In C++, a class with a const
data member may have a copy constructor.
#include <iostream>
class A
{
private:
const int k_;
public:
A(int k) : k_(k) {}
A() = delete;
A(const A& other) : k_(other.k_) {}
int get_k() const { return k_; }
};
int main(int argc, char** argv)
{
A a1(5);
A a2(a1);
std::cout << "a1.k_ = " << a1.get_k() << "\n";
std::cout << "a2.k_ = " << a2.get_k() << "\n";
}
Output:
a1.k_ = 5
a2.k_ = 5
In C++, a class with a const
data member may not use the default assignment operator.
class A
{
private:
const int k_;
public:
A(int k) : k_(k) {}
A() = delete;
A(const A& other) : k_(other.k_) {}
int get_k() const { return k_; }
};
int main(int argc, char** argv)
{
A a1(5);
A a2(0);
a2 = a1;
}
Yields a compile time error:
const_copy_constructor.cpp: In function ‘int main(int, char**)’:
const_copy_constructor.cpp:18:10: error: use of deleted function ‘A& A::operator=(const A&)’
18 | a2 = a1;
| ^~
const_copy_constructor.cpp:1:7: note: ‘A& A::operator=(const A&)’ is implicitly deleted because the default definition would be ill-formed:
1 | class A
| ^
const_copy_constructor.cpp:1:7: error: non-static const member ‘const int A::k_’, can’t use default assignment operator
In C++, a class with a const
data member may use a non-default assignment operator as long as you don’t attempt to change the const
data member, but you better think long and hard about what it means to use this assignment operator if one of the underlying members cannot be modified.
class A
{
private:
const int k_;
public:
A(int k) : k_(k) {}
A() = delete;
A(const A& other) : k_(other.k_) {}
A& operator=(A const& other)
{
// do nothing
return *this;
}
int get_k() const { return k_; }
};
int main(int argc, char** argv)
{
A a1(5);
A a2(0);
a2 = a1;
}
Yields no compile time errors.