C++ struct memory allocation

Question

in C++ if I define a structure like this

struct ComplexFloat {
  float r;
  float i;
};

and I declare two variables like this

struct ComplexFloat cf;
float *f=(float*)cf;

can I safely assume that the following condition will always be true

(&(cf.r)==&(f[0]) && &(cf.i)==&(f[1]))

?

In other words, can I safely assume that in a struct containing floats, its elements will occupy contiguous positions in memory and will be ordered according to the order in which they appear in the definition of the struct?

I tested and it is the case with gcc 4.8.2 on Ubuntu, I just want to make sure it is always valid.

That expression won't even compile, `cf` is not an array or a pointer. — Some programmer dude, Jun 18 '15 at 19:52
If this is for academic curiosity, it's fine but if it is for production code, you might want to rethink your design. — R Sahu, Jun 18 '15 at 19:56
Also, this will violate the [strict aliasing rule](http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule). — Some programmer dude, Jun 19 '15 at 05:17

score 2 · Answer 1 · answered Jun 18 '15 at 19:54

2

No you cannot assume that i will be in the right place unless you control the alignment of data in the struct. There might be alignment padding between r and i

answered Jun 18 '15 at 19:54

pm100

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They must be in the same order though – M.M Jun 19 '15 at 00:49

C++ struct memory allocation

1 Answers1