error: cannot convert 'float*' to 'qreal* {aka double*} in initialization - c++

Question

I'm trying to compile old Qt project and i encounter this error:
error: cannot convert 'float*' to 'qreal* {aka double*}' in
initialization
Here's the fragment of code:
void Camera::loadProjectionMatrix()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
qreal *dataMat = projectionMatrix_.data();
GLfloat matriceArray[16];
for (int i= 0; i < 16; ++i)
matriceArray[i] = dataMat[i];
glMultMatrixf(matriceArray);
}
What are my options to overcome this error?

Answer 1

The projection matrix will return float* to you as per documentation:
float * QMatrix4x4::​data()
Returns a pointer to the raw data of this matrix.
The best practice would be to eliminate the qreal usage in your codebase regardless this case. When the contributors went through the Qt 5 refactoring, the qreal ancient concept was dropped as much as possible and definitely should not be used much in new code where the API deals with float.
The recommendation is to use float these days in such cases. This is a bit historical, really. Back then, it made sense to define qreal to double where available, but float where not, e.g. ARM platforms. See the old documentation:
typedef qreal
Typedef for double on all platforms except for those using CPUs with ARM architectures. On ARM-based platforms, qreal is a typedef for float for performance reasons.
In Qt 5, the documentation is slightly different, although the main concept seems to have remained the same:
typedef qreal
Typedef for double unless Qt is configured with the -qreal float option.
I would fix your code the following way:
void Camera::loadProjectionMatrix()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
float *dataMat = projectionMatrix_.data();
GLfloat matriceArray[16];
for (int i= 0; i < 16; ++i)
matriceArray[i] = dataMat[i];
glMultMatrixf(matriceArray);
}
Strictly speaking, you could also go an alternative way to solve the issue, namely by using this method rather than data():
float & QMatrix4x4::​operator()(int row, int column)
Returns a reference to the element at position (row, column) in this matrix so that the element can be assigned to.
In which case, you could even eliminate the dataMat variable and assign the items directly to your matriceArray in the iteration.
Going even further than that, you should consider using a Qt library for this common task, namely e.g. the opengl classes either in QtGui or Qt3D. It would make more sense to mess with low-level opengl API calls if you do something custom.

Answer 2

Apparently, projectionMatrix_.data() returns a float*, and you cannot assign a float* to a double* (which is what qreal* is in this case).
Use
float *dataMat = projectionMatrix_.data();
or
auto dataMat = projectionMatrix_.data();
instead. The latter sometimes has the advantage that it might still be correct code if the return type of the function changes for some reason, although that is nothing to expect from a mature library. Additionally, you cannot get the type wrong on accident.