How to convert vector to array C++

Question

I want to convert vector of double to array of double. Can any one help me to do this

Answer 1

vector<double> thevector;
//...
double *thearray = &thevector[0];

This is guaranteed to work by the standard, however there are some caveats: in particular take care to only use thearray while thevector is in scope.

Answer 2

There's a fairly simple trick to do so, since the spec now guarantees vectors store their elements contiguously:

std::vector<double> v;
double* a = &v[0];

Answer 3

Vectors effectively are arrays under the skin. If you have a function:

void f( double a[]);

you can call it like this:

vector <double> v;
v.push_back( 1.23 )
f( &v[0] );

You should not ever need to convert a vector into an actual array instance.

Answer 4

What for? You need to clarify: Do you need a pointer to the first element of an array, or an array?

If you're calling an API function that expects the former, you can do do_something(&v[0], v.size()), where v is a vector of doubles. The elements of a vector are contiguous.

Otherwise, you just have to copy each element:

double arr[100];
std::copy(arr, arr + 100, v.begin());

Of course, ensure v actually has enough elements to copy.

---

Utility function, to help with arrays:

template <typename T, size_t N>
T* end(T (&pArray)[N])
{
    return &pArray[0] + N;
}

Giving:

double arr[100];
std::copy(arr, end(arr), v.begin());