std::map crashes when doing m[0]

Question

The following crashes:

  std::map<std::string, int> m1{ {"0", 0}, { "1", 1 }};
  // auto melem = m1["0"]; // OK
  auto melem = m1[0];

Why is that?

Why shouldn't it crash? Argument to `operator[]` must of type `key_type` for given map. — Yksisarvinen, May 30 '19 at 16:30
Because `"0"` is one of the keys and `0` is not. Why do you expect it not to crash, or what do you expect to happen instead? — mkrieger1, May 30 '19 at 16:32
basic_string( const CharT* s,const Allocator& alloc = Allocator() );: Constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The length of the string is determined by the first null character. *The behavior is undefined if [s, s + Traits::length(s)) is not a valid range (for example, if s is a null pointer)*. — Matthieu Brucher, May 30 '19 at 16:39
@mkrieger1 To be fair, a "crash" for something like this is not logically to be expected. — Lightness Races in Orbit, May 30 '19 at 16:44

score 7 · Accepted Answer · answered May 30 '19 at 16:42

Unfortunately (thanks C!) it is "possible" to construct a std::string from the integer 0, because it counts as a null pointer literal.

However, it's not really possible:

Constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The length of the string is determined by the first null character. The behavior is undefined if [s, s + Traits::length(s)) is not a valid range (for example, if s is a null pointer).

You got a crash because you tried to create a std::string from a null pointer.

At least with GCC the result of this contract violation is an [unhandled] exception with a descriptive name:

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid

Unfortunately, though, you can't rely on this, and you don't get any warnings during build.

Thanks, this is really helping. Indeed, no warning, just a crash. I though this had something to do with the map, but actually not. doing something like std::string s = 0 is enough to make the program crash. Thanks! — Korchkidu, May 30 '19 at 19:41

std::map crashes when doing m[0]

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