Copy constructors

 

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A copy constructor of class T is a non-template constructor whose first parameter is T&, const T&, volatile T&, or const volatile T&, and either there are no other parameters, or the rest of the parameters all have default values. A type with a public copy constructor is CopyConstructible.

Syntax

class_name ( const class_name & )	(1)
class_name ( const class_name & ) = default;	(1)
class_name ( const class_name & ) = delete;	(1)

Explanation

1. Typical declaration of a copy constructor
2. Forcing a copy constructor to be generated by the compiler
3. Avoiding implicit default constructor

The copy constructor is called whenever an object is initialized from another object of the same type, which includes

• initialization, T a = b; or T a(b);, where b is of type T
• function argument passing: f(a);, where a is of type T and f is void f(T t)
• function return: return a; inside a function such as T f(), where a is of type T, which has no move constructor.

Implicitly-declared copy constructor

If no user-defined copy constructors are provided for a class type (struct, class, or union), the compiler will always declare a copy constructor as an inline public member of its class. This implicitly-declared copy constructor has the form T::T(const T&) if all of the following is true:

• all direct and virtual bases of T have copy constructors with references to const or to const volatile as their first parameters
• all non-static members of T have copy constructors with references to const or to const volatile as their first parameters

Otherwise, the implicitly-declared copy constructor is T::T(T&). (Note that due to these rules, the implicitly-declared copy constructor cannot bind to a volatile lvalue argument)

A class can have multiple copy constructors, e.g. both T::T(const T&) and T::T(T&). If some user-defined copy constructors are present, the user may still force the generation of the implicitly declared copy constructor with the keyword default (since C++11).

Deleted implicitly-declared copy constructor

The implicitly-declared or defaulted copy constructor for class T is undefined (until C++11) / defined as deleted (since C++11) in any of the following is true:

• T has non-static data members that cannot be copied (have deleted, inaccessible, or ambiguous copy constructors)
• T has direct or virtual base class that cannot be copied (has deleted, inaccessible, or ambiguous copy constructors)
• T has direct or virtual base class with a deleted or inaccessible destructor
• T has a user-defined move constructor or move assignment operator (since C++11)
• T is a union and has a variant member with non-trivial copy constructor (since C++11)
• T has a data member of rvalue reference type (since C++11)

Trivial copy constructor

The implicitly-declared copy constructor for class T is trivial if all of the following is true:

• T has no virtual member functions
• T has no virtual base classes
• The copy constructor selected for every direct base of T is trivial
• The copy constructor selected for every non-static class type (or array of class type) memeber of T is trivial

A trivial copy constructor is a constructor that creates a bytewise copy of the object representation of the argument, and performs no other action. Objects with trivial copy constructors can be copied by copying their object representations manually, e.g. with std::memmove. All data types compatible with the C language (POD types) are trivially copyable.

Implicitly-defined copy constructor

If the implicitly-declared copy constructor is not deleted or trivial, it is defined (that is, a function body is generated and compiled) by the compiler. For union types, the implicitly-defined copy constructor copies the object representation (as by std::memmove). For non-union class types (class and struct), the implicitly performs full member-wise copy of the object's bases and non-static members, in their initialization order, using direct initialization.

Example

struct A {
    int n;
    A(int n=1) : n(n) {}
    A(const A& a) : n(a.n) {} // user-defined copy ctor
};
 
struct B : A {
    // implicit default ctor B::B()
    // implicit copy ctor B::B(const B&) 
};
 
struct C : B {
     C() : B() {}
 private:
     C(const C&); // non-copiable, C++98 style
};
 
int main()
{
    A a1(7);
    A a2(a1); // calls the copy ctor
    B b;
    B b2 = b;
    A a3 = b; // conversion to A& and copy ctor
    volatile A va(10);
    // A a4 = va; // compile error
 
    C c;
    // C c2 = c; // compile error
}