Strange outputs of std::to_string in C++11

Question

I have this small snippet of C++ code:

#include <array>
#include <string>
#include <iostream>

int main() 
{
  std::string name = "mario";
  std::cerr << "Hello world! " + name + "\n";

  std::array<float, 4> arr = {12, 12.3, 13, 14};
  std::cerr << "first item is: " + std::to_string(arr.front()) << std::endl;
  std::cerr << "last item is: " + std::to_string(arr[-1]) << std::endl;
  return 0;
}

It compiles and outputs the following:

work ❯ c++ -std=c++11 -o hello_world hello.cpp
work ❯ ./hello_world
Hello world! mario
first item is: 12.000000
last item is: 0.000000

But, if I comment out the first two lines like:

#include <array>
#include <string>
#include <iostream>

int main() 
{
  //std::string name = "mario";
  //std::cerr << "Hello world! " + name + "\n";

  std::array<float, 4> arr = {12, 12.3, 13, 14};
  std::cerr << "first item is: " + std::to_string(arr.front()) << std::endl;
  std::cerr << "last item is: " + std::to_string(arr[-1]) << std::endl;
  return 0;
}

And compile and run it. Then it outputs the following:

work ❯ c++ -std=c++11 -o hello_world hello.cpp
work ❯ ./hello_world
first item is: 12.000000
last item is: 12.000000

I have three questions:

why do we get 0.000 in the first case, when using arr[-1]?
why do we get 12.000 in the second case when using arr[-1]?
why do we get different output for arr[-1] in the second case, when we comment out first two statements?

Edit: Based on comments, I understand that arr[-1] would be undefined behavior and hence returns 0.000 in the first case. But, how does commenting out the other statements change this behavior? This is completely confusing to me since I come from Python world.

@NeilButterworth I'm simply trying to access the last element in the array and print it. — kmario23, Jun 08 '18 at 22:26
Undefined behavior, getting junk from memory. If you want the last element, just like you have front() you also have back() — Samer Tufail, Jun 08 '18 at 22:27
You are accessing an invalid position which will result in undefined behavior. — Ervin Szilagyi, Jun 08 '18 at 22:27
`arr[-1]` would access `arr[18446744073709551615]` element but your array is only 4 elements, so slight mismatch here. — Killzone Kid, Jun 08 '18 at 22:44
In C++ "undefined behavior" means that it may conjure demons out of your nose. How? Why? Who cares. Just don't do it. — Öö Tiib, Jun 08 '18 at 22:54
@kmario23 *Strange outputs of std::to_string in C++11* -- You know you're doing something that leads to undefined behavior, so why would you expect the output to be "not strange"? — PaulMcKenzie, Jun 08 '18 at 23:04

score 6 · Accepted Answer · edited Jun 08 '18 at 22:44

6

This is because of Undefined behavior, as std::array::operator[] does not perform any bounds checking, and you are accessing something that is not there.

std::array::operator[] Returns a reference to the element at specified location pos. No bounds checking is performed.

Therefore, no matter what you change or comment, the UB will still be UB.

edited Jun 08 '18 at 22:44

Remy Lebeau

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answered Jun 08 '18 at 22:31

JeJo

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@kmario23 Just for example, see this(https://www.ideone.com/TScRGs) and this( https://www.ideone.com/xqcsAI) two outputs of your codes. Now you have for both, same outcomes. How? that's the magic of UB. You can not define it properly. The only solution is adopt good coding standards(https://stackoverflow.com/questions/3016077/how-to-spot-undefined-behavior) – JeJo Jun 08 '18 at 23:08

Strange outputs of std::to_string in C++11

1 Answers1