Can shrinking a std::string throw an exception?

Raymond Chen

Raymond

I had a C++ string that I wanted to truncate. Say, something like this:

void remove_extension(std::string& s)
{
 auto pos = s.rfind('.');
 if (pos != std::string::npos) {
  s.resize(pos);
 }
}

The question is whether this function can throw an exception. Can the call to resize throw an exception when used to make a string smaller?

And the answer appears to be yes, at least in C++17.

The specification of the resize(n) method in C++17 says that in the case where n <= size(), “the function replaces the string designated by *this with a string of length n whose elements are a copy of the initial elements of the original string designated by *this.”

In other words, the resize(n) method, when shrinking a string (or leaving the size unchanged), behaves as if a new string is created, which replaces the current string. And creating a new string may throw bad_alloc.

Of course, implementations may use the as-if rule and resize the string in place, but the standard does not require them to do so.

But wait, all is not lost. Because another way to shrink a string is to use the erase(n) method.

  • [basic.string]: basic_string is a contiguous container.
  • [container.requirements.general] (11): Unless otherwise specified…, all container types defined in this Clause meet the following additional requirements:
  • [container.requirements.general] (11.3): No erase()… function throws an exception.
  • [string.erase]: Throws: length_error if n > max_size().

There are a few things referenced in the “…” portion of [container.requirements.general] (11), but they do not apply to basic_string.

Hooray, we can use the erase method to shrink the string and avoid an exception.

void remove_extension(std::string& s)
{
 auto pos = s.rfind('.');
 if (pos != std::string::npos) {
  s.erase(pos);
 }
}

Bonus chatter: It appears that the issue of resize() throwing an exception when trimming was brought up¹ by Stephan T. Lavavej and fixed by Tim Song in P1148R0: Starting in C++20, if you call the resize() method to shrink the string (or keep it the same size), the behavior is defined in terms of erasure and therefore does not throw an exception.

¹ I could have written “raised” but I didn’t.²

² Except that I just did.

 

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