Mocking C++ classes that copy constructors - c++

Question

I'm coming from C# to C++ and when I try to construct tests, copy constructors are making it hard for me to mock the usual way I am accustomed to.
#include <iostream>
using namespace std;
class DB {
public:
virtual int getValue() { return 42; }
DB(const DB& source) { }
DB() { }
};
class DB_Mock: public DB {
public:
virtual int getValue() { return 999; }
};
class A {
public:
A(DB db) {
m_db = db;
}
void doIt() {
cout << "Result:" << m_db.getValue() << endl;
}
private:
DB m_db;
};
int main() {
/* prints 42 as expected */
DB db;
A a(db);
a.doIt();
/* prints 42, expected 999 */
DB_Mock db_mock;
A a2(db_mock);
a2.doIt();
return 0;
}
How do you approach problems like this?

Answer 1

m_db is not a reference or pointer, c++ polymorphism is work with only base class pointer or reference.
A(DB db) {
m_db = db; // m_db just sliced copy of base part of db.
}
Change this to reference like
class A
{
public:
explicit A(DB &db) : m_db(db) // it is initializing not copy
{
}
void doIt()
{
std::cout << "Result:" << m_db.getValue() << std::endl;
}
private:
DB &m_db; // Now m_db is reference
};
Compile and Run Here
Above will work But, You can not change reference value if you want to change use smart pointer[Recommended].
Reference :
Constructors and member initializer lists
Smart pointer
override
See virtual table to understand v_ptr
Edit:
As Pete Becker said be aware of lifetimes, if you pass local variable reference like
A make()
{
DB db;
A a(db);
return a;
}
int main()
{
const A &a = make();
// Wrong db is destroyed (dangling reference).
a.doIt(); // assume doIt(), getValue() are declared as const
return 0;
}
Above code is Wrong (undefined behaviour), So i Recommend to use smart pointer(Heap storage) to avoid this problems.
Thanks.