How does gnuplot C++ api know where a pointer ends?

Question

I want to draw a heatmap in C++ and I came across this code : Plotting heatmap with Gnuplot in C++

In gp.send2d(frame); all what the function send2d is seeing is a float*, so how does it know the dimensions (4x4) ? Or even that it can access safely 16 elements ?

Answer 1

The size of the array is part of its type. In the example you link to frame is a float[4][4]. Since send2d() is declared as

template <typename T> Gnuplot &send2d(const T &arg)

It will deduce the type of T to be float[4][4] and make arg a reference to that. Expanded out that would look like

Gnuplot &send2d(const float (&arg)[4][4])

Which shows that arg is a reference 2d array. There is no decay to a pointer here since we have a reference.

Here is an example that shows that using references maintains the array trype instead of decaying to a pointer

template<typename T>
void foo(T & arr_2d)
{
    for (auto& row : arr_2d)
    {
        for (auto& col : row)
            std::cout << col << " ";
        std::cout << "\n";
    }
}


int main() 
{
    int bar[2][2] = {1,2,3,4};
    foo(bar);
}

Output:

1 2 
3 4 

Live Example