C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 35:
35.1 What's the idea behind templates?
35.2 What's the syntax / semantics for a "class template"?
35.3 What's the syntax / semantics for a "function template"?
35.4 How do I explicitly select which version of a function template should get called?
35.5 What is a "parameterized type"?
35.6 What is "genericity"?
35.7 My template function does something special when the template type T is int or std::string; how do I write my template so it uses the special code when T is one of those specific types?
35.8 Huh? Can you provide an example of template specialization that doesn't use foo and bar?
35.9 But most of the code in my template function is the same; is there some way to get the benefits of template specialization without duplicating all that source code?
35.10 All those templates and template specializations must slow down my program, right?
35.11 So templates are overloading, right?
35.12 Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
35.13 How can I avoid linker errors with my template functions? Updated!
35.14 How does the C++ keyword export help with template linker errors? Updated!
35.15 How can I avoid linker errors with my template classes? Updated!
35.16 Why do I get linker errors when I use template friends?
35.17 How can any human hope to understand these overly verbose template-based error messages?
35.18 Why am I getting errors when my template-derived-class uses a nested type it inherits from its template-base-class?
35.19 Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?
35.20 Can the previous problem hurt me silently? Is it possible that the compiler will silently generate the wrong code?
35.21 How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?
35.22 Follow-up to previous: can I pass in the underlying structure and the element-type separately?
35.23 Related: all those proxies must negatively reflect on the speed of my program. Don't they?
[35.19] Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?

Perhaps surprisingly, the following code is not valid C++, even though some compilers accept it:

template<typename T>
class B {
public:
  void f() { }   member of class B<T>
};

template<typename T>
class D : public B<T> {
public:
  void g()
  {
    f();   bad (even though some compilers erroneously (temporarily?) accept it)
  }
};
This might hurt your head; better if you sit down.

Within D<T>::g(), the name f does not depend on template parameter T, so f is known as a nondependent name. On the other hand, B<T> is dependent on template parameter T so B<T> is called a dependent name.

Here's the rule: the compiler does not look in dependent base classes (like B<T>) when looking up nondependent names (like f).

This doesn't mean that inheritance doesn't work. Class D<int> is still derived from class B<int>, the compiler still lets you implicitly do the is-a conversions (e.g., D<int>* to B<int>*), dynamic binding still works when virtual functions are invoked, etc. But there is an issue about how names are looked up.

Workarounds:

  • Change the call from f() to this->f(). Since this is always implicitly dependent in a template, this->f is dependent and the lookup is therefore deferred until the template is actually instantiated, at which point all base classes are considered.
  • Insert using B<T>::f; just prior to calling f().
  • Change the call from f() to B<T>::f(). Note however that this might not give you what you want if f() is virtual, since it inhibits the virtual dispatch mechanism.