C++ default arguments: which statement is correct about redefinition and using function calls as defaults?

Introduction / Context:
Default arguments simplify function calls by supplying omitted values. Two frequent sources of confusion are whether the default can be a function call and whether a default can be specified again in another declaration. Clarifying these rules avoids ODR and ambiguity problems in headers and source files.

Given Data / Assumptions:

• We may declare the same function more than once across translation units.
• A default argument is substituted at the call site.
• Expressions used as defaults must be visible at the point of declaration.

Concept / Approach:

C++ does allow a default argument to be any valid expression (subject to scope), including a function call such as int f(int x = get_default()); as long as the callee is declared beforehand. However, C++ forbids redefining the default for the same parameter in another declaration; you should place the default once (typically in a header) and omit it from later declarations/definitions. Thus, statement A is false, statement B is false, and statement D is the correct choice: redefinition of a default parameter is not allowed.

Step-by-Step Solution:

Verification / Alternative check:

Write two declarations: one with = get_default() and a later one trying = 42. The compiler rejects the second. Ensure get_default is declared before the defaulted parameter; compilation then succeeds, proving A is false.

Why Other Options Are Wrong:

A: contradicts the language; defaults are ordinary expressions evaluated at the call site.

B: contradicts ODR-style rules for defaults.

C: a combination of two false statements.

Common Pitfalls:

• Putting the same default both in the header and the definition file.
• Using non-visible names in the default expression.

Final Answer:

C++ does not allow the redefinition of a default parameter.