c++ - copy and assignment operator with constant size arrays - Stack Overflow

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Suppose this class in C++

class Message {
  char msg[64];
};

Why are operator= and default copy already correct

Message & operator=(const Message &o) {
  if (this != &o)
    this->msg = o.msg;
  }
  return *this;
}

In assignment should be wrong?

Isn't it the default semantics to use assignment operator field per field to implement default assignment and copy construction

Suppose this class in C++

class Message {
  char msg[64];
};

Why are operator= and default copy already correct

Message & operator=(const Message &o) {
  if (this != &o)
    this->msg = o.msg;
  }
  return *this;
}

In assignment should be wrong?

Isn't it the default semantics to use assignment operator field per field to implement default assignment and copy construction

• c++
• language-lawyer

Share Improve this question Follow edited Jan 17 at 17:18 Yann TM asked Jan 17 at 15:04 Yann TMYann TM 2,0751515 silver badges2323 bronze badges 11

• 2 Please provide a minimal reproducible example with godbolt.. We don't know what is o.mg – Louis Go Commented Jan 17 at 15:07
• "For non-union class types, the [Implicitly-defined copy assignment] operator performs member-wise copy assignment of the object's direct bases and non-static data members, in their initialization order, using built-in assignment for the scalars, memberwise copy-assignment for arrays, and copy assignment operator for class types (called non-virtually)." from cppreference, emphasis mine. – paleonix Commented Jan 17 at 15:13
• 1 @YannTM The standard doesn't say that there's a case that uses memcpy. In any case, the member that's an array of objects that are not trivially copyable works just as well. – Igor Tandetnik Commented Jan 17 at 15:25
• 1 @YannTM "sizeof is known" is the only possibility in C++, there are no Variable Length Arrays in standard. And tip for future questions: if you want answers based on standard, add language-lawyer tag to your question (added here now). – Yksisarvinen Commented Jan 17 at 15:25
• 2 This question is similar to: Indirect array assignment works, while direct array assignment does not. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. – paleonix Commented Jan 17 at 15:34

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1 Answer 1

Sorted by: Reset to default 9

The implicitly-defined copy assignment operator is not as simple as "apply assignment to each member". Arrays are specifically taken into account as a special case.

[class.copy.assign]/12 The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobjects... Each subobject is assigned in the manner appropriate to its type:
...
(12.2) if the subobject is an array, each element is assigned, in the manner appropriate to the element type;
...

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