When writing C++, understanding the exact sequence of events during object creation is crucial for both performance and correctness. A common point of confusion arises when comparing a built-in data member (like an int) initialized using a member initializer list versus assigned inside the constructor body.

Does a primitive variable briefly hold a "garbage" value before being initialized? How does the C++ object model handle memory allocation and object lifetimes? Let's break down the exact timeline for both approaches.

Case 1: Assignment Inside the Constructor Body

Consider the following class definition:

class Account {
    int balance;
public:
    Account(int x) {
        balance = x; // Assignment inside constructor body
    }
};

In this scenario, the exact timeline according to the C++ standard is as follows:

  1. Storage Allocation: Memory for the Account object (including space for balance) is allocated on the stack, heap, or static storage.
  2. Default Initialization: Before the constructor body runs, all data members must be initialized. Because balance is not in the initializer list, it undergoes default-initialization. For built-in types like int, default-initialization does nothing—meaning balance contains an indeterminate (garbage) value.
  3. Lifetime Begins: The lifetime of the balance object officially begins once its default-initialization completes (even though its value is indeterminate).
  4. Constructor Body Execution: The constructor body executes.
  5. Assignment: The statement balance = x; runs, assigning the value x to the already existing int object, replacing the indeterminate value.

Case 2: Initialization via Member Initializer List

Now consider using a member initializer list:

class Account {
    int balance;
public:
    Account(int x)
        : balance(x) // Member initializer list
    {
    }
};

Here is the exact step-by-step timeline defined by the C++ Object Model:

  1. Storage Allocation: Memory for the Account object is allocated. At this exact moment, raw memory exists, but the balance subobject's lifetime has not started yet.
  2. Direct Initialization: The member initializer list evaluates x and performs direct-initialization on balance. The value x is written directly into the memory location reserved for balance as its initial value.
  3. Lifetime Begins: The lifetime of balance begins at the moment its initialization with x completes.
  4. Constructor Body Execution: The (empty) constructor body begins executing.

Does balance Ever Hold a "Garbage" Value in an Initializer List?

No. When using a member initializer list, balance never holds an indeterminate or garbage value from the C++ abstract machine's perspective.

Although the raw physical memory hardware previously contained whatever bits were left over, the C++ object model does not consider the object initialized until the initializer expression is evaluated. The first value balance ever holds as a valid C++ object is x.

Key Differences Summary

  • Constructor Body Assignment: Default-initialization (indeterminate value) → Lifetime starts → Assignment (value overwritten).
  • Initializer List: Direct-initialization (initialized directly with x) → Lifetime starts.

Why This Distinction Matters

For primitive types like int, modern optimizing compilers (like GCC, Clang, or MSVC) will often produce identical machine code for both cases when optimizations are enabled. However, the distinction is vital for two reasons:

  • Class Types & Non-PODs: If balance were a complex class type (e.g., std::string), assignment in the constructor body would call the default constructor first, followed by the copy/move assignment operator. Using the initializer list invokes only the direct constructor, avoiding redundant operations.
  • Const and Reference Members: Members declared as const or reference types (int&) must be initialized in the member initializer list because they cannot be assigned to after creation.