unique_ptr array access Segmentation fault

Question

When I access array elements through unique_ptr, a segfault occurs,Through vs debugging, I found that the type and data of std::unique_ptr<T[]> p is strange,I think it should be an array, but it looks like a string,No matter how many elements I push, the data of p points to "to", and other elements cannot be seen.

code

#include <memory>
#include <string>
#include <assert.h>
#include<vector>
#include<iostream>
#include <stack>
#include <string>
#include <sstream>

template <typename T>
class FixedCapacityStockOfStrings {
public:
    FixedCapacityStockOfStrings(const int cap) {
        p = std::make_unique<T[]>(cap);
        MAX = cap;
    } 
    bool isEmpty() {
        return N == 0;
    }
    size_t const size() { return N; }
    void push(T& item){
        //assert(N < MAX - 1);
        if (N == MAX-1) resize(2 * MAX);
        p[N++] = item;
    }
    T pop() {
        assert(N > 0);
        T item = p[--N];
        p[N] = nullptr;//Segmentation fault is here
        if ( N <= MAX / 4) resize(MAX / 2);
        return item;
    }
    size_t max() const { return MAX; }
    void clear() {
        N = 0;
    }
private:
    void resize(int max) {
        auto t = std::make_unique<T[]>(max);
        for (int i = 0; i < N; i++) {
            t[i] = p[i];
        }
        p.reset();
        p = std::move(t);
        MAX = max;
    }
    std::unique_ptr<T[]> p;
    size_t N,MAX;
};

int main() {
    FixedCapacityStockOfStrings<std::string> s(100);
    std::string line,item;
    while (std::getline(std::cin, line)) {
        std::istringstream items(line);
        while (items >> item) {
            if (item != "-")
                s.push(item);
            else if (!s.isEmpty()) std::cout << s.pop() << " ";
        }
        std::cout << "(" << s.size() << " left on stack)" << " max stack : " << s.max() << std::endl;
        s.clear();
    }
}

Answer 1

Note that p[N] has type std::string& for T = std::string , so what

p[N] = nullptr;

does is call std::string::operator=(const char*) with parameter nullptr . This is not a parameter you're allowed to pass to this assignment operator; it expects a 0-terminated string.

Edit: Improved based on suggestion by @Remy Lebeau

You should go with

p[N] = T{};

instead.

Answer 2

You forgot to initialize N in the constructor, so it is a garbage value and reading it is undefined behavior.

Answer 3

p contains an array of std::string. When you assign p[N] = nullptr , you assign a C string to std::string. C string is a pointer to a null-terminated character array and nullptr is not a valid C string.

Answer 4

In the statement p[N] = nullptr; , you are assigning a nullptr to a std::string , which is Undefined Behavior .