B.a[i][j] is not defined at any point since you haven't told your vector how large it will be. You either have to use vector::push_back(), which will increase the size of the vector to accommodate, or assign B.a to a usable length first. There are two easy ways to do the latter. First, just to assign your output to the input or argument from the get-go. You can declare and define 'Matrix B = A;' to assign the whole object, or you can just add line 'B.a = this->a;' to your current code to pass only a. The second and even easier route: you can just pass the argument by value and skip the entire step, since you never have to define Matrix B at all. If you really want, you can also manually resize the vectors to suit your needs:

int sizeOfA = A.a.size();
B.a.resize(sizeOfA);
for(int i=0; i < sizeOfA; i++) {
    B.a[i].resize(sizeOfA); }

Obviously, that would be terribly verbose and generally unnecessary, but you can do it and it would also solve your segmentation fault issue. Nevertheless, the strategy nicely demonstrates the nature of your problem. Otherwise, the code works. Had to do a double-take on a[i][j], but that will technically work as well since it defaults to the current object. I personally use this->a[i][j], though.