std::to_string return empty string when using std::allocator

Question

I got the empty return of std::to_string when I use std::allocator. The code is:

#include <iostream>
#include <memory>
#include <string>

int main()
{
    std::allocator<std::string> allocatorStr;
    auto const pStr = allocatorStr.allocate(5);
    for (int i = 0; i < 5; ++i) { *(pStr + i) = std::to_string(i); }
    for (int i = 0; i < 5; ++i) { std::cout << *(pStr + i) << std::endl; }

    return 0;
}

But if I change the line *(pStr + i) = std::to_string(i); to *(pStr + i) = "string"; I could get non-empty output. I don't know where is wrong. Thank you very much for your help.

Answer 1

To quote from cppreference , (emphasis mine):

[ allocate ] ... allocates n * sizeof(T) bytes of uninitialized storage

In other words, there are no valid std::string objects there yet and trying to use them (even as the target of a copy) invokes undefined behaviour (I got a segfault).

To fix this, use placement new to construct the needed std::string objects immediately after calling allocate :

auto pStr = allocatorStr.allocate(5);
for (int i = 0; i < 5; ++i) { new (pStr + i) std::string; }
...

Then, it works