Segmentation fault when Using std::initializer_list

Hi i am trying to understand how constructors work and so reading different examples. I have a class constructor that takes an initializer_list but it keeps giving Segmentation fault. The files that i have are as follows:

1. strvec.h

class StrVec {
public:
    StrVec(): elements(nullptr), first_free(nullptr), cap(nullptr) {

    }
    StrVec(const StrVec&);
    StrVec(std::initializer_list<std::string> il);
    StrVec &operator=(const StrVec&);
    ~StrVec();
    void push_back(const std::string&);
    void pop_back();
    void reserve(std::size_t);
    void resize(std::size_t, const std::string& = std::string());
    bool empty() const {
        return begin() == end();
    }
    
    
    std::size_t size() const {
        return first_free - elements;
    }
    std::size_t capacity() const{
        return cap - elements;
    }
    std::string *begin() const {
        return elements;
    }
    std::string *end() const {
        return first_free;
    }

    
private:
    static std::allocator<std::string> alloc;
    void chk_n_alloc() {
        if (size() == capacity()){
            reallocate();
        }
    }
    std::pair<std::string*, std::string*> alloc_n_copy(const std::string*, const std::string*);
    void free();
    void reallocate();

    std::string *elements;
    std::string *first_free;
    std::string *cap;
};

2. strvec.cpp

StrVec::StrVec(const StrVec &s){
    auto newdata = alloc_n_copy(s.begin(), s.end());
    elements     = newdata.first;
    first_free   = cap = newdata.second;
}

StrVec::StrVec(std::initializer_list<std::string> il){
    for(const auto &s:il){
        push_back(s);
    }
}
std::pair<std::string*, std::string*> StrVec::alloc_n_copy(const std::string *b, const std::string *e){
    auto data = alloc.allocate(e - b);
    return {data, uninitialized_copy(b, e, data)};
}

3. mainfile.cpp

int main() {
StrVec sv10 { "il1", "il2", "il3", "il4", "il5" };
return 0;
}

My question is how can i resolve this issue and why am i getting this Segmentation fault and what does it mean so that i can avoid it in the future?

PS: I know that the error is due to the StrVec(std::initializer_list<std::string> il); constructor since if i remove this and its use in the mainfile.cpp then Segmentation fault goes away.