Preventing pass-by-value
“But,” you say, “if I
don’t make a copy-constructor, the compiler will create one for me. So how
do I know that an object will never be passed by value?”
There’s a simple technique for
preventing pass-by-value: declare a private
copy-constructor. You don’t even need to create a
definition, unless one of your member functions or a friend function
needs to perform a pass-by-value. If the user tries to pass or return the object
by value, the compiler will produce an error message because the
copy-constructor is private. It can no longer create a default
copy-constructor because you’ve explicitly stated that you’re taking
over that job.
//: C11:NoCopyConstruction.cpp
// Preventing copy-construction
class NoCC {
int i;
NoCC(const NoCC&); // No definition
public:
NoCC(int ii = 0) : i(ii) {}
};
void f(NoCC);
int main() {
NoCC n;
//! f(n); // Error: copy-constructor called
//! NoCC n2 = n; // Error: c-c called
//! NoCC n3(n); // Error: c-c called
} ///:~
Notice the use of the more general form
NoCC(const NoCC&);
