Question

I want to make a dynamic array of foo, with the number of items being x. Arguments y and z are to be passed to the constructor of the item foo. I was hoping to do something similar to:

Foo* bar = new Foo(y, z)[x];

However that produced the following compiler error:

 error: expected `;' before '[' token

So after speaking with an experienced friend, he gave me this, which he admitted was a lazy way of doing it, but it works. I was wondering, is there a better/proper way?

Foo* bar = (Foo*) new int[x];
for (int i = 0; i < x; i++) {
    bar[i] = Foo(y, z);
}

Answer 1

“I want to make a dynamic array” So use a std::vector, it exists for a reason.

std::vector<foo> bar(x, foo(y, z));

This creates a dynamic array with x elements initialized to foo(y, z).

---

The above makes copies of the second parameter, x times. If you want to generate values for the vector, use generate_n:

std::vector<double> weights;
std::generate_n(std::back_inserter(weights), x, ...);

You replace ... with a function or functor to call, that returns a value. Generally you make a functor:

struct generate_weight
{
    double operator()() const
    {
        return random(1e-3);
    }
};

Giving:

std::generate_n(std::back_inserter(weights), x, generate_weight());

If your compiler supports C++0x, you can take advantage of lambda’s. These do the same thing, except they keep the code concise and localized:

std::generate_n(std::back_inserter(weights), x,
                [](){ return random(1e-3); } );