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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 unchecked (no exceptiosn are thrown).

Non-Compliant Example

The behavior of this non-compliant example is undefined becuase 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] = 'X';
}	  

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

Compliant Solution

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

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

void f() {
  std::string bss("01234567");
  try {
    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  for (int i = 0get_index();
  if (i < 100; i++s.length()) {
    bs.at(i)s[i] = '\01';
  } else {
}
catch (...) {
  cerr << "Index out of range" << endl;
}

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    // 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 read 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

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References

Section 21.3.4 basic_string element access