C++ object lifecycle: when is a constructor invoked for a class object?

Introduction / Context:
Constructors initialize objects to a valid state. Knowing precisely when they run helps you reason about resource acquisition and initialization order, which is critical for RAII patterns in C++ (Resource Acquisition Is Initialization).

Given Data / Assumptions:

• We discuss automatic (stack), static, and dynamic objects (via new).
• “Declaration of an object” is taken to mean its definition/creation.
• We ignore forward declarations without definitions.

Concept / Approach:

Constructors are invoked when an object is created: for automatic objects, upon entering their scope at the definition; for static-duration objects, before main (or on first use for function-local statics with initialization guards); for dynamic objects, immediately when new allocates and constructs. Mere use of an already-constructed object does not call the constructor again. Defining a class type itself does not trigger constructors; only creating instances does.

Step-by-Step Solution:

Verification / Alternative check:

Instrument a constructor with logging and observe when messages print: at creation, not at arbitrary “use” points nor at type declaration time.

Why Other Options Are Wrong:

“time of use of an object”: use does not trigger construction.

“time of declaration/use of a class”: classes are types; only objects have constructors called.

Common Pitfalls:

• Confusing declaration with forward declaration; only definitions create objects.
• Assuming copy elision rules always call or skip constructors—modern C++ may elide but construction still conceptually occurs.

Final Answer:

A constructor is called at the time of declaration of an object.