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The Library Working Group, responsible for the wording of the Standard Library section of the C++ Standard, has an unresolved issue on the definition of user-defined type. Although the Library Working Group has no official stance on the definition [INCITS 2014], we define it to be any class, struct, union, or enum that is not defined within namespace std or a namespace contained within namespace std. Effectively, it is a user-provided type instead of a standard library–provided type.

Noncompliant Code Example

In this noncompliant code example, the declaration of x is added to the namespace std, resulting in undefined behavior:

namespace std {
int x;
}

Compliant Solution

This compliant solution assumes the intention of the programmer was to place the declaration of x into a namespace to prevent collisions with other global identifiers. Instead of placing the declaration into the namespace std, the declaration is placed into a namespace without a reserved name.

namespace nonstd {
int x;
}

Noncompliant Code Example

In this noncompliant code example, a template specialization of std::plus is added to the namespace std in an attempt to allow std::plus to concatenate a std::string and MyString object. However, because the template specialization is of a standard library–provided type (std::string), this code results in undefined behavior.

#include <functional>
#include <iostream>
#include <string>

class MyString {
  std::string data;
  
public:
  MyString(const std::string &data) : data(data) {}
  
  const std::string &get_data() const { return data; }
};

namespace std {
template <>
struct plus<string> : binary_function<string, MyString, string> {
  string operator()(const string &lhs, const MyString &rhs) const {
    return lhs + rhs.get_data();
  }
};
}

void f() {
  std::string s1("My String");
  MyString s2(" + Your String");
  std::plus<std::string> p;
  
  std::cout << p(s1, s2) << std::endl;
}

Compliant Solution

The interface for std::plus requires that both arguments to the function call operator and the return type are of the same type. Because the attempted specialization in the noncompliant code example results in undefined behavior, this compliant solution defines a new std::binary_function derivative that can add a std::string to a MyString object without requiring modification of the namespace std.

#include <functional>
#include <iostream>
#include <string>

class MyString {
  std::string data;
  
public:
  MyString(const std::string &data) : data(data) {}
  
  const std::string &get_data() const { return data; }
};

struct my_plus : std::binary_function<std::string, MyString, std::string> {
  std::string operator()(const std::string &lhs, const MyString &rhs) const {
    return lhs + rhs.get_data();
  }
};

void f() {
  std::string s1("My String");
  MyString s2(" + Your String");
  my_plus p;
  
  std::cout << p(s1, s2) << std::endl;
}

Compliant Solution

In this compliant solution, a specialization of std::plus is added to the std namespace, but the specialization depends on a user-defined type and meets the Standard Template Library requirements for the original template, and so it complies with this rule. However, because MyString can be constructed from std::string, this compliant solution involves invoking a converting constructor whereas the previous compliant solution does not.

#include <functional>
#include <iostream>
#include <string>
 
class MyString {
  std::string data;
   
public:
  MyString(const std::string &data) : data(data) {}
   
  const std::string &get_data() const { return data; }
};
 
namespace std {
template <>
struct plus<MyString> {
  MyString operator()(const MyString &lhs, const MyString &rhs) const {
    return lhs.get_data() + rhs.get_data();
  }
};
}
 
void f() {
  std::string s1("My String");
  MyString s2(" + Your String");
  std::plus<MyString> p;
   
  std::cout << p(s1, s2).get_data() << std::endl;
}

Risk Assessment

Altering the standard namespace can cause undefined behavior in the C++ standard library.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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