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The index operators:

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std::string index operators const_reference

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operator[](size_type

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)

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const

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 and reference

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operator[](size_type

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return ) return the character stored at the specified position if , pos < size(). If When pos =>= size(), the const
version returns the terminating null character type value. Otherwise, the behavior is undefined.In any case, the behavior of the index operators is a reference to an object of type charT with value charT() is returned. The index operators are unchecked (no exceptions are thrown for range errors).

Non-Compliant Code Example

The behavior of this non-compliant example is undefined because the size() of bs is 8 but the index used to reference bs ranges from 0 through 99.

string bs("01234567");
for (int i=0; i<100; i++) {
  bs[i] = '\0';
}

This program does not typically raise an exception and is likely to crash.

Compliant Solution

This compliant solution uses the {{basic_string at()}} method which behaves in a similar fashion to the index {{operator\[\]}} but throws an {{out_of_range}} exception if {{pos >= size()}}.

, and attempting to modify the resulting out-of-range object results in undefined behavior.

Similarly, the std::string::back() and std::string::front() functions are unchecked as they are defined to call through to the appropriate operator[]() without throwing.

Do not pass an out-of-range value as an argument to std::string::operator[](). Similarly, do not call std::string::back() or std::string::front() on an empty string. This rule is a specific instance of CTR50-CPP. Guarantee that container indices and iterators are within the valid range.

Noncompliant Code Example

In this noncompliant code example, the value returned by the call to get_index() may be greater than the number of elements stored in the string, resulting in undefined behavior.

#include <string>
 
extern std::size_t get_index();
 
void f() {
  std::string s("01234567");
  s[get_index()] = '1';
}

Compliant Solution (try/catch)

This compliant solution uses the std::basic_string::at() function, which behaves in a similar fashion to the index operator[] but throws a std::out_of_range exception if pos >= size().

#include <stdexcept>
#include <string>
extern std::size_t get_index();

void f() {
  std::string s

string bs("01234567");
  try {
  for (int i=0; i<100; i++) {
    bs.at(i) = '\0';
  }
}
catch (...) {
  cerr << "Index out of range" << endl;
}

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  s.at(get_index()) = '1';
  } catch (std::out_of_range &) {
    // Handle error
  }
}

Compliant Solution (Range Check)

This compliant solution checks that the value returned by get_index() is within a valid range before calling operator[]().

#include <string>

extern std::size_t get_index();

void f() {
  std::string s("01234567");
  std::size_t i = get_index();
  if (i < s.length()) {
    s[i] = '1';
  } else {
    // Handle error
  }
}

Noncompliant Code Example

This noncompliant code example attempts to replace the initial character in the string with a capitalized equivalent. However, if the given string is empty, the behavior is undefined.

#include <string>
#include <locale>

void capitalize(std::string &s) {
  std::locale loc;
  s.front() = std::use_facet<std::ctype<char>>(loc).toupper(s.front());
}

Compliant Solution

In this compliant solution, the call to std::string::front() is made only if the string is not empty.

#include <string>
#include <locale>

void capitalize(std::string &s) {
  if (s.empty()) {
    return;
  }

  std::locale loc;
  s.front() = std::use_facet<std::ctype<char>>(loc).toupper(s.front());
}

Risk Assessment

Unchecked element access can lead to out-of-bounds bound reads and writes and write-anywhere exploits. These exploits can, in turn, lead to the execution of arbitrary code with the permissions of the vulnerable process.

Automated Detection

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Related Vulnerabilities

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

Related Guidelines

Bibliography

References

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