Why don't C++ compilers define operator== and operator!=?


Question

I am a big fan of letting the compiler do as much work for you as possible. When writing a simple class the compiler can give you the following for 'free':

  • A default (empty) constructor
  • A copy constructor
  • A destructor
  • An assignment operator (operator=)

But it cannot seem to give you any comparison operators - such as operator== or operator!=. For example:

class foo
{
public:
    std::string str_;
    int n_;
};

foo f1;        // Works
foo f2(f1);    // Works
foo f3;
f3 = f2;       // Works

if (f3 == f2)  // Fails
{ }

if (f3 != f2)  // Fails
{ }

Is there a good reason for this? Why would performing a member-by-member comparison be a problem? Obviously if the class allocates memory then you'd want to be careful, but for a simple class surely the compiler could do this for you?

1
281
10/21/2008 8:12:23 AM

Accepted Answer

The compiler wouldn't know whether you wanted a pointer comparison or a deep (internal) comparison.

It's safer to just not implement it and let the programmer do that themselves. Then they can make all the assumptions they like.

68
10/20/2008 9:53:07 AM

The argument that if the compiler can provide a default copy constructor, it should be able to provide a similar default operator==() makes a certain amount of sense. I think that the reason for the decision not to provide a compiler-generated default for this operator can be guessed by what Stroustrup said about the default copy constructor in "The Design and Evolution of C++" (Section 11.4.1 - Control of Copying):

I personally consider it unfortunate that copy operations are defined by default and I prohibit copying of objects of many of my classes. However, C++ inherited its default assignment and copy constructors from C, and they are frequently used.

So instead of "why doesn't C++ have a default operator==()?", the question should have been "why does C++ have a default assignment and copy constructor?", with the answer being those items were included reluctantly by Stroustrup for backwards compatibility with C (probably the cause of most of C++'s warts, but also probably the primary reason for C++'s popularity).

For my own purposes, in my IDE the snippet I use for new classes contains declarations for a private assignment operator and copy constructor so that when I gen up a new class I get no default assignment and copy operations - I have to explicitly remove the declaration of those operations from the private: section if I want the compiler to be able to generate them for me.


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