STR52-CPP. Use valid references, pointers, and iterators to reference elements of a basic_string

Since std::basic_string is a container of characters, this rule is a specific instance of CTR51-CPP. Use valid references, pointers, and iterators to reference elements of a container. As a container, it supports iterators just like other containers in the Standard Template Library. However, the std::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 an invalidated reference, pointer, or iterator because doing so results in undefined behavior.

Noncompliant Code Example

This noncompliant code example copies input into a std::string, replacing semicolon (;) 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;

  // Copy input into email converting ";" to " "
  std::string::iterator loc = email.begin();
  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;

  // Copy input into email converting ";" to " "
  std::string::iterator loc = email.begin();
  for (auto i = input.begin(), e = input.end(); i != e; ++i, ++loc) {
    loc = email.insert(loc, *i != ';' ? *i : ' ');
  }
}

Compliant Solution (std::replace())

This compliant solution uses a standard algorithm to perform the replacement. When possible, using a generic algorithm is preferable to inventing your own solution.

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

Noncompliant Code Example

In this noncompliant code example, data is invalidated after the call to replace(), and so its use in g() is undefined behavior:

#include <iostream>
#include <string>
 
extern void g(const char *);
 
void f(std::string &exampleString) {
  const char *data = exampleString.data();
  // ...
  exampleString.replace(0, 2, "bb");
  // ...
  g(data);
}

Compliant Solution

In this compliant solution, the pointer to exampleString's internal buffer is not generated until after the modification from replace() has completed:

#include <iostream>
#include <string>

extern void g(const char *);

void f(std::string &exampleString) {
  // ...
  exampleString.replace(0, 2, "bb");
  // ...
  g(exampleString.data());
}

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