Smart Pointers(shared_ptr)

Question

i try to learn how to use smart pointers. I use for the long time normal pointers and i think i need some upgrade of my skills.

I make some research, i understand some aspects of smart pointers, but i try to implement in a clear project to see how smart pointers work. I try:

#include <iostream>
#include <array>
#include <memory>

class Entity
{
public:
    Entity()
    {
        std::cout << "Entity called!" << std::endl;
    }
    ~Entity()
    {
        std::cout << "Entity destroyed!" << std::endl;
    }
    void print() { std::cout << "Message!"; }
};

int main()
{
    std::shared_ptr<int>f1(new int[100]);

    f1.get()[1] = 1;
    std::cout << f1.get()[1];
}

all good, message it's print. But when i try:

#include <iostream>
#include <array>
#include <memory>

class Entity
{
public:
    Entity()
    {
        std::cout << "Entity called!" << std::endl;
    }
    ~Entity()
    {
        std::cout << "Entity destroyed!" << std::endl;
    }
    void print() { std::cout << "Message!"; };
};

int main()
{
    std::shared_ptr<Entity>f1(new Entity[100]);

    f1.get()[1].print();
}

I get this error: [img]https://i.stack.imgur.com/hMDIZ.png

int main()
{
    std::shared_ptr<Entity>f1(new Entity[100]);

    (f1.get() + 1)->print();
}

same error.

I try to use std::make_shared:

#include <iostream>
#include <array>
#include <memory>

class Entity
{
public:
    Entity()
    {
        std::cout << "Entity called!" << std::endl;
    }
    ~Entity()
    {
        std::cout << "Entity destroyed!" << std::endl;
    }
    void print() { std::cout << "Message!"; };
};

int main()
{
    std::shared_ptr<Entity>f1 = std::make_shared<Entity>();

    f1->print();
}

everything it's ok.

I try to alocate continue memory with int:

#include <iostream>
#include <array>
#include <memory>

class Entity
{
public:
    Entity()
    {
        std::cout << "Entity called!" << std::endl;
    }
    ~Entity()
    {
        std::cout << "Entity destroyed!" << std::endl;
    }
    void print() { std::cout << "Message!"; };
};

int main()
{
    std::shared_ptr<int>f1 = std::make_shared<int>(100);

    f1.get()[1] = 10;
    std::cout << f1.get()[1];
}

message it's printed, output: 10 but error: https://i.imgur.com/UPu7VZo.png

i try in another way:

int main()
{
    std::shared_ptr<int>f1 = std::make_shared<int>(100);

    *(f1.get() +1) = 10;
    std::cout << *(f1.get() + 1);
}

same error.

std::shared_ptr<Entity[]>f1 = std::make_shared<Entity[]>(new Entity[100]);

i have error...

What i try to make something like this:

int main()
{
    Entity* f1 = new Entity[100];

    f1[0].print();
    f1[1].print();
    f1[2].print();

}

but i want to use smart pointers.

for int i want to make some assigned value like this:

int main()
{
    int* f1 = new int[100];

    f1[0] = 14;
    f1[1] = 20;
    f1[2] = 5;
}

How can i make something like this with smart pointers. I want to use: std::make_shared(new Entity[100]) or something like this.

I can try with library like vector or array but i don't want to use this library for the moment. I want to keep my code clear for the moment. After i understand 100% smart pointers i will use array and vector library

Error messages are text. Paste the text here; don't link to an image of text. — tenfour, Mar 30 '20 at 11:54
You should use `std::shared_ptr` or the container will try to call `delete` instead of `delete[]` which is wrong https://stackoverflow.com/questions/1553382/is-delete-equal-to-delete — Cory Kramer, Mar 30 '20 at 11:57
Does this answer your question? [shared\_ptr to an array : should it be used?](https://stackoverflow.com/questions/13061979/shared-ptr-to-an-array-should-it-be-used) — a1ezh, Mar 30 '20 at 12:01
`make_shared` creates ("makes") a shared object. It does not take a pointer to an object and "make it shared". — molbdnilo, Mar 30 '20 at 12:02
While `std::shared_ptr` is possible, `std::vector` and `std::array` are more common. Smart pointers are only one of the replacements of dumb pointers. The standard containers are another replacement. — MSalters, Mar 30 '20 at 12:02

john · Answer 1 · 2020-03-30T12:22:42.523

5

Not

std::shared_ptr<Entity[]> f1 = std::make_shared<Entity[]>(new Entity[100]);

but

std::shared_ptr<Entity[]> f1 = std::make_shared<Entity[]>(100);

or even simpler

auto f1 = std::make_shared<Entity[]>(100);

When you use make_shared you should not use new. One of the points of make_shared is to allocate memory more efficiently that it would be if you used new.

Note: the array form of make_shared requires C++20.

edited Mar 30 '20 at 12:22

answered Mar 30 '20 at 11:56

john

85,011
4
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I have errorrs :warning C4200: nonstandard extension used: zero-sized array in struct/union – Claudiu Andries Mar 30 '20 at 12:00
What line of code does that error refer to? What version of C++ are you compiling to? – john Mar 30 '20 at 12:00
@ClaudiuAndries what compiler are you using? – Mgetz Mar 30 '20 at 12:01
IDE it's visual studio comunity, but i think it's C++2017/2020 – Claudiu Andries Mar 30 '20 at 12:02
i compilate:int main() { std::shared_ptrf1 = std::make_shared(100); } and after:https://pastebin.com/3mEjjb8W – Claudiu Andries Mar 30 '20 at 12:03
@ClaudiuAndries in visual studio you need to set according to what version of standard the compiler compiles your code. https://learn.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=vs-2019 – Öö Tiib Mar 30 '20 at 12:05
@ClaudiuAndries Apparently using an array type in make_shared is not supported until C++20, apologies. – john Mar 30 '20 at 12:06
@Öö Tiib it's C++17; – Claudiu Andries Mar 30 '20 at 12:09
@john i use c++17 – Claudiu Andries Mar 30 '20 at 12:10
@ClaudiuAndries: For C++17, see NadavS's answer. It's slightly more verbose. (Or use `std::vector`, which is supported since 1998) – MSalters Mar 30 '20 at 12:15
using shared_ptr with arrays is just bad practice. – Waqar Mar 30 '20 at 12:28
@Waqar why? tell me more, i want to understand that – Claudiu Andries Mar 30 '20 at 12:33
This post explains is much better than I ever could: https://stackoverflow.com/questions/3266443/can-you-use-a-shared-ptr-for-raii-of-c-style-arrays – Waqar Mar 30 '20 at 14:49
@Waqar That post is out of date, since C++11 shared_ptr can safely be used on arrays. – john Mar 30 '20 at 15:01

score 2 · Answer 2 · answered Mar 30 '20 at 12:10

2

The code sample that should work for you is:

int main() {
    std::shared_ptr<Entity[]>f1(new Entity[100]);

    f1.get()[1].print();
}

As for make_shared, you're out of luck - the current CPP standard, 17, doesn't seem to support it (see https://en.cppreference.com/w/cpp/memory/shared_ptr/make_shared) However, make_shared is only used to eliminate a race condition in allocation and can be implemented manually or just omitted. I encourage reading more about it.

answered Mar 30 '20 at 12:10

NadavS

779
3
12

your code work.. but i don't understand exacly: auto sp = std::make_shared(12); static_assert(std::is_same_v>); std::cout << sp->i << '\n'; in my case C++17: auto f2 = std::make_shared(100); //compile without errors how can i acces f2.get()[1].printf()? i need to make static_assert? – Claudiu Andries Mar 30 '20 at 12:31
and question 2: how can i take care about race condition in allocation, how can i implement manual? – Claudiu Andries Mar 30 '20 at 12:35
I don't understand question 1. – NadavS Mar 30 '20 at 12:41
In question 2, the issue is that if the constructor of `shared_ptr` fails you leak your allocation. AFAIK, the code to solve it is: – NadavS Mar 30 '20 at 12:44
shared_ptr p; T* temp_ptr = new T(); try { p = temp_ptr; } catch(...) { delete temp_ptr; throw; } – NadavS Mar 30 '20 at 12:45
question 1: i see an exemple on cppreference : auto sp = std::make_shared(12); how can i get the acces on sp[1] sp[2]? – Claudiu Andries Mar 30 '20 at 12:47
in this example, if I understand correcty, there is only one `C`, not an array of `C` s. Therefore, it is not what you're looking fore. – NadavS Mar 30 '20 at 12:49

Smart Pointers(shared_ptr)

2 Answers2