debug assertion "string subscript out of range" is incorrect for [size()]

debug assertion "string subscript out of range" is incorrect for [size()] by hhoppe

Closed

as Fixed

Type:	Bug
ID:	733729
Opened:	3/27/2012 2:47:36 PM
Access Restriction:	Public
Moderator Decision:	Sent to Engineering Team for consideration

Workaround(s)

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User(s) can reproduce this bug

Given an empty string, operator[](0) shows a debug assertion error "string subscript out of range".

The C++ standard (both before and after C++11) specify that operator[](size()) should return a reference to the value CharT() (the null character):

See http://en.cppreference.com/w/cpp/string/basic_string/operator_at :

If pos==size(),
The const version returns a reference to the character with value CharT() (the null character). (until C++11)
Both versions returns a reference to the character with value CharT() (the null character). Modifying the null character through non-const reference results in undefined behavior. (since C++11)

In other words, no assertion error should be reported when pos==size() .

Details (expand)

Visual Studio 2010 SP1

Simple program:

string s;
fprintf(stderr,"%d\n",int(s[0]));

English

Windows 7

English

Debug assertion error window pops up

No assertion error.

0 attachments

Hi,

Thanks for reporting this bug. We've already fixed it, and you can observe the fix in the VC11 Beta.

This actually changed between C++03 and C++11. C++03 21.3.4 [lib.string.access]/1 said:

"const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
Returns: If pos < size(), returns data()[pos]. Otherwise, if pos == size(), the const version returns charT(). Otherwise, the behavior is undefined."

VC10's debug assertions strictly enforced these rules. In particular, they allowed s[s.size()] for const strings, but prohibited it for modifiable strings.

Now, C++11 21.4.5 [string.access]/1-2 says:

"const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
Requires: pos <= size().
Returns: *(begin() + pos) if pos < size(), otherwise a reference to an object of type T with value charT(); the referenced value shall not be modified."

Therefore, we changed VC11 to allow s[s.size()] for both const and modifiable strings.

If you need to work around this in VC10, you can say: static_cast<const string&>(s)[s.size()]

If you have any further questions, feel free to E-mail me at stl@microsoft.com .

Stephan T. Lavavej
Visual C++ Libraries Developer