QtOpenGLWidget subclass does not draw anything - qt

Question

I have a simple vertex and fragment shader. I use them in a class derived from QtOpenGLWidget.the program is supposed to draw a triangle, but it does not show anything. The glClearColor works fine and I am able to change the background color. When I run the program from command line, the program outputs one single line, but draws nothing:
QOpenGLShaderProgram::attributeLocation( position ): shader program is not linked
When I insert getGLError after gl commands, none of them report an error.
here is the class definition:
class GLCanvas:public QOpenGLWidget , protected QOpenGLFunctions
{
QMatrix4x4 matrix;
QOpenGLShaderProgram* shader;
GLuint posLocation;
GLuint colLocation;
GLuint matLocation;
void cleanup();
public:
GLCanvas(QWidget* parent);
void initializeGL() override;
void paintGL() override;
void resizeGL(int w, int h) override;
QStringList getGLinfo();
};
static const char *vertexShaderSource =
R"(
#version 330
in vec3 position;
void main()
{
gl_Position = vec4(position, 1);
})";
static const char *fragmentShaderSource =
R"(
#version 330
void main()
{
gl_FragColor = vec4(0.4,0.4,0.8,1.0);
})";
GLCanvas::GLCanvas(QWidget* parent): QOpenGLWidget(parent)
{
setFormat(QSurfaceFormat::defaultFormat());
matrix.setToIdentity();
posLocation = -1;
colLocation = -1;
matLocation = -1;
}
void GLCanvas::initializeGL()
{
makeCurrent();
connect(context(), &QOpenGLContext::aboutToBeDestroyed, this, &GLCanvas::cleanup);
constexpr int glNoError = 0;
GLenum error = -1;
initializeOpenGLFunctions();
glClearColor(.1f, .1f, .1f, 1.0f);
error = glGetError();
if (error != glNoError)
{
qDebug() << "glclearcolor failed";
}
shader = new QOpenGLShaderProgram(this);
shader->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
error = glGetError();
if (error != glNoError)
{
qDebug() << "failed after adding vertex shader.";
}
shader->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
error = glGetError();
if (error != glNoError)
{
qDebug() << "failed after adding fragment shader.";
}
Q_ASSERT(shader->link());
error = glGetError();
if (error != glNoError)
{
qDebug() << "gl error at link.";
}
//colLocation = shader->attributeLocation("color");
//matLocation = shader->uniformLocation("matrix");
//shader->release();
posLocation = shader->attributeLocation("position");
error = glGetError();
if (error != glNoError)
{
qDebug() << "gl error at link.";
}
glEnable(GL_DEPTH_TEST);
//colLocation = shader->attributeLocation("color");
//matLocation = shader->uniformLocation("matrix");
//shader->release();
//glEnable(GL_CULL_FACE);
}
//###################################################################
void GLCanvas::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
GLfloat vertices[] = { 0.0f, 0.707f, .0f, 0.5f, -0.5f, .0f ,-0.5f, -0.5f, .0f};
Q_ASSERT(shader->bind());
shader->setAttributeArray(posLocation, vertices, 3, 0);
glEnableVertexAttribArray(posLocation);
glDrawArrays(GL_LINES, 0, 3);
/*
GLfloat lvertices[] = { 0.907f, 0.907f, 0.5f, -0.957f, -0.957f, 0.5f };
shader->setAttributeArray(posLocation, lvertices, 0, 3);
glDrawArrays(GL_LINE_STRIP, 0, 2);
glDisableVertexAttribArray(posLocation);
*/
//glDisableVertexAttribArray(colLocation);
shader->release();
// qDebug() << getGLinfo();
}
void GLCanvas::resizeGL(int w, int h)
{
Q_UNUSED(w);
Q_UNUSED(h);
//matrix.setToIdentity();
//matrix.perspective(45.0f, w / float(h), 0.01f, 1000.0f);
glViewport(0, 0, w, h);
}
/*
void GLCanvas::mousePressEvent(QMouseEvent *event)
{
Q_UNUSED(event);
}
void GLCanvas::mouseMoveEvent(QMouseEvent *event)
{
Q_UNUSED(event);
}
*/
void GLCanvas::cleanup()
{
if (shader == nullptr)
return;
makeCurrent();
delete shader;
//shader = nullptr;
doneCurrent();
}

Answer 1

I did everything carefully from the beginning and things worked. probably a stupid mistake was the cause of it all. Thank you all.