It's called dependency injection via constructor injection: class A gets the dependency as an argument to its constructor and saves the reference to dependent class as a private variable.
There's an interesting introduction on wikipedia.
For const-correctness I'd write:
using T = int;
class A
{
public:
A(const T &thing) : m_thing(thing) {}
// ...
private:
const T &m_thing;
};
but a problem with this class is that it accepts references to temporary objects:
T t;
A a1{t}; // this is ok, but...
A a2{T()}; // ... this is BAD.
It's better to add (requires C++11 at least):
class A
{
public:
A(const T &thing) : m_thing(thing) {}
A(const T &&) = delete; // prevents rvalue binding
// ...
private:
const T &m_thing;
};
Anyway if you change the constructor:
class A
{
public:
A(const T *thing) : m_thing(*thing) { assert(thing); }
// ...
private:
const T &m_thing;
};
it's pretty much guaranteed that you won't have a pointer to a temporary.
Also, since the constructor takes a pointer, it's clearer to users of A that they need to pay attention to the lifetime of the object they pass.
Somewhat related topics are: