Do you want to declare a copy constructor in a implicit way ? Implicitly-Declared Copy Constructor helps you to do this, here is the full post;
The Constructor in C++ is a function, a method in the class, but it is a ‘special method’ that is automatically called when an object of a class is created. We don’t need to call this function. Whenever a new object of a class is created, the Constructor allows the class to initialize member variables or allocate storage. This is why the name Constructor is given to this special method. Here is a simple constructor class example below,
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class myclass { public: myclass() { std::cout << "myclass is constructed!\n"; }; }; |
There are different constructor types in classes and the Copy Constructor is one of these. Copy Constructors not only used in classes but also used with struct and union data types.
The Copy Constructor in classes (class_name) is a non-template constructor whose first parameter is class_name&, const class_name&, volatile class_name&, or const volatile class_name& . It can be used with no other parameters or with the rest of the parameters all have default values.
The Copy Constructor is a constructor type for classes that class_name must name the current class, or it should be a qualified class name when it is declared at namespace scope or in a friend declaration.
Implicitly-Declared Copy Constructor
In C++, the compiler declares a Copy Constructor as a non-explicit inline public member of its class If a copy constructor is not defined for a class type (struct, class, or union), We can Implicitly declare a copy constructor when defining a new class. Remember that copy constructor has this syntax,
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class_name (const class_name& ) // Copy Constructor { }; |
and this Copy Constructor will be declared implicitly when declaring a new class as below,
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class new_class_name : access_type class_name { // Copy Constructor will be Declared Implicitly }; |
Here is an example,
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class myclass { public: myclass() // Default Constructor { }; myclass(const myclass& ) // Copy Constructor { }; }; class my_otherclass : public myclass // Implicitly-Declared Copy Constructor { }; |
Here is the full example,
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#include <iostream> class myclass { public: int param; myclass() // Default Constructor { }; myclass(const myclass& ) // Copy Constructor { }; }; class my_otherclass : public myclass // Implicitly-Declared Copy Constructor { }; int main() { myclass class1; class1.param=100; std::cout << class1.param << '\n' ; // Using Implicitly-Declared Copy Constructor my_otherclass class2; std::cout << class2.param << '\n' ; getchar(); return 0; } |
output will be like this.
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100 4358220 |
As in this example, Implicit declared copy constructor does not copy values.
If there are user-defined copy constructors, user can force the generation of the implicitly declared copy constructor with the ‘default’ keyword. Both Implicitly-declared or defaulted copy constructors have an exception specification. Using forced copy construction in myclass as below also outs same results;
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myclass(const myclass& )= default; |
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