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Thinking in C++
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Automatic operator= creation

Because assigning an object to another object of the same type is an activity most people expect to be possible, the compiler will automatically create a type::operator=(type) if you don’t make one. The behavior of this operator mimics that of the automatically created copy-constructor; if the class contains objects (or is inherited from another class), the operator= for those objects is called recursively. This is called memberwise assignment. For example,

//: C12:AutomaticOperatorEquals.cpp
#include <iostream>
using namespace std;

class Cargo {
  Cargo& operator=(const Cargo&) {
    cout << "inside Cargo::operator=()" << endl;
    return *this;

class Truck {
  Cargo b;

int main() {
  Truck a, b;
  a = b; // Prints: "inside Cargo::operator=()"
} ///:~

The automatically generated operator= for Truck calls Cargo::operator=.

In general, you don’t want to let the compiler do this for you. With classes of any sophistication (especially if they contain pointers!) you want to explicitly create an operator=. If you really don’t want people to perform assignment, declare operator= as a private function. (You don’t need to define it unless you’re using it inside the class.)

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire