EXP34-CPP. Do not access an object outside of its lifetime

Every object has a lifetime in which it can be used in a well-defined manner. The lifetime of an object begins when sufficient, properly-aligned storage has been obtained for it, and its initialization is complete. The lifetime of an object ends when a nontrivial destructor, if any, is called for the object, and the storage for the object has been reused or released. Use of an object, or a pointer to an object, outside of its lifetime frequently results in undefined behavior.

The C++ Standard, [basic.life], paragraph 5 [ISO/IEC 14882-2014], describes the lifetime rules for pointers:

Before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. For an object under construction or destruction, see 12.7. Otherwise, such a pointer refers to allocated storage, and using the pointer as if the pointer were of type void*, is well-defined. Indirection through such a pointer is permitted but the resulting lvalue may only be used in limited ways, as described below. The program has undefined behavior if:
    — the object will be or was of a class type with a non-trivial destructor and the pointer is used as the operand of a delete-expression,
    — the pointer is used to access a non-static data member or call a non-static member function of the object, or
    — the pointer is implicitly converted to a pointer to a virtual base class, or
    — the pointer is used as the operand of a static_cast, except when the conversion is to pointer to cv void, or to pointer to cv void and subsequently to pointer to either cv char or cv unsigned char, or
    — the pointer is used as the operand of a dynamic_cast.

Paragraph 6 similarly describes the lifetime rules for nonpointers:

Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any glvalue that refers to the original object may be used but only in limited ways. For an object under construction or destruction, see 12.7. Otherwise, such a glvalue refers to allocated storage, and using the properties of the glvalue that do not depend on its value is well-defined. The program has undefined behavior if:
    — an lvalue-to-rvalue conversion is applied to such a glvalue,
    — the glvalue is used to access a non-static data member or call a non-static member function of the object, or
    — the glvalue is bound to a reference to a virtual base class, or
    — the glvalue is used as the operand of a dynamic_cast or as the operand of typeid.

Do not use an object outside of its lifetime, except in the ways described above as being well-defined.

Noncompliant Code Example

In this noncompliant code example, a pointer to an object is used, prior to its lifetime starting, to call a non-static member function of the object, resulting in undefined behavior:

struct S {
  void mem_fn();
};
 
void f() {
  S *s;
  s->mem_fn();
}

Compliant Solution

In this compliant solution, storage is obtained for the pointer prior to calling S::mem_fn():

struct S {
  void mem_fn();
};
 
void f() {
  S *s = new S;
  s->mem_fn();
  delete s;
}

An improved compliant solution would not dynamically allocate memory directly, but would instead use an automatic local variable to obtain the storage and perform initialization. If a pointer was required, use of a smart pointer like std::unique_ptr would be a marked improvement. However, these suggested compliant solutions would distract from the lifetime demonstration of this compliant solution, and are consequently not shown.

Noncompliant Code Example

In this noncompliant code example, a pointer to an object is implicitly converted to a virtual base class after the object's lifetime has ended, resulting in undefined behavior:

struct B {};
 
struct D1 : virtual B {};
struct D2 : virtual B {};
 
struct S : D1, D2 {};
 
void f(const B *b) {}
 
void g() {
  S *s = new S;
  // Use s
  delete s;
 
  f(s);
}

Despite the fact that f() never makes use of the object, the fact that it is passed as an argument to f() is sufficient to trigger undefined behavior.

Compliant Solution

In this compliant solution, the lifetime of s is extended to cover the call to f():

struct B {};
 
struct D1 : virtual B {};
struct D2 : virtual B {};
 
struct S : D1, D2 {};
 
void f(const B *b) {}
 
void g() {
  S *s = new S;
  // Use s
  f(s);
 
  delete s;
}

Noncompliant Code Example

In this noncompliant code example, the address of a local variable is returned from f(). When the resulting pointer is passed to h(), the lvalue-to-rvalue conversion applied to i results in undefined behavior:

int *g() {
  int i = 12;
  return &i;
}
 
void h(int *i);
 
void f() {
  int *i = g();
  h(i);
}

Some compilers generate a diagnostic message when a pointer to an object with automatic storage duration is returned from a function, as in this example.

Compliant Solution

In this compliant solution, the local variable returned from g() has static storage duration instead of automatic storage duration, extending its lifetime sufficiently for use within f():

int *g() {
  static int i = 12;
  return &i;
}
 
void h(int *i);
 
void f() {
  int *i = g();
  h(i);
}

Noncompliant Code Example

In this noncompliant code example, the function g() returns a lambda which captures the automatic local variable i by reference. When that lambda is returned from the call, the reference it captured will refer to a variable whose lifetime has ended. As a result, when the lambda is executed in f(), the use of the dangling reference in the lambda results in undefined behavior. As a general rule of thumb, functions returning lambdas should not capture by reference. 

auto g() {
  int i = 12;
  return [&] {
    i = 100;
    return i;
  };
}

void f() {
  int i = g()();
}

Compliant Solution

In this compliant solution, the lambda does not capture i by reference, but instead captures it by copy. Consequently, the lambda contains an implicit data member named i whose lifetime is that of the lambda.

auto g() {
  int i = 12;
  return [=] () mutable {
    i = 100;
    return i;
  };
}

void f() {
  int i = g()();
}

Noncompliant Code Example

A std::initializer_list<> object is constructed from an initializer list as if the implementation allocated a temporary array and passed it to the std::initializer_list<> constructor. This temporary array has the same lifetime as other temporary objects, except that initializing a std::initializer_list<> object from the array extends the lifetime of the array exactly like binding a reference to a temporary [ISO/IEC 14882-2014]. In this noncompliant code example, a member variable of type std::initializer_list<int> is list initialized within the constructor's ctor-initializier. Under these circumstances, the conceptual temporary array's lifetime ends once the constructor exits, and so accessing any elements of the std::initializer_list<int> member variable results in undefined behavior.

#include <initializer_list>
#include <iostream>

class C {
  std::initializer_list<int> L;
  
public:
  C() : L{1, 2, 3} {}
  
  int first() const { return *L.begin(); }
};

void f() {
  C c;
  std::cout << c.first();
}

Compliant Solution

In this compliant solution, the std::initializer_list<int> member variable is replaced with a std::vector<int>, which copies the elements of the initializer list to the container instead of relying on a dangling reference to the temporary array.

#include <initializer_list>
#include <iostream>
#include <vector>
 
class C {
  std::vector<int> L;
  
public:
  C() : L{1, 2, 3} {}
  
  int first() const { return *L.begin(); }
};
 
void f() {
  C c;
  std::cout << c.first();
}

Risk Assessment

Referencing an object outside of its lifetime can result in an attacker being able 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