Is there a way to initialize a array of some objects with the same parameter for the constructer function?

Question

For example,

class JJ
{
public:
    JJ(int c)
    {
        a = c;
    }
    int a;
};

I can write

JJ z[2] = {6,6};

But for JJ z[100], I can't write {6,6,6....,6}, so what can I do with that?

Are you using a compiler supporting C++11 or even C++14 ? If so have a look at std::array — Jojje
– Jojje, Commented Mar 9, 2016 at 10:18
You could use std::vector<JJ> z(100, 6); to create 100 elements with value 6. — Bo Persson
– Bo Persson, Commented Mar 9, 2016 at 10:24
@BoPersson Yes, but what if there are more arguments in the constructor? Besides, there is no way no use JJ z[100] to make it without stl? — Jason
– Jason, Commented Mar 9, 2016 at 10:29
You might want to specify what you mean by "array". It has more than one meaning in C++. For example, a C-style array, the std::array templated container type in the standard library, possibly even usage of other standard containers (like std::vector). Each choice has different advantages, depending on what you are ACTUALLY doing. Containers are often considered vastly preferable to C-style arrays, even if beginners think otherwise. — Peter
– Peter, Commented Mar 9, 2016 at 10:29
@Peter The information in your last sentence informs me sth I didn't know. I meant C-style arrays. It seems with C-style arrays, it can't be done. — Jason
– Jason, Commented Mar 9, 2016 at 10:34

Jarod42 · Accepted Answer · 2016-03-09 10:36:11Z

3

If std::vector doesn't fit your needs with

std::vector<JJ> v(100, JJ(6));

You may use std::array

namespace detail
{

    template <std::size_t N, std::size_t...Is, typename T>
    std::array<T, N> make_array(std::index_sequence<Is...>, const T& t)
    {
        return {{(static_cast<void>(Is), t)...}};
    }

}


template <std::size_t N, typename T>
std::array<T, N> make_array(const T& t)
{
    return detail::make_array<N>(std::make_index_sequence<N>{}, t);
}

And then

std::array<JJ, 100> a = make_array<100>(JJ(6));

Demo

edited Mar 9, 2016 at 10:36

answered Mar 9, 2016 at 10:29

Jarod42

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2 Comments

Peter Over a year ago

The usage of namespace detail is unnecessary.

Jarod42 Over a year ago

@Peter: It does compile Demo.

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