What do constructors and destructors do?

A constructor runs when an object is created and initialises its members. A destructor runs when it is destroyed and releases resources. Constructors share the class name and have no return type; destructors are ~Class(), take no arguments and there is exactly one.

class Buffer {
public:
    explicit Buffer(std::size_t n)
        : data_(new char[n]), size_(n) {}

    ~Buffer() { delete[] data_; }

    Buffer(const Buffer&) = delete;
    Buffer& operator=(const Buffer&) = delete;

private:
    char* data_;
    std::size_t size_;
};

Prefer the member initialiser list over assignment in the constructor body. It initialises directly instead of default-constructing then assigning, and it is required for const and reference members. Initialisation order follows declaration order, not the order in the list, which can surprise you.

A destructor that reliably frees resources is what makes RAII work, so cleanup happens on scope exit and during stack unwinding from an exception.