定义赋值运算符和复制构造函数

Question

Suppose I have a class A. I have defined a copy constructor and an assignment operator overloading function. When I do

Class A; Class B=A;

Then while defining Class B, is the copy constructor invoked or the assignment operator?

Thanks in advance.

EDIT: Sorry, I mentioned wrong code. It should be:

A a; A b=a;

Answer 1

IIRC T t = value invokes the copy constructor, you can verify that by outputting a string in the constructors to determine which method is used. IIRC when the declaration and assignment are on the same line, it is not called assignment but initialization.

On the other hand, what you've posted does not make sense, you cannot assign one type to another type, you can only assign to type instances.

EDIT: Even if you have a case of two different types (the context of your question is not clear on this one):

class A {};

class B {
public:
   B(const A& other)  { cout << "copy"; }
   B& operator=(const A& other) { cout << "assign"; }
};

int main() {
   A a;
   B b = a; // copy con
   B b1(a); // same as above
   b = a;   // assign op
}

Even then, when both the "copy constructor" and assignment operators take in another type, the copy constructor will still be invoked rather than the assignment operator.

Answer 2

Assuming you actually mean something like:

A a;
A b = a;

The copy constructor is invoked. The Standard allows = this special meaning in this usage.

Answer 3

Create a simple class with both, and debug what function is executed by setting a breakpoint in both. Then youll see, and youll also learn a little bit of debugging.

Answer 4

Let's try it out!

#include <iostream>

class A {
 public:
  A() {
    std::cout << "default constructor\n";
  }
  A(const A& other) {
    std::cout << "copy constructor\n";
  }
  A& operator=(const A& rhs) {
    std::cout << "assignment operator\n";
    return *this;
  }
};

int main(int argc, char** argv) {
  std::cout << "first declaration: ";
  A a;
  std::cout << "second declaration: ";
  A b(a);
  std::cout << "third declaration: ";
  A c = a;
  std::cout << "fourth declaration: ";
  A d;
  std::cout << "copying? ";
  d = a;
  return 0;
}

This prints:

first declaration: default constructor
second declaration: copy constructor
third declaration: copy constructor
fourth declaration: default constructor
copying? assignment operator

Working example here: http://codepad.org/DNCpqK2E

Answer 5

when you are defining a new object with assigning another object into it. It invokes copy constructor. eg, Copy constructor call:

Class A;
Class B=A;

Assignment operator call:

Class A;
Class B;
B=A;

You can always test this, by writing a "print" statement in both the methods to find out which one is being called.

Hope it helped...