vertices)/sizeof(vertices[0]));

  // Build and compile our shader program
    // Vertex shader
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    // Check for compile time errors
    GLint success;
    GLchar infoLog[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Fragment shader
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    // Check for compile time errors
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Link shaders
    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    // Check for linking errors
    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
    if (!success) {
        glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

   GLfloat vertices[] = {
         0.5f,  0.5f, 0.0f,  // Top Right
         0.5f, -0.5f, 0.0f,  // Bottom Right
        -0.5f, -0.5f, 0.0f,  // Bottom Left
        -0.5f,  0.5f, 0.0f   // Top Left
    };
    GLuint indices[] = {  // Note that we start from 0!
        0, 1, 3,  // First Triangle
        1, 2, 3   // Second Triangle
    };
    GLuint VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    // Bind the Vertex Array Object first, then bind and set vertex buffer(s) and attribute pointer(s).
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, 0); // Note that this is allowed, the call to glVertexAttribPointer registered VBO as the currently bound vertex buffer object so afterwards we can safely unbind

    glBindVertexArray(0); // Unbind VAO (it's always a good thing to unbind any buffer/array to prevent strange bugs), remember: do NOT unbind the EBO, keep it bound to this VAO



  while(d.isDisplayActive()== true)
  {
         d.Update();
        // Render
        // Clear the colorbuffer
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // Draw our first triangle
        glUseProgram(shaderProgram);
        glBindVertexArray(VAO);
        //glDrawArrays(GL_TRIANGLES, 0, 6);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
        glBindVertexArray(0);

        // Swap the screen buffers
       // glfwSwapBuffers(window);
        SDL_GL_SwapWindow(d.m_window);


  }
   // Properly de-allocate all resources once they've outlived their purpose
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    // Terminate GLFW, clearing any resources allocated by GLFW.
    //glfwTerminate();




  SDL_Quit();
  return 0;
}