OOP37-CPP. Write constructor member initializers in the canonical order

The member initializer list for a class constructor allows members to be initialized to specified values as well as base class constructors to be called with specific arguments. However, the order in which initialization occurs is fixed and does not depend on the order written in the member initializer list. According to the C++ Standard, [class.base.init], paragraph 11 [ISO/IEC 14882-2014]:

In a non-delegating constructor, initialization proceeds in the following order:
— First, and only for the constructor of the most derived class, virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
— Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
— Finally, the compound-statement of the constructor body is executed.
[Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. —end note]

Consequently, the order in which member initializers appear in the member initializer list is irrelevant. The order in which members are initialized, including base class initialization, is determined by the declaration order of the class member variables, or the base class specifier list. Writing member initializers in an order other than the canonical one can result in undefined behavior, such as reading uninitialized memory.

Always write member initializers in a constructor in the canonical order: first, direct base classes in the order they appear in the base-specifier-list for the class, then nonstatic data members in the order they are declared in the class definition.

Noncompliant Code Example

In this noncompliant code example, the member initializer list for C::C() attempts to initialize SomeVal first, and then initialize DependsOnSomeVal to a value dependent on SomeVal. However, since the declaration order of the member variables does not match the member initializer order, attempting to read the value of SomeVal results in an unspecified value being stored into DependsOnSomeVal.

class C {
  int DependsOnSomeVal;
  int SomeVal;
 
public:
  C(int val) : SomeVal(val), DependsOnSomeVal(SomeVal + 1) {}
};

Compliant Solution

This compliant solution changes the declaration order of the class member variables so that the dependency can be ordered properly in the constructor's member initializer list:

class C {
  int SomeVal;
  int DependsOnSomeVal;
 
public:
  C(int val) : SomeVal(val), DependsOnSomeVal(SomeVal + 1) {}
};

Note that it is reasonable for initializers to depend on previously initialized values.

Noncompliant Code Example

In this noncompliant code example, the derived class, D, attempts to initialize the base class, B1, with a value obtained from the base class, B2. However, because B1 is initialized before B2 due to the declaration order in the base class specifier list, the results are undefined.

class B1 {
  int Val;
 
public:
  B1(int V) : Val(V) {}
};

class B2 {
  int OtherVal;
 
public:
  B2(int V) : OtherVal(V) {}
  int getOtherVal() const { return OtherVal; }
};

class D : B1, B2 {
  D(int A) : B2(A), B1(getOtherVal()) {}
};

Compliant Solution

This compliant solution initializes both base classes using the same value from the constructor's parameter list, instead of relying on the initialization order of the base classes:

class B1 {
  int Val;
 
public:
  B1(int V) : Val(V) {}
};

class B2 {
  int OtherVal;
 
public:
  B2(int V) : OtherVal(V) {}
};

class D : B1, B2 {
  D(int A) : B1(A), B2(A) {}
};

Risk Assessment

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography