Is it possible to edit template name at compile time? [closed]

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. 开发者_运维问答 Closed 10 years ago.

I am trying to modify template type given at compile time but unable to do it. Let see if you have some idea.

Lets consider we have 2 classes class A and A_test and one template class B. Will implemented class B as it is having a function b_f() which is internaly creating an object of template class and calling public funcion A_f()/A_test_f() of class A or Class A_test.

from main, will be creating class B obj; obj.b_f(); but I want to creat an object of class A_test not class A.

Let me know if it is possible.

Basicaly I wanted to do object injection. Please let me know if it is possible.

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The best solution by far is to try to rename A_test_f() to be the same as A_f().

If that proves impossible, the next thing I'd try is to specialize class B:

template<class AT>
class B {
public: b_f() {
    AT m_A;
    m_A.A_f();
};

template<>
class B<A_test> {
public: b_f() {
    AT m_A;
    m_A.A_test_f();
};

If B is too complicated, there are other things to try, but you probably want to rethink what you're doing first. If all else fails, do what I coded above here, but call it B_HELPER instead of B and then b_f() can do: B_HELPER<AT> m_A; m_A.b_f(); That way you don't have to recode all of B.

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The common way to inject dependencies into templates at compile-time is through type traits. This allows customizing the templates externally through another struct or class which has specific knowledge of the class. Examples in the standard library include std::char_traits<> and std::iterator_traits<>. Boost also defines some, including boost::type_traits<>.

Traits involve defining a struct for the general case and specializing it for alternate cases when necessary.

  // general case: select method named "f".
template<class T> struct b_traits
{
    typedef void(T*F)();
    static const F f = &T::f;
};

  // template type that forwards method selection to "b_traits" struct.
template<class AT>
class B {
public: b_f() {
    AT m_A;
    (m_A.*(b_traits<AT>::f))();
};

class A_test { ... };

  // special case: select method named "A_test_f".
template<> struct b_traits<A_test>
{
    typedef void(T*F)();
    static const F f = &A_test::A_test_f;
};

int main ()
{
    B<A_test> b;
    b.b_f(); // will invoke "A_test::A_test_f()" rather than "A_test::f()".
}