{"id":103349,"date":"2020-01-20T07:00:00","date_gmt":"2020-01-20T15:00:00","guid":{"rendered":"http:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103349"},"modified":"2020-01-19T16:47:04","modified_gmt":"2020-01-20T00:47:04","slug":"20200120-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200120-00\/?p=103349","title":{"rendered":"Can shrinking a std::string<\/CODE> throw an exception?"},"content":{"rendered":"

I had a C++ string that I wanted to truncate. Say, something like this:<\/p>\n

void remove_extension(std::string& s)\r\n{\r\n auto pos = s.rfind('.');\r\n if (pos != std::string::npos) {\r\n  s.resize(pos);\r\n }\r\n}\r\n<\/pre>\n
The question is whether this function can throw an exception. Can the call to resize<\/code> throw an exception when used to make a string smaller?<\/p>\n
And the answer appears to be yes<\/i>, at least in C++17.<\/p>\n
The specification of the resize(n)<\/code> method in C++17 says that in the case where n <= size()<\/code>, “the function replaces the string designated by *this<\/code> with a string of length n<\/code> whose elements are a copy of the initial elements of the original string designated by *this<\/code>.”<\/p>\n
In other words, the resize(n)<\/code> 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<\/code>.<\/p>\n
Of course, implementations may use the as-if<\/i> rule and resize the string in place, but the standard does not require them to do so.<\/p>\n
But wait, all is not lost. Because another way to shrink a string is to use the erase(n)<\/code> method.<\/p>\n