STR38-CPP. Use valid references, pointers, and iterators to reference elements of a basic_string

The basic_string template class has unusual invalidation semantics. According to the C++ Standard, [string.require], paragraph 5 [ISO/IEC 14882-2014]:

References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated by the following uses of that basic_string object:

•  as an argument to any standard library function taking a reference to non-const basic_string as an argument.
• Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.

Examples of standard library functions taking a reference to non-const std::basic_string are: std::swap(), ::operator>>(basic_istream &, string &), and std::getline().

Do not use a reference, pointer, or iterator that has been invalidated, as that results in undefined behavior. This rule is a specific instance of CTR32-CPP. Use valid references, pointers, and iterators to reference elements of a container.

Noncompliant Code Example

This noncompliant code example copies input into a std::string, replacing ';' characters with spaces. This example is noncompliant because the iterator loc is invalidated after the first call to insert(). The behavior of subsequent calls to insert() is undefined.

#include <string>
 
void f(const std::string &input) {
  std::string email;
  std::string::iterator loc = email.begin();

  // copy input into email converting ";" to " "
  for (auto I = input.begin(), E = input.end(); I != E; ++I, ++loc) {
    email.insert(loc, *I != ';' ? *I : ' ');
  }
}

Compliant Solution (std::string::insert())

In this compliant solution, the value of the iterator loc is updated as a result of each call to insert() so that the invalidated iterator is never accessed. The updated iterator is then incremented at the end of the loop.

#include <string>
 
void f(const std::string &input) {
  std::string email;
  std::string::iterator loc = email.begin();

  // copy input into email converting ";" to " "
  for (auto I = input.begin(), E = input.end(); I != E; ++I, ++loc) {
    loc = email.insert(loc, *I != ';' ? *I : ' ');
  }
}

Compliant Solution (std::replace())

In this compliant solution, the manual loop is replaced with a standard algorithm that performs the replacement. Using generic algorithms is generally preferable to inventing your own solution when possible.

#include <algorithm>
#include <string>
 
void f(const std::string &input) {
  std::string email{input};
  std::replace(email.begin(), email.end(), ';', ' ');
}

Risk Assessment

Using an invalid reference, pointer or iterator to a string object could allow an attacker to run arbitrary code.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

      007. Characters and Strings (STR)      STR39-CPP. Range check element access