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To default-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type, the default constructor for T is called (and the initialization is ill-formed if T has no default constructor or overload resolution results in an ambiguity or in a function that is deleted or inaccessible from the context of the initialization);
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.
If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

As a result, objects of type T with automatic or dynamic storage duration must be explicitly initialized prior to having their value read as part of an expression, except if T is a class type or array thereof, or an unsigned narrow character type. If T is an unsigned narrow character type, it may be used to initialize an object of unsigned narrow character type, which results in both objects having an indeterminate value. This can be used to implement copy operations like std::memcpy() without triggering undefined behavior.

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Objects of static or thread storage duration are zero-initialized before any other initialization takes place [ISO/IEC 14882-2014], and need not be explicitly initialized prior to having their value read.

Noncompliant Code Example

In this noncompliant code example, an uninitialized local variable is evaluated as part of an expression to print its value, resulting in undefined behavior:

#include <iostream>
 
void f() {
  int i;
  std::cout << i;
}

Compliant Solution

In this compliant solution, the object is initialized prior to printing its value:

#include <iostream>
 
void f() {
  int i = 0;
  std::cout << i;
}

Noncompliant Code Example

In this noncompliant code example, an int * object is allocated by a new-expression, but the memory is points to is not initialized. The object's pointer value, and the value it points to are printed to the standard output stream. Printing the pointer value is well-defined, but attempting to print the value pointed to yields an indeterminate value, resulting in undefined behavior.

#include <iostream>
 
void f() {
  int *i = new int;
  std::cout << i << ", " << *i;
}

Compliant Solution

In this compliant solution, the memory is properly initialized prior to printing its value:

#include <iostream>
 
void f() {
  int *i = new int;
  *i = 12;
  std::cout << i << ", " << *i;
}

Noncompliant Code Example

In this noncompliant code example, the class member variable C is not initialized by the default constructor. Despite the local variable o being default-initialized, the use of C within the call to S::f() results in the use of an indeterminate value.

class S {
  int C;
 
public:
  int f(int I) const { return I + C; }
};
 
void f() {
  S o;
  int i = o.f(10);
}

Compliant Solution

In this compliant solution, S is given a default constructor that initializes the class member variable C:

class S {
  int C;
 
public:
  S() : C(0) {}
  int f(int I) const { return I + C; }
};
 
void f() {
  S o;
  int i = o.f(10);
}

Risk Assessment

Reading uninitialized variables is undefined behavior and can result in unexpected program behavior. In some cases, these security flaws may allow the execution of arbitrary code.

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Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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