c++ - Memory layout of vector of POD objects -

suppose have simple c++ class,

class data {      public:           float data[3];                    void clear() { data[0] = 0.0f; data[1] = 0.0f; data[2] = 0.0f }  } 

and vector of data's,

std::vector<data> v(10); 

is safe assume &v[0].data[0] points array of 30 floats?

from standard

23.3.6.1 class template vector overview

the elements of vector stored contiguously, meaning if v vector t type other bool, obeys identity &v[n] == &v[0] + n 0 <= n < v.size()

so &v[0] indeed points beginning of 10 continuous data objects.

but layout of data have

9.2.13 class members

nonstatic data members of (non-union) class same access control (clause 11) allocated later members have higher addresses within class object. order of allocation of non-static data members different access control unspecified (11). implementation alignment requirements might cause 2 adjacent members not allocated after each other; so might requirements space managing virtual functions (10.3) , virtual base classes (10.1).

so cannot sure sizeof(data) == 3*sizeof(float), therefore general answer should be: it's not save assume 30 continuous floats.