C++ memcpy from double array to float array

Question

Is is possible to memcpy from a double array to a float array safely?

Answer 1

Depends on what you want. The values certainly won't be preserved. If you need that, use std::copy.

#include <algorithm>

int main()
{
    double a[] = {1.618, 3.1416, 2.7, 0.707, 1.0};
    float b[5];
    std::copy(a, a + 5, b);
}

Answer 2

No.

Answer 3

The problem is that there is no guarantee that the compiler's binary representation of a double is the equivalent representation of a float. In order to use memcpy for multi-byte types is that the underlying representation must be the same (same layout). You can safely copy float to float, int to int and double to double.

You are destined for undefined behavior when the source type does not match the destination type, such as copying from long to char or float to double. The memcpy function does not make any conversion or perform any promotions. It just copies.

Answer 4

In general case - no.

In specific cases, on a given platform the representation of float and double might be the same, and the copy will succeed. But it doesn't make any practical sense anyway.