C++ copying array into pointer argument

Question

Everyone,

I am writing a class in C++ which calculates points in space. I am designing this class to be independent from the application, so basically what I want -

Initialize object with input parameters -> calculate the values -> retrieve the calculated array

I prefer using C-like array (not the std::array). I'm writing the getter function -

// The definition of the ctrlpoints array
GLfloat ctrlpoints[4][4][3];

void GlCircle::getControlPoints(GLfloat* controlPoints) {
    std::copy(std::begin(ctrlpoints), std::end(ctrlpoints), std::begin(controlPoints));
}

Basically it should copy all values from ctrlpoints array to controlPoints target array. Of course compiler does not allow me to do just that (it doesn't like the type for std::begin(controlPoints)). Maybe someone can suggest an idea how should I return a copy of the array? Maybe the idea can realized in some other way, please let me know.

Thanks in advance.

"I prefer using C-like array (not the std::array)." Can you give any sensible reasoning for this? — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Feb 15, 2015 at 18:54
I am using opengl for drawing the points and it accepts only that kind of array (as far as I know). So it would a more elegant solution, otherwise it requires type convertions.. — rock-ass
– rock-ass, Commented Feb 15, 2015 at 19:13
@rock-ass: Nope, you can obtain a pointer to the raw data of a std::array just fine. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Feb 15, 2015 at 19:19

R Sahu · Accepted Answer · 2015-02-15 19:05:26Z

3

void GlCircle::getControlPoints(GLfloat* controlPoints) {
    std::copy(std::begin(ctrlpoints), std::end(ctrlpoints), std::begin(controlPoints));
}

does not work since std::begin(ctrlpoints) and std::end(ctrlpoints) can be used to iterate over the 2D arrays of a 3D array, it doesn't automatically flatten the 3D array so you can treat it like a 1D array.

You can use three for loops to get the data from the member variable to the output argument.

void GlCircle::getControlPoints(GLfloat* controlPoints) {
    int index = 0;
    for ( int i = 0; i < 4; ++i )
    {
       for ( int j = 0; j < 4; ++j )
       {
          for ( int k = 0; k < 3; ++k )
          {
             controlPoints[index++] = ctrlpoints[i][j][k];
          }
       }
    }
}

answered Feb 15, 2015 at 19:05

R Sahu

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1 Comment

rock-ass Over a year ago

This simple solution fits my needs and works. Thank you.

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