Triangle in OpenGL without vertex data

main.cpp

#include <SDL.h>

#define GLEW_STATIC
#include <GL/glew.h>

const char* vertShaderSrc =
    "#version 440\n"
    "void main() {"
        "if (gl_VertexID == 0) gl_Position = vec4( 0.0,  0.5, 0, 1);"
        "if (gl_VertexID == 1) gl_Position = vec4( 0.5, -0.5, 0, 1);"
        "if (gl_VertexID == 2) gl_Position = vec4(-0.5, -0.5, 0, 1);"
    "}";

const char* fragShaderSrc =
    "#version 440\n"
    "out vec4 color;"
    "void main() {"
        "color = vec4(1, 0, 0, 1);"
    "}";

int main(int argc, char* argv[]) {
    SDL_Init(SDL_INIT_VIDEO);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 4);

    SDL_Window* window = SDL_CreateWindow("Triangle without vertex data", 100, 100, 800, 600, SDL_WINDOW_OPENGL);
    SDL_GLContext context = SDL_GL_CreateContext(window);

    glewExperimental = GL_TRUE;
    glewInit();

    GLuint vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    GLuint vertShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertShader, 1, &vertShaderSrc, nullptr);
    glCompileShader(vertShader);

    GLuint fragShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragShader, 1, &fragShaderSrc, nullptr);
    glCompileShader(fragShader);

    GLuint program = glCreateProgram();
    glAttachShader(program, vertShader);
    glAttachShader(program, fragShader);
    glLinkProgram(program);
    glUseProgram(program);

    SDL_Event windowEvent;
    while (true) {
        if (SDL_PollEvent(&windowEvent)) {
            if (windowEvent.type == SDL_QUIT) break;
        }

        glClearColor(0, 0, 0, 1);
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawArrays(GL_TRIANGLES, 0, 3);

        SDL_GL_SwapWindow(window);
    }

    return 0;
}