getting a segfault when I try to deep copy `unique_ptr`s - c++

Question

Why can't I return a unique_ptr from a clone function? I thought I could do this.
I have a base class for different mathematical functions called transform. I have a container of pointers to this type because I am using polymorphism. For example, all these derived classes have a different implementation of log_jacobian that is useful for statistical algorithms.
I am using unique_ptrs to this transform class, and so I have made a (pure virtual) clone function that makes new unique_ptr pointing to a deep copy of the same mathematical transform object. This new object is of the same type that derives from transform<float_t>, but it's a separate because you can't have two unique_ptrs pointing to the same thing.
template<typename float_t>
class transform{
...
virtual std::unique_ptr<transform<float_t>> clone() const = 0;
...
};
My transform_container class holds a few of these at a time. After all, most statistical models have more than one parameter.
template<typename float_t, size_t numelem>
class transform_container{
private:
using array_ptrs = std::array<std::unique_ptr<transform<float_t>>, numelem>;
array_ptrs m_ts;
unsigned m_add_idx;
...
auto get_transforms() const -> array_ptrs;
};
I'm not sure why the deep copy function get_transforms isn't working, though. It is used to make copies, and to access individual transformations from the container. I get a segfault when I run some tests. If I run it in gdb, it explicitly tells me the line with a comment after it is bad.
template<typename float_t, size_t numelem>
auto transform_container<float_t,numelem>::get_transforms() const -> array_ptrs
{
array_ptrs deep_cpy;
for(size_t i = 0; i < numelem; ++i){
deep_cpy[i] = m_ts[i]->clone(); // this line
}
return deep_cpy;
}
I've also tried std::moveing it into deep_cpy[i] and using unique_ptr::reset, but to no avail.
Edit:
here are some other relevant methods: a method that adds transforms to transform_container, and the factory method for the individual transform:
template<typename float_t>
std::unique_ptr<transform<float_t> > transform<float_t>::create(trans_type tt)
{
if(tt == trans_type::TT_null){
return std::unique_ptr<transform<float_t> >(new null_trans<float_t> );
}else if(tt == trans_type::TT_twice_fisher){
return std::unique_ptr<transform<float_t> >(new twice_fisher_trans<float_t> );
}else if(tt == trans_type::TT_logit){
return std::unique_ptr<transform<float_t> >(new logit_trans<float_t> );
}else if(tt == trans_type::TT_log){
return std::unique_ptr<transform<float_t> >(new log_trans<float_t> );
}else{
throw std::invalid_argument("that transform type was not accounted for");
}
}
template<typename float_t, size_t numelem>
void transform_container<float_t, numelem>::add_transform(trans_type tt)
{
m_ts[m_add_idx] = transform<float_t>::create(tt);
m_add_idx++;
}

Answer 1

In get_transforms(), you are looping over the entire m_ts[] array, calling clone() on all elements - even ones that have not been assigned by add_transform() yet! Unassigned unique_ptrs will be holding a nullptr pointer, and it is undefined behavior to call a non-static class method through a nullptr.
The easiest fix is to change the loop in get_transforms() to use m_add_idx instead of numelem:
template<typename float_t, size_t numelem>
auto transform_container<float_t,numelem>::get_transforms() const -> array_ptrs
{
array_ptrs deep_cpy;
for(size_t i = 0; i < m_add_idx; ++i){ // <-- here
deep_cpy[i] = m_ts[i]->clone();
}
return deep_cpy;
}
Otherwise, you would have to manually ignore any nullptr elements, eg:
template<typename float_t, size_t numelem>
auto transform_container<float_t,numelem>::get_transforms() const -> array_ptrs
{
array_ptrs deep_cpy;
for(size_t i = 0, j = 0; i < numelem; ++i){
if (m_ts[i]) {
deep_cpy[j++] = m_ts[i]->clone();
}
}
return deep_cpy;
}
Either way, you should update add_transform() to validate that m_add_idx will not exceed numelem:
template<typename float_t, size_t numelem>
void transform_container<float_t, numelem>::add_transform(trans_type tt)
{
if (m_add_idx >= numelem) throw std::length_error("cant add any more transforms"); // <-- here
m_ts[m_add_idx] = transform<float_t>::create(tt);
++m_add_idx;
}
That being said, since transform_container can have a variable number of transforms assigned to it, I would suggest changing transform_container to use a std::vector instead of a std::array, eg:
template<typename float_t>
class transform_container{
private:
using vector_ptrs = std::vector<std::unique_ptr<transform<float_t>>>;
vector_ptrs m_ts;
...
auto get_transforms() const -> vector_ptrs;
};
template<typename float_t>
auto transform_container<float_t>::get_transforms() const -> vector_ptrs
{
vector_ptrs deep_cpy;
deep_cpy.reserve(m_ts.size());
for(const auto &elem : m_ts){
deep_cpy.push_back(elem->clone());
}
return deep_cpy;
}
template<typename float_t>
std::unique_ptr<transform<float_t>> transform<float_t>::create(trans_type tt)
{
switch (tt) {
case trans_type::TT_null:
return std::make_unique<null_trans<float_t>>();
case trans_type::TT_twice_fisher:
return std::make_unique<twice_fisher_trans<float_t>>();
case trans_type::TT_logit:
return std::make_unique<logit_trans<float_t>>();
case trans_type::TT_log:
return std::make_unique<log_trans<float_t>>();
}
throw std::invalid_argument("that transform type was not accounted for");
}
template<typename float_t>
void transform_container<float_t>::add_transform(trans_type tt)
{
m_ts.push_back(transform<float_t>::create(tt));
}