Class contiguous data

Question

I have a C++ class which has four private floats and a bunch of nonstatic public functions that operate on this data.

Is it guaranteed, or possible to make it so, that the four floats are contiguous and there is no padding. This would make the class the size of four floats, and it's address would be that of the first float.

Answer 1

That depends on your compiler.

You can use #pragma pack(1) with e.g. MSVC and gcc, or #pragma pack 1 with aCC.

For example, assuming MSVC/gcc:

#pragma pack(1)
class FourFloats
{
    float f1, f2, f3, f4;
};

Or better:

#pragma pack(push, 1)
class FourFloats
{
    float f1, f2, f3, f4;
};
#pragma pack(pop)

That basically disables padding and guarantees that the floats are contiguous. However, to ensure that the size of your class is actually 4 * sizeof(float), it must not have a vtbl, which means virtual members are off-limits.