CERT C++: STR52-CPP

Use valid references, pointers, and iterators to reference elements of a basic_string

Since R2022b

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Description

Rule Definition

Use valid references, pointers, and iterators to reference elements of a basic_string.¹

Polyspace Implementation

The rule checker checks for the issue Use of invalid string iterator.

Examples

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Use of Invalid String Iterator

Issue

This issue occurs when you use an iterator, pointer, or reference to an element of a basic_string object that has been invalidated by actions such as insertion or erasure. This issue is a specific instance of the issue Use of invalid iterator, which causes violations of CERT C++: CTR51-CPP. Consider this code:

	std::string str = "Basic String";
	auto it_begin = str.begin();
	//...
	str.replace(0,5,"Advanced");
	str.insert(it_begin,'A'); // Violation

Here, the iterator it_begin is invalidated after the call to string::replace. Using this invalid iterator in string::insert causes a violation of this rule.

Functions that invalidate an STL iterator also invalidate basic_string iterators. In addition, these functions invalidate a basic_string iterator:

std::swap()
Operator>>(std::basic_istream&, std::string&)
std::getline()

Risk

Dereferencing the invalid iterator might produce unexpected results. If the invalidated iterator becomes a dangling pointer or an uninitialized pointer, dereferencing it might cause sudden termination of the program.

Fix

The C++ standard defines which operations invalidate the iterators of the standard library containers. These operations also invalidate a string operator. See Containers library. When you perform actions that might invalidate an iterator, revalidate the iterator. For instance, recalculate the iterator after an insertion or erasure.

Example — Avoid Using Invalid Iterators

#include <string>

  
void foo(const std::string& raw, std::string& processed) {
   
  // Process the raw string and store in processed
  auto loc = processed.begin();
  for (auto i = raw.begin(); i != raw.end(); ++i, ++loc) {
    processed.insert(loc, *i != ';' ? *i : ' ');//Noncompliant
  }
}

In this example, the first processed.insert operation in the for loop invalidates the iterator i. The subsequent use of the iterator i might result in undefined behavior. Polyspace^® reports a violation of this rule.

Correction — Update the Iterator

When you use iterators after an insertion or erasure, update the iterator to avoid undefined behavior. For instance, in this code, the iterator i is updated after each insertion so that the iterator remains valid.

#include <string>

  
void foo(const std::string& raw, std::string& processed) {
   
  // Process the raw string and store in processed
  auto loc = processed.begin();
  for (auto i = raw.begin(); i != raw.end(); ++i, ++loc) {
    loc = processed.insert(loc, *i != ';' ? *i : ' ');//Compliant
  }
}

Check Information

Group: 05. Characters and Strings (STR)

Version History

Introduced in R2022b

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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