Can't use pointer as a default template parameter

Question

I am trying to use default raw pointer as a default template parameter. I read that non type template parameters are restricted to integral types, enums, pointers and references. With references I had no issues, but when I tried to use pointer I am facing such error:

error: non-type template argument of type 'Engine *' is not a constant expression.

Here is my code:

#include <iostream>
#include <memory>

using std::cout;
using std::endl;

class Engine
{
public:
    void startEngine()
    {
        m_started = true;
        cout << "Engine started!" << endl;
    }
private:
    bool m_started = false;
};

template<typename T, T* DEFAULT>
class Car
{
public:
    explicit Car(const uint64_t uid) : m_uid(uid), engine(DEFAULT)
    {
        engine->startEngine();
    }

private:
    uint64_t m_uid;
    T* engine;
};

namespace
{
    std::shared_ptr<Engine> engine = std::make_shared<Engine>();
    Engine* e = engine.get();
}

int main()
{
    Car<Engine, e> lambo(0);
    return 0;
}

The only restriction as I see now is that second template argument needs to have static storage duration and external or internal linkage but the code fits these requirements. Appreciate any help.

Answer 1

non-type template arguments must be a constant expression:

A template-argument for a non-type template-parameter shall be a converted constant expression (5.20) of the type of the template-parameter.

---

Perhaps you can workaround this making the object you point has static storage duration and linkage (see):

For pointers to objects, the template arguments have to designate the address of a complete object with static storage duration and a linkage (either internal or external), or a constant expression that evaluates to the appropriate null pointer or std::nullptr_t value.

Example:

namespace{
    Engine e;
}

int main(){
    Car<Engine, &e> lambo(0);
    return 0;
}

Answer 2

As with any non-type template parameter, a pointer template parameter has to be known at compile time, or, in C++ Standard parlance, be a constant expression. And your pointer is not - compiler has no way of knowing what this address going to be when the program is executed.