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C++ Coding Standard

Rule:  STR51-CPPChecked automatically with code checker

Synopsis:Do not attempt to create a std::string from a null pointer
Severity Level:1


The  std::basic_string type uses the traits design pattern to handle implementation details of the various string types, resulting in a series of string-like classes with a common, underlying implementation. Specifically, the  std::basic_string class is paired with  std::char_traits to create the  std::stringstd::wstringstd::u16string , and  std::u32string classes. The  std::char_traits class is explicitly specialized to provide policy-based implementation details to the  std::basic_string type. One such implementation detail is the  std::char_traits::length() function, which is frequently used to determine the number of characters in a null-terminated string. According to the C++ Standard, [char.traits.require], Table 62 [ISO/IEC 14882-2014], passing a null pointer to this function is undefined behavior because it would result in dereferencing a null pointer.

The following  std::basic_string member functions result in a call to  std::char_traits::length() :

  • basic_string::basic_string(const charT *, const Allocator &)
  • basic_string &basic_string::append(const charT *) 
  • basic_string &basic_string::assign(const charT *)
  • basic_string &basic_string::insert(size_type, const charT *)
  • basic_string &basic_string::replace(size_type, size_type, const charT *)
  • basic_string &basic_string::replace(const_iterator, const_iterator, const charT *)
  • size_type basic_string::find(const charT *, size_type)
  • size_type basic_string::rfind(const charT *, size_type)
  • size_type basic_string::find_first_of(const charT *, size_type)
  • size_type basic_string::find_last_of(const charT *, size_type)
  • size_type basic_string::find_first_not_of(const charT *, size_type)
  • size_type basic_string::find_last_not_of(const charT *, size_type)
  • int basic_string::compare(const charT *)
  • int basic_string::compare(size_type, size_type, const charT *)
  • basic_string &basic_string::operator=(const charT *)
  • basic_string &basic_string::operator+=(const charT *)

The following  std::basic_string nonmember functions result in a call to to  std::char_traits::length() :

  • basic_string operator+(const charT *, const basic_string&)
  • basic_string operator+(const charT *, basic_string &&)
  • basic_string operator+(const basic_string &, const charT *)
  • basic_string operator+(basic_string &&, const charT *)
  • bool operator==(const charT *, const basic_string &)
  • bool operator==(const basic_string &, const charT *)
  • bool operator!=(const charT *, const basic_string &)
  • bool operator!=(const basic_string &, const charT *)
  • bool operator<(const charT *, const basic_string &)
  • bool operator<(const basic_string &, const charT *)
  • bool operator>(const charT *, const basic_string &)
  • bool operator>(const basic_string &, const charT *)
  • bool operator<=(const charT *, const basic_string &)
  • bool operator<=(const basic_string &, const charT *)
  • bool operator>=(const charT *, const basic_string &)
  • bool operator>=(const basic_string &, const charT *)

Do not call any of the preceding functions with a null pointer as the  const charT * argument.

This rule is a specific instance of EXP34-C. Do not dereference null pointers.

Implementation Details

Some standard library vendors, such as libstdc++, throw a  std::logic_error when a null pointer is used in the above function calls, though not when calling std::char_traits::length() . However, std::logic_error  is not a requirement of the C++ Standard, and some vendors (e.g., libc++ and the Microsoft Visual Studio STL) do not implement this behavior. For portability, you should not rely on this behavior.

Noncompliant Code Example

In this noncompliant code example, a  std::string object is created from the results of a call to  std::getenv() . However, because std::getenv() returns a null pointer on failure, this code can lead to undefined behavior when the environment variable does not exist (or some other error occurs).

#include <cstdlib>
#include <string>
void f() {
   std::string tmp(std:: getenv ( "TMP" ));
   if (!tmp.empty()) {
     // ...

Compliant Solution

In this compliant solution, the results from the call to  std::getenv() are checked for null before the  std::string object is constructed.

#include <cstdlib>
#include <string>
void f() {
   const char *tmpPtrVal = std:: getenv ( "TMP" );
   std::string tmp(tmpPtrVal ? tmpPtrVal : "" );
   if (!tmp.empty()) {
     // ...