Why auto_ptr in c++ was removed.

• std::auto_ptr

auto_ptr was deprecated from gcc 4.7.1 and above or when compiled with --std=c++11 will start to give warning, and is completely removed from --std=c++17.

The main reason is, it moves from lvalue via copy/assignment operator.

For Eg:

#include <iostream>
#include <memory>

int main()
{
    auto printNull = [] (std::auto_ptr<int> &value, std::string name) 
    {
        (value.get() == nullptr) ? std::cout << name << " is nullptr\n" 
                        : std::cout << name << " is not nullptr, & contains " << *value << "\n";
    };

    std::auto_ptr<int> p_int(new int{5});
    printNull(p_int, "p_int");
    std::auto_ptr<int> p_new_int;
    p_new_int = p_int;  /*Moves instead of copy*/
    printNull(p_new_int, "p_new_int");
    printNull(p_int, "p_int");

    return 0;
}


****OUTPUT****

p_int is not nullptr, & contains 5
p_new_int is not nullptr, & contains 5
p_int is nullptr

So it's definitely not a good thing, and it was not aligning itself with modern cpp ideologies.
In most cases auto_ptr can be directly swapped with std::unique_ptr<>, but in larger systems caution is advised before changing.

Even thought it is removed from c++17, I can still compile it in gcc 13.1 link this was kind of strange.

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