Create an array of objects using std::unique_ptr

Question

I am currently learning smart pointers and am trying to do the following:

#include <memory>
#include <string>

int main() {
    std::unique_ptr<std::string[]> str_array(new std::string[5]);
    for (int i = 0; i < 5; i++) {
        std::getline(std::cin, str_array.get()[i]);
    }
    return 0;
}

This code simply scans 5 std::string objects into an array. However, the debugger shows that the memory is not getting reserved for 5 objects:

What am I doing wrong?

Alberto · Accepted Answer · 2020-07-19 10:21:31Z

5

It's caused by the fact that new std::string[5] is a dynamic array allocation, and so it has no (fixed/defined/calculable) size (if you don't save it before the allocation), infact

auto p = new std::string[5];
std::cout << sizeof(p);

Will print 8 that is the size of a pointer.

And so the debugger sees that ptr as a pointer, and can't figure out that there are other 4 elements after the first one (and so can't figure out that is an array, and not just a pointer to a string)

Seems like that this is a "C++ POV", and that the debugger can have more informations, and so should be able to figure out that it is an array of string and not just a pointer to string (thanks to @Konrad Rudolph)

edited Jul 19, 2020 at 10:21

answered Jul 19, 2020 at 10:07

Alberto

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2 Comments

Konrad Rudolph Over a year ago

To be fair the debugger could (and arguably should) see this, as it potentially has more information than the C++ type system. The fact that the debugger displays the pointer type as std::string* rather than std::string[] or even std::string[5] is a debugger QOI issue.

Alberto Over a year ago

@KonradRudolph thanks, added a note at the bottom, feel free to edite the answer if you think that there is more to say, i'll gladly approve and accept it

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