const vector of Pointers to C-Style Array Elements

Question

Say I have a C-style array like this: int foo[]{1, 2, 3, 4, 5};

Now I want to construct a const std::vector<int*> pFoo{&foo[0], &foo[1], &foo[2], &foo[3], &foo[4]};

I can use the initializer_list as long as I know all the elements. But say that I was just passed foo and it's size. Can I initialize pFoo without knowing the size of foo at design time?

do you mean size as in number of elements in array (5) or size of int? — Jimmy
– Jimmy, Commented Oct 20, 2014 at 12:47
you can initialise a vector by using start and end iterators vector<int*>(int* start,int* end) - this assumes they are contiguous in memory — Jimmy
– Jimmy, Commented Oct 20, 2014 at 12:49
@Jimmy You're talking about using std::vector's iterator ctor I believe, which won't work cause I'm constructing a std::vector of int*s not a std::vector of ints. — Jonathan Mee
– Jonathan Mee, Commented Oct 20, 2014 at 12:53
You can probably use boost:: transform_iterator to adapt begin(foo) and end(foo). — n. m. could be an AI
– n. m. could be an AI, Commented Oct 21, 2014 at 4:28

user4164058 · Accepted Answer · 2014-10-21 04:44:52Z

1

You can create a "proxy" function that initializes your vector. This uses template deduction to find the size of the array automatically.

template <typename T, std::size_t N>
std::vector<int*> init_vector(T (&foo)[N])
{
    std::vector<int*> vec;
    for (std::size_t i = 0; i < N; ++i)
    {
        vec.push_back(&foo[i]);
    }
    return vec;
}

int main()
{
    int foo[] {1, 2, 3, 4, 5};
    const std::vector<int*> vec = init_vector(foo);
    for (auto v : vec) std::cout << *v << " ";
}

Alternatively, if you're able to use Boost, you can use boost::make_transform_iterator:

int* convert_to_ptr(int& i)
{
    return &i;
}

int main()
{
    int foo[] {1, 2, 3, 4, 5};
    const std::vector<int*> vec { 
        boost::make_transform_iterator(std::begin(foo), convert_to_ptr),
        boost::make_transform_iterator(std::end(foo),   convert_to_ptr)
    };

edited Oct 21, 2014 at 4:44

answered Oct 21, 2014 at 4:11

user4164058

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2 Comments

Jonathan Mee Over a year ago

I have to say that template deduction is pretty cool. However, not quite what I was looking for. That can be just above the definition of vec without a templatized function. It requires a copy construction, even if a separate function isn't called. The boost::make_transform_iterator is exactly what I'm looking for... but I don't have access to boost :(

user3920237 Over a year ago

@JonathanMee I believe copy elision is in effect here.

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