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':
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?
How does C++ compiler differs between overloaded postfix and prefix operators? (D) By making prefix ++ as a global function and postfix as a member function.
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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