C++ Pointer within parentheses

Question

I am a bit confused about this syntax.

void function ( float (*points)[2]); 

Is this declaring an array of float pointers? If so, why the following code returns an error:

void foo( float (*points)[2]) {}

float *p[2];
foo(p); // error

float (*p)[2];
foo(p); // okay

Why do you need the parentheses?

Answer 1

float *p[2] defines an array of float*, with size 2.

float (*p)[2] defines a pointer to an array of float, with size 2:

int main(int argc, char* argv[])
{

    float* p[2];
    p[0] = new float(0.0);
    p[1] = new float(1.0);
    std::cout << *(p[0]) << " , " << *(p[1]) << "\n";

    float Q[2] = { 0.0, 1.0 };
    float(*q)[2] = &Q;
    std::cout << (*q)[0] << " , " << (*q)[1] << "\n";

    delete p[0];
    delete p[1];
}

Notice that if the sizes of Q and q doesn't match, you get an error:

float Q[3] = { 0.0, 1.0, 2.0 };
float(*q)[2] = &Q; //error C2440: 'initializing' : cannot convert from 'float (*)[3]' to 'float (*)[2]'

Answer 2

This is a pointer to an array of two float values. Its practical purpose is an ability to pass a 2-D array with one dimension fixed to 2:

float data[][2] = {{1.2, 3.4}, {5.6, 7.8}, {9.1, 10.2}};
foo(data);

Typically a function like that would also take the overall size of the array.

Alternatively, a function like that could be used to allocate an array of two floats, and set the pointer to the result of allocation.