C++Opengl三角形正在绘制或根本不可见

我的代码根本没有显示三角形。我试着一步一步地调试它,但没有成功。我的代码出了什么问题?

#include <gl/glew.h>
#include <GLFW/glfw3.h>
#include <iostream>
static unsigned int CompileShader(unsigned int type, const std::string& source)
{
    unsigned int id = glCreateShader(type);
    const char* src = source.c_str();
    glShaderSource(id, 1, &src, nullptr);
    glCompileShader(id);

    int result;
    glGetShaderiv(id, GL_COMPILE_STATUS, &result);

    if (result == GL_FALSE)
    {
        int length;
        glGetShaderiv(id, GL_INFO_LOG_LENGTH, &length);
        char* message = (char*)alloca(length * sizeof(char));
        glGetShaderInfoLog(id, length, &length, message);
        std::cout << "Failed to compile" << (type == GL_VERTEX_SHADER ? "vertex" : "fragment") << " shader" << std::endl;
        std::cout << message << std::endl;
        glDeleteShader(id);
        return 0;
    }
    return id;
}

static unsigned CreateShader(const std::string& vertexShader, const std::string& fragmentShader)
{
    unsigned int program = glCreateProgram();
    unsigned int vs = CompileShader(GL_VERTEX_SHADER, vertexShader);
    unsigned int fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);

    glAttachShader(program, vs);
    glAttachShader(program, fs);
    glLinkProgram(program);
    glValidateProgram(program);

    glDeleteShader(vs);
    glDeleteShader(fs);

    return program;
}

int main(void)
{
    GLFWwindow* window;

    /* Initialize the library */
    if (!glfwInit())
        return -1;

    /* Create a windowed mode window and its OpenGL context */
    window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    /* Make the window's context current */
    glfwMakeContextCurrent(window);

    if (glewInit() != GLEW_OK)
        std::cout << "Error!" << std::endl;

    float positions[6] = {-0.5f, -0.5f, 0.0f, 0.0f, 0.5f, 0.5f};

    unsigned int buffer = 0;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), positions, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, 0);

    std::string vertexShader =
        "#version 330 core\n"
        "\n"
        "layout(location=0)in vec4 position;\n"
        "\n"
        "void main()\n"
        "{\n"
        "   gl_Position = position;\n"
        "}\n";

    std::string fragmentShader =
        "#version 330 core\n"
        "\n"
        "layout(location=0)out vec4 color;\n"
        "\n"
        "void main()\n"
        "{\n"
        "   color = vec4(0.0, 0.0, 1.0, 1.0);\n"
        "}\n";

    unsigned int shader = CreateShader(vertexShader, fragmentShader);
    glUseProgram(shader);
    /* Loop until the user closes the window */
    while (!glfwWindowShouldClose(window))
    {
        /* Render here */
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glBindVertexArray(buffer);

        glDrawArrays(GL_TRIANGLES, 0, 3);
        glfwPollEvents();
        /* Swap front and back buffers */
        glfwSwapBuffers(window);

        /* Poll for and process events */
        glfwPollEvents();
    }
    glDeleteShader(shader);

    glfwTerminate();
    return 0;
}