Functions of a class like callbacks in OpenGL c ++

I have had a problem in the development of a graphic engine using OpenGL. The fact is that I have defined the class DB in which I implement the method initContext(int argc, char** argv) in which I must make the corresponding calls to the callbacks. The fact is that these callbacks must be functions without a member, and as I try to introduce them through functions of the class, they are with member and give error. I would like to know how this can be solved. I attach the code of .h and .cpp

CODE OF db.h

#pragma once
#include "mesh.h"

class DB {
    std::vector<Mesh> modelos;
    std::vector<Programa> programas;
    Camara *camara;
public:
    DB();
    void initContext(int argc, char** argv);
    void initOGL();
    void mainLoop();
    void addMesh(Mesh modelo);
    void idle();
    void render();
    void resize(int width, int height);
    void mouse(int button, int state, int x, int y);
    void keyboard(unsigned char key, int x, int y);
};

CODE OF db.cpp

#include "db.h"



DB::DB() 
{
    camara = new Camara();
}



void DB::initContext(int argc, char** argv)
{
    //Creamos el contexto
    glutInit(&argc, argv);
    glutInitContextVersion(3, 3);
    glutInitContextFlags(GLUT_FORWARD_COMPATIBLE);
    glutInitContextProfile(GLUT_CORE_PROFILE);
    //Definimos el FrameBuffer y creamos la ventana
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(0, 0);
    glutCreateWindow("Prácticas OGL");
    //Cargamos todas las extensiones
    glewExperimental = GL_TRUE;
    GLenum err = glewInit();
    if (GLEW_OK != err)
    {
        std::cout << "Error: " << glewGetErrorString(err) << std::endl;
        exit(-1);
    }
    const GLubyte *oglVersion = glGetString(GL_VERSION);
    std::cout << "This system supports OpenGL Version: " << oglVersion << std::endl;
    //Definimos los callback
    glutReshapeFunc(resize);
    glutIdleFunc(idle);
    glutDisplayFunc(render);
    glutMouseFunc(mouse);
    glutKeyboardFunc(keyboard);
}

void DB::resize(int width, int height)
{
    glViewport(0, 0, width, height);
    DB::camara->setProj(glm::perspective(glm::radians(60.0f), float(width) / float(height), 1.0f, 50.0f));
    glutPostRedisplay();
}

void DB::initOGL()
{
    //Activamos el test de profundidad y establecemos el color del fondo
    glEnable(GL_DEPTH_TEST);
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    //Indica la orientación de la cara front, configura la etapa de rasterizado y activa el culling
    glFrontFace(GL_CCW);
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    glEnable(GL_CULL_FACE);
    //Define la matriz de vista y la matriz de proyección
    camara->setProj(camara->getProj() * (glm::perspective(glm::radians(60.0f), 1.0f, 0.1f, 50.0f)));
    camara->getView()[3].z = -6;
}

void DB::idle()
{
    for (int i = 0; i < modelos.size(); i++)
    {
        modelos[i].setModel(glm::rotate(glm::mat4(1), 0.0f, glm::vec3(0, 1, 0))* glm::translate(glm::mat4(1), glm::vec3(0, 0, 3)));
    }
    glutPostRedisplay();
}

void DB::render()
{
    for (int i = 0; i < modelos.size(); i++)
    {
        modelos[i].drawMesh(*(DB::camara));
    }
}

void DB::mainLoop()
{
    glutMainLoop();
}

void DB::addMesh(Mesh modelo)
{
    modelos.push_back(modelo);
}


void DB::mouse(int button, int state, int x, int y)
{

}

void DB::keyboard(unsigned char key, int x, int y)
{

}

The problem arises in the initContext method, specifically in the last five lines, that is, in relation to the callbacks. I would like to know how I can convert these functions into functions without a member.