How can I use unique pointers for an array of parameterized objects in C++ using Visual Studio 2017?

Question

Consider a class whose default constructor takes in the file path as a parameter.

class Test
{
    public:
         Test(const std::string& filepath);
    ...
    ...
};

Now I wish to create and initialize an array of Test objects using unique pointers in VS2017.

int main()
{
    std::unique_ptr<Test[]> m_Tests;
    int testCount = 2;
    std::string path1, path2;

    m_Tests = std::make_unique<Test[]>(testCount);    // This line gives a compilation error
    m_Tests[0] = std::make_unique<Test>(path1);
    m_Tests[1] = std::make_unique<Test>(path2);
}

How can I make this work?

Related reading: [make_unique arrays, original proposal vs. final](https://stackoverflow.com/q/16755415/1553090) — paddy, Feb 25 '20 at 06:30
This would be much easier if you used `std::vector` instead of `std::unique_ptr`. Is there any reason you are not using `std::vector`? — walnut, Feb 25 '20 at 06:32
FYI: [SO: How to initialize elements of an array managed by a unique_ptr?](https://stackoverflow.com/a/52217532/7478597) — Scheff's Cat, Feb 25 '20 at 06:33

Jędrzej Dudkiewicz · Accepted Answer · 2020-02-25T08:05:00.750

2

g++ 9.2.0 tells me that you lack default constructor, i.e. one without parameters. Adding such constructor works fine. If it's not what you want, you can create array of unique_ptr's, so std::unique_ptr<std::unique_ptr<Test>[]> and after that initialize each element by hand, something similar to this:

#include <memory>
#include <algorithm>
#include <iostream>

struct Test {
std::string str_;
Test(std::string const& str) : str_(str) { }
void print() { std::cout << str_ << '\n'; }
};

int main()
{
    std::unique_ptr<std::unique_ptr<Test>[]> m_Tests;
    int testCount = 2;
    std::string path1{"a"}, path2{"b"};

    m_Tests = std::make_unique<std::unique_ptr<Test>[]>(testCount);
    std::array<std::string, 2> paths{path1, path2};
    std::transform(paths.begin(), paths.end(), &m_Tests[0],
        [](auto const& p) { return std::make_unique<Test>(p); });

    for (int i = 0 ; i < testCount ; ++i) {
            m_Tests[i]->print();
    }
}

edited Feb 25 '20 at 08:05

answered Feb 25 '20 at 06:36

Jędrzej Dudkiewicz

1,053
8
21

The default constructor could be added with a small modification easily: `Test(std::string const &str = std::string())`. – Scheff's Cat Feb 25 '20 at 06:39
Yes, yet this might not be what OP wants, that's why I posted "longer" version. – Jędrzej Dudkiewicz Feb 25 '20 at 06:40
@JędrzejDudkiewicz This is exactly the approach I wanted. In your solution, each path is set separately. Can it be done at once? – Majis Feb 25 '20 at 07:44
1

Added, but I don't think this is great solution. – Jędrzej Dudkiewicz Feb 25 '20 at 08:05

walnut · Answer 2 · 2020-02-25T07:16:40.453

There is no overload of std::make_unique that does this, so you would need to use new directly:

m_Tests.reset(new Test[testCount]{path1, path2});

This will however only compile if testCount is a constant expression, so you need to change the definition int testCount = 2; to const int or constexpr int.

If testCount is not a constant expression, there needs to be a default constructor defined for the case that testCount is smaller than 2 at runtime.

So, really, you probably want to ignore testCount and just let the array size be deduced:

m_Tests.reset(new Test[]{path1, path2});

It would be much easier if you just used std::vector:

std::vector<Test> m_Tests;

//...

m_Tests.emplace_back(path1);
m_Tests.emplace_back(path2);

score -1 · Answer 3 · answered Feb 25 '20 at 06:51

-1

How about you use std::array and can you get rid of testCount (or use it as constexp) then the code can be like below.

class Test
{
public:
    Test(const std::string& filepath){}
};

int main()
{
    constexpr int testCount = 2;
    std::unique_ptr<std::array<Test, testCount>> m_Tests;
    std::string path1, path2;

    m_Tests = std::make_unique<std::array<Test, testCount>>(std::array<Test, testCount>{path1,path2});
}

answered Feb 25 '20 at 06:51

Hakan Batmaz

64
4

This answer assumes that `testCount` is known at compile time. – fwyzard Nov 12 '21 at 08:46

How can I use unique pointers for an array of parameterized objects in C++ using Visual Studio 2017?

3 Answers3