Create a unique_ptr to an array with type double in c++

Question

I am writing a constructor for a class containing the following:

// my_class.hpp
private:
    std::unique_ptr<double[]> my_list;

and I am facing some problems when trying to initialize my_list

my_class::my_class(const int new_size, const double new_values){
    // creating an array with new_values as initial value.
    auto size = static_cast<const size_t>(new_size);
    my_list = std::make_unique<double[]>(size);          // <-- Error
    for (size_t i = 0; i < size; i++){
        my_list[i] = new_values;
    }
}

Error Message: "do not declare C-style arrays, use std::array<> instead [modernize-avoid-c-arrays,-warnings-as-errors]"

I have tried the following:

    my_list = std::make_unique<std::array<double>>(size);    // <-- Error
    // Error Message: "too few template arguments for class template 'array' [clang-diagnostic-error]"

and

    my_list = std::make_unique<std::array<double,size>>(size);    // <-- Error
    // Error Message: "non-type template argument is not a constant expression [clang-diagnostic-error]"

How can I fix this problem and also is there any way to initialize such an array without looping through each element?

An std::array is a fixed size array. You probably want an std::vector. Read on both options and decide for yourself. — Benny K
– Benny K, Commented Jul 6, 2020 at 8:23
I've never really seen the use-case for std::unique_ptr<T[]> when we have std::vector<T>. If ownership semantics is wanted then std::unique_ptr<std::vector<T>> is probably better (and a good use-case for pointers to containers, which is generally not common). — Some programmer dude
– Some programmer dude, Commented Jul 6, 2020 at 8:25
@Someprogrammerdude It can be useful when your legacy code hands you a pointer that you get to own and you want to avoid copying. I don't see a case for ever creating one "from scratch", though. — molbdnilo
– molbdnilo, Commented Jul 6, 2020 at 8:57
Your error is not coming from standard c++ and might be incorrect. It looks like it wants you to use only vectors in your code. — Daniel
– Daniel, Commented Jul 6, 2020 at 9:36

Caleth · Accepted Answer · 2020-07-06 08:39:30Z

2

How can I fix this problem

Switch std::unique_ptr<double[]> to std::vector<double>.

Is there any way to initialize such an array without looping through each element?

my_class::my_class(std::size_t new_size, double new_values) 
: my_list(new_size, new_values) 
{}

answered Jul 6, 2020 at 8:39

Caleth

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Tiger Yu · Accepted Answer · 2020-07-06 11:02:28Z

1

There are two ways to fix this problem.

keep your std::unique_ptr<double[]>, and you do not need to alter the rest of your code, but you need to replace these two lines

    auto size = static_cast<const size_t>(new_size);
    my_list = std::make_unique<double[]>(size);          // <-- Error

with this:

    my_list = std::make_unique<double[]>(new_size);  // allocates new_size of doubles

As Caleth pointed out, you can change the type of data member my_list from std::unique_ptr<double[]> to std::vector<double>, but then you need to change all the placed that access my_list.

How can I fix this problem and also is there any way to initialize such an array without looping through each element?

If you keep my_list as unique_ptr, then you can use std::fill_n(my_list.get(), new_size, new_values) to initialize your array.

edited Jul 6, 2020 at 11:02

answered Jul 6, 2020 at 9:37

Tiger Yu

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3 Comments

StevenC Over a year ago

I am using solution 1 but I end up with a different error message "no matching function for call to make_unique"

Tiger Yu Over a year ago

Sorry, syntax error, it should be std::make_unique<double[]>(new_size);

JHBonarius Over a year ago

solution 1 is not really required. What he's doing is redundant, but valid.

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