OpenGL - Cannot find mistake - opengl

Question

I tried to code a small OpenGL app, but on the window will be displayed nothing.
But I cannot find any mistakes.
What did I wrong ?
And all error checks are negative, so there occur no errors.
I generated the OpenGL 3.3 header with this tool: https://bitbucket.org/alfonse/glloadgen/wiki/Home
#include "OpenGL.hpp"
#include <iostream>
#include <GLFW/glfw3.h>
#include "World.hpp"
int main(void)
{
glfwInit();
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
GLFWwindow * window = glfwCreateWindow(800, 600, "Project X", nullptr, nullptr);
glfwMakeContextCurrent(window);
world = new World();
do
{
world->Render();
glfwSwapBuffers(window);
glfwPollEvents();
}
while(glfwWindowShouldClose(window) == false);
delete world;
glfwTerminate();
return 0;
}
#include "World.hpp"
World::World()
{
glClearColor(0.5f, 0.5f, 0.5f, 1);
GLuint vertex_shader = glCreateShader(GL_VERTEX_SHADER);
const GLchar * vertex_source = "#version 330\n"
"layout(location = 0)in vec3 in_vertex_position;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(in_vertex_position, 1);\n"
"}\n";
glShaderSource(vertex_shader, 1, &vertex_source, nullptr);
glCompileShader(vertex_shader);
GLint vertex_result;
glGetShaderiv(vertex_shader, GL_COMPILE_STATUS, &vertex_result);
if(vertex_result != GL_TRUE)
{
std::cerr<<"Could not compile vertex shader !"<<std::endl;
}
GLuint fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
const GLchar * fragment_source = "#version 330\n"
"out vec4 out_fragment_color;\n"
"void main()\n"
"{\n"
"out_fragment_color = vec4(0.3, 1.0, 1.0, 1.0);\n"
"}\n";
glShaderSource(fragment_shader, 1, &fragment_source, nullptr);
glCompileShader(fragment_shader);
GLint fragment_result;
glGetShaderiv(fragment_shader, GL_COMPILE_STATUS, &fragment_result);
if(fragment_result != GL_TRUE)
{
std::cerr<<"Could not compile fragment shader !"<<std::endl;
}
program = glCreateProgram();
glAttachShader(program, vertex_shader);
glAttachShader(program, fragment_shader);
glLinkProgram(program);
GLint result;
glGetProgramiv(program, GL_LINK_STATUS, &result);
if(result != GL_TRUE)
{
std::cerr<<"Could not link program !"<<std::endl;
}
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
GLfloat vertices_positions[] = {0, 0, 0, 1, 0, 0, 1, 1, 0};
glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(GL_FLOAT), vertices_positions, GL_STATIC_DRAW);
}
World::~World()
{
}
void World::Render()
{
glClear(GL_COLOR_BUFFER_BIT);
glUseProgram(program);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableVertexAttribArray(0);
}