C++ const-reference semantics?
- by Kristoffer
Consider the sample application below. It demonstrates what I would call a flawed class design.
#include <iostream>
using namespace std;
struct B
{
B() : m_value(1) {}
long m_value;
};
struct A
{
const B& GetB() const { return m_B; }
void Foo(const B &b)
{
// assert(this != &b);
m_B.m_value += b.m_value;
m_B.m_value += b.m_value;
}
protected:
B m_B;
};
int main(int argc, char* argv[])
{
A a;
cout << "Original value: " << a.GetB().m_value << endl;
cout << "Expected value: 3" << endl;
a.Foo(a.GetB());
cout << "Actual value: " << a.GetB().m_value << endl;
return 0;
}
Output:
Original value: 1
Expected value: 3
Actual value: 4
Obviously, the programmer is fooled by the constness of b. By mistake b points to this, which yields the undesired behavior.
My question: What const-rules should you follow when designing getters/setters?
My suggestion: Never return a reference to a member variable if it can be set by reference through a member function. Hence, either return by value or pass parameters by value. (Modern compilers will optimize away the extra copy anyway.)
