unique_ptr or vector?

Question

If you don\'t need dynamic growth and don\'t know the size of the buffer at compile time, when should unique_ptr be used instead of vector<

Answer 1

If you're in a position where vector is even a possibility, you probably want to go with that except in extreme and rare circumstances. And even then, a custom type instead of unique_ptr may well be the best answer.

So what the heck is unique_ptr good for? :-)

unique_ptr really shines in two circumstances:

1. You need to handle a malloc/free resource from some legacy function and you would like to do it in a modern exception safe style:

void
foo()
{
    std::unique_ptr p(strdup("some text"), std::free);
    for (unsigned i = 0; p[i]; ++i)
        std::cout << p[i];
    std::cout << '\n';
}

2. You've need to temporarily secure a new[] resource before transferring it onto another owner:

class X
{
    int* data_;
    std::string name_;

    static void validate(const std::string& nm);
public:
    ~X() {delete [] data_;}

    X(int* data, const std::string& name_of_data)
        : data_(nullptr),
          name_()
    {
        std::unique_ptr hold(data);  // noexcept
        name_ = name_of_data;               // might throw
        validate(name_);                    // might throw
        data_ = hold.release();             // noexcept
    }
};

In the above scenario, X owns the pointer passed to it, whether or not the constructor succeeds. This particular example assumes a noexcept default constructor for std::string which is not mandated. However:

1. This point is generalizable to circumstances not involving std::string.
2. A std::string default constructor that throws is lame.