← Trexquant Interview Insights
First clarify that the question is about the implicitly-declared default constructor, not the default constructor in general. Then explain the conditions under which it is declared (when no user-declared constructors exist) and the conditions under which it is defined as deleted (e.g., due to non-default-constructible members or bases, or reference members without initializers). Finally, emphasize that a deleted default constructor still participates in overload resolution.
Pro tip: Mention that the implicitly-declared default constructor is defined as deleted if any non-static data member or base class has a non-trivial default constructor that is deleted or inaccessible, or if the class has a reference member without a default initializer. This shows deep understanding of C++ object model rules.
State that you are discussing the implicitly-declared default constructor, which is a special member function. Distinguish it from the default constructor that might be user-provided.
Explain that the compiler implicitly declares a default constructor if the class has no user-declared constructors. Note that this declaration is public and inline.
List the cases where the implicitly-declared default constructor is defined as deleted: a non-static data member or base class with a deleted or inaccessible default constructor, a reference member without a default initializer, a const member without a default initializer and without a user-provided default constructor, or a member of a type with a non-trivial default constructor that is not accessible.
Explain that a deleted default constructor still exists and participates in overload resolution, but using it is ill-formed. This means the class is not default-constructible.
Provide a concise example, such as a class with a reference member or a const member without initializer, to illustrate when the default constructor is deleted.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Clarify that the default constructor is only implicitly declared if there are no user-declared constructors, and that other special member functions (copy/move operations, destructor) do not suppress it. Then explain the nuanced interactions: a user-declared destructor or copy/move operation can affect move constructor generation, but not the default constructor. Finally, mention that if any constructor is user-declared, the default constructor is not generated, but it can be explicitly defaulted.
Pro tip: Emphasize that the rules are about *user-declared* constructors, not other special member functions, and that the default constructor is unique in this regard. Also note that 'user-declared' includes explicitly defaulted or deleted functions.
State that a default constructor is one callable with no arguments, and the compiler implicitly declares one only if the class has no user-declared constructors.
Separate constructors (default, copy, move) from assignment operators and destructor, noting that only constructors affect default constructor generation.
Clarify that any user-declared constructor (including copy/move constructors) suppresses the implicit default constructor, but user-declared copy/move assignment operators or destructor do not.
Mention that a user-declared destructor or copy operation inhibits implicit move constructor/assignment, but this does not impact default constructor generation.
Conclude that only user-declared constructors suppress the default constructor, and that you can use '= default' to explicitly request it if needed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by defining the implicit default constructor and the conditions under which the compiler generates it. Then explain that it is not generated (or is defined as deleted) when any member or base class lacks an accessible default constructor, or when a member is a reference or const-qualified without a default initializer. Finally, discuss the consequences: the class becomes non-default-constructible, and any attempt to default-construct it results in a compile-time error.
Pro tip: Mention that in C++11 and later, you can use `= default` to explicitly request the compiler-generated default constructor, but it will still be deleted if the class has non-default-constructible members. Also note that the rule applies recursively: if a base class's default constructor is deleted, the derived class's implicit default constructor is also deleted.
Explain that the compiler implicitly declares a default constructor if no user-declared constructors exist. It is used to default-initialize objects.
List the cases: a member or base class has no default constructor (or it is inaccessible/deleted), a member is a reference, or a member is const-qualified without a default member initializer.
Clarify that in such cases, the implicit default constructor is defined as deleted (since C++11) or not generated at all (pre-C++11). This makes the class non-default-constructible.
Mention that any attempt to default-construct the class will fail at compile time. To fix, provide a user-defined default constructor that initializes the problematic members, or use default member initializers.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Straightforward once you've answered the theory parts.
Start by clarifying that the implicit default constructor is only generated when no user-declared constructors exist, then give one clear example for each case: a class with no constructors, a class with any user-declared constructor, and a class with a member or base that makes the default constructor deleted. Keep the examples minimal and focus on the underlying rules rather than memorized trivia.
Pro tip: Mention that a deleted default constructor is still a declared constructor, so it participates in overload resolution and can cause a compile error if used—this shows you understand the difference between 'not declared' and 'declared as deleted'.
Explain that the compiler generates a default constructor only if the class has no user-declared constructors. Also note that it is defined as deleted if a non-static data member or base class has a deleted or inaccessible default constructor, or if it has a reference member without a default initializer.
Provide a simple class with no user-declared constructors, e.g., `struct A { int x; };`. The compiler implicitly declares `A::A()`.
Provide a class with any user-declared constructor, e.g., `struct B { B(int) {} };`. Because a constructor is user-declared, no implicit default constructor is generated.
Provide a class with a member that has a deleted default constructor, e.g., `struct C { std::mutex m; };` or `struct D { D() = delete; };`. The compiler declares the default constructor as deleted.
Emphasize that 'not declared' means the constructor does not exist in overload resolution, while 'deleted' means it exists but cannot be used. This affects error messages and SFINAE contexts.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.