Fact Check: Is "implicitly deleted because the default definition would be ill-formed"?

Analysis of the Claim: "Is 'implicitly deleted because the default definition would be ill-formed'?"

1. Introduction

The claim in question revolves around a technical assertion in C++ programming, specifically regarding the behavior of class constructors and assignment operators. The phrase "implicitly deleted because the default definition would be ill-formed" suggests that certain classes cannot utilize the default copy or assignment operations due to their member variables or structure. This assertion is often encountered in discussions about C++ class design and error messages generated by the compiler when certain conditions are met.

2. What We Know

In C++, when a class contains members that are either non-copyable or non-movable, the compiler may implicitly delete the default copy constructor or assignment operator. This is often indicated by error messages that state the default definition would be ill-formed. Here are some relevant points derived from various sources:

• Implicit Deletion: The C++ standard specifies that if a class has a member that is non-copyable (e.g., a reference type), the compiler will implicitly delete the copy constructor and assignment operator for that class. This is to prevent the creation of objects that cannot be copied correctly, leading to potential runtime errors such as double deletion or memory corruption 15.

• Common Scenarios: Several discussions on platforms like Stack Overflow highlight scenarios where users encounter this error. For instance, if a class contains a reference as a member, it cannot be copied, and thus the default copy behavior is deleted 236.

• Workarounds: Developers often suggest alternatives, such as using pointers instead of references, to avoid this issue. This allows for more flexible memory management and avoids the implicit deletion of the copy constructor 15.

3. Analysis

The sources consulted provide a range of insights into the claim, but they vary in terms of reliability and depth of explanation:

• Technical Forums: Many of the sources, such as Stack Overflow and GitHub issues, are user-generated content. While they can provide practical insights and real-world examples, they may also reflect individual experiences that are not universally applicable. For instance, the discussion on Stack Overflow 2 provides a specific coding scenario but lacks a formal citation of C++ standards, which could lend more authority to the claims made.

• Expert Opinions: Some posts, like those from established programming forums 8, present more structured discussions, often including multiple user responses that can help clarify the issue. However, these discussions can also be biased by the personal experiences of the contributors, which may not represent the broader programming community.

• Lack of Formal References: None of the sources provide direct citations from the C++ standard or authoritative textbooks on C++. This absence makes it difficult to fully validate the claims being made, as the foundational rules of C++ regarding class behavior are not explicitly referenced.

• Potential Conflicts of Interest: Some discussions may arise from users seeking help for specific coding problems, which could lead to anecdotal evidence rather than systematic analysis. This is particularly true in forums where users may be more focused on finding quick solutions rather than providing rigorous explanations.

4. Conclusion

Verdict: Mostly True

The claim that certain classes in C++ are "implicitly deleted because the default definition would be ill-formed" is largely supported by the evidence reviewed. The C++ standard indeed stipulates that if a class contains non-copyable members, the compiler will implicitly delete the default copy constructor and assignment operator to prevent ill-formed operations. This behavior is frequently encountered in practical programming scenarios, as illustrated by user discussions on platforms like Stack Overflow.

However, the conclusion is nuanced by the limitations of the evidence. Most of the sources consulted are anecdotal and lack formal citations from the C++ standard or authoritative texts, which raises questions about their reliability. Additionally, the variability in user experiences and the absence of comprehensive formal references mean that while the claim holds true in many cases, it may not apply universally to all programming contexts.

Readers are encouraged to critically evaluate the information presented and consult authoritative sources for a deeper understanding of C++ class behavior and the implications of implicit deletion.

5. Sources

1. "is implicitly deleted because the default definition would be ill-formed:" - PJRC Forum. Link
2. "c++ - is implicitly deleted because the default definition would be ill ..." - Stack Overflow. Link
3. "person()' is implicitly deleted because the default definition ..." - Stack Overflow. Link
4. "C++ Atomics: operator= is implicitly deleted because the ..." - GitHub. Link
5. "c++ - error: implicitly deleted because the default definition would be ..." - Stack Overflow. Link
6. "c++ - Class implicitly deleted because the default definition ..." - Stack Overflow. Link
7. "Installation fails compiling C++ code 'implicitly deleted ..." - GitHub. Link
8. "Assignment operator implicitly deleted - C++ Forum." Link
9. "Node() is implicitly deleted because the default definition ..." - GitHub. Link
10. "How resolve problem of use of deleted fu - C++ Forum." Link

In summary, while the claim regarding implicit deletion in C++ is supported by various user discussions and examples, the lack of formal references and the anecdotal nature of many sources necessitate a careful evaluation of the information presented. Further authoritative sources, such as the C++ standard documentation or recognized programming textbooks, would be beneficial for a more comprehensive understanding.