pointer to a vector element, caution !

have a pointer to a vector element, then just push_back element to the vector, then the pointer will be invalid when vector enlarge the content. 

e.g:

vector<A> vec;

vec.push_back(A(1));

A* p_a = &vec[0];

vec.push_back(A(2));

vec.push_back(A(3));

...

then try to access p_a, it will report wild pointer error ... 

2 hours debug ... T, T

being too flexible sometime means disaster.