Error 1282 if I attach a fragment shader to the program

I am trying to run a program using shaders. In Another question I did , I found that I was mixing fixed pipeline functions with newer stuff (from OpenGL 2.0), so I tried to remove all "old stuff" "like glMatrixMode, but I'm not sure if I'm doing it right In doubt, I also removed calls like glLightfv, which I'm not entirely sure if they would be allowed if I were using a programmable pipeline.

This is the main code:

#include <GL/glew.h>
#include <GL/glut.h>
#include <iostream>
#include <vector>
#include "utility.hpp"
#include "program.hpp"

GLfloat width=600, height=800;
Program* program_ref;

void init()
{   
    glewInit();
    Shader vertex_shader= Shader("vertex_shader",GL_VERTEX_SHADER);
    // This just reads the file "vertex_shader", creates the shader and compiles it
    Shader geometry_shader= Shader(); // This means that the shader is empty
    // So the program class will be enough smart to recognize it and don't attach it
    Shader fragment_shader= Shader("fragment_shader",GL_FRAGMENT_SHADER);
    program_ref= new Program(vertex_shader,geometry_shader,fragment_shader);
    // This creates a program, attaches the shaders to it, links and uses it
}

void display()
{
    vector<GLfloat> quad{-0.5,-0.5,0.5,-0.5,0.5,0.5,-0.5,0.5};
    glClearColor(0,0,0,0);
    glClear(GL_COLOR_BUFFER_BIT);
    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(2,GL_FLOAT,0,quad.data());
    glDrawArrays(GL_QUADS,0,4);
    glDisableClientState(GL_VERTEX_ARRAY);
    glutSwapBuffers();
    cout << glGetError() << endl;
}

void reshape(int w, int h)
{
    width=w;
    height=h;
    glutPostRedisplay();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
    glutInitWindowPosition(100,100);
    glutInitWindowSize(500,500);
    glutCreateWindow("test");
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    init();
    glewInit();
    glutMainLoop();
    return 0;
}

      

        
        
        
      

    

( More classes to see , in the comments I wrote, how programs and shaders behave)

shader.hpp:

#ifndef shader_hpp
#define shader_hpp

#include "utility.hpp"
#include "shader.hpp"
#include <fstream>

#define MAX_SIZE (size_t)1.0e5


class Shader
{
private:
    GLuint id;
    GLenum type;
    bool null;
public:
    Shader()
    {
        id=-1;
        type= -1;
        null= true;
    }
    Shader(const string& filename,GLenum type)
    {
        GLchar* data= new GLchar[MAX_SIZE];
        streamsize count;
        this->type= type;
        ifstream is;
        is.open(filename);
        is.read(data,MAX_SIZE);
        count= is.gcount();
        is.close();
        id=glCreateShader(type);
        glShaderSource(id,1,(const GLchar**)&data,&count);
        glCompileShader(id);
        delete[] data;
        null= false;
    }
    GLuint getId() const
    {
        return id;
    }
    GLenum getType() const
    {
        return type;
    }
    bool isNull() const
    {
        return null;
    }
};

#endif

      

        
        
        
      

    

program.hpp:

#ifndef program_hpp
#define program_hpp

#include "utility.hpp"
#include "shader.hpp"

class Program
{
private:
    GLuint id;
public:
    Program(const Shader& vertex_shader, const Shader& geometry_shader, const Shader& fragment_shader)
    {
    id= glCreateProgram();
    if(!vertex_shader.isNull())
    {
        glAttachShader(id,vertex_shader.getId());
    }
    if(!geometry_shader.isNull())
    {
        glAttachShader(id,geometry_shader.getId());
    }
    if(!fragment_shader.isNull())
    {
        glAttachShader(id,fragment_shader.getId());
    }
    glLinkProgram(id);
    glUseProgram(id);
    }
    GLuint getId()
    {
    return id;
    }
};

#endif

      

        
        
        
      

    

The two classes here are just simplifying things by using glew to compile the shaders and attach them to the program.

The problem is this: if I just attach the vertex shader and not the fragment shader, everything is fine and I see a square with the correct color (red because in the vertex shader below I set the color to red). If instead I also attach the fragment shader, I see that the square is white. I also tried to figure out what was wrong and I printed the result to glGetError (). If I enable the fragment shader I get error 1282, if I don’t enable it I don’t get any error (0).

Vertex shader:

void main()
{
    gl_Position    = gl_ModelViewProjectionMatrix * gl_Vertex;
    gl_FrontColor  = vec4(1,0,0,1);
    // The only relevant thing I do is to set the color to red
    // I see the quad red if I attach only the vertex shader and not the fragment shader
    gl_TexCoord[0] = gl_MultiTexCoord0;
}

      

        
        
        
      

    

Fragment shader:

void main()
{
    gl_fragColor= gl_Color;
}