Enforce functions to implement for template argument class?

Should I define an interface which explicitly informs the user what all he/she should implement in order to use the class as template argument or let the compiler warn him when the functionality is not implemented ?

template <Class C1, Class C2> 
SomeClass
{
  ...
}

Class C1 has to implement certain methods and operators, compiler won't warn until they are used. Should I rely on compiler to warn or make sure that I do:

Class C1 : public SomeI开发者_如何学GonterfaceEnforcedFunctions
{
   // Class C1 has to implement them either way 
   // but this is explicit? am I right or being 
   // redundant ?
}

---

Ideally, you should use a concept to specify the requirements on the type used as a template argument. Unfortunately, neither the current nor the upcoming standard includes concepts.

Absent that, there are various methods available for enforcing such requirements. You might want to read Eric Neibler's article about how to enforce requirements on template arguments.

I'd agree with Eric's assertion that leaving it all to the compiler is generally unacceptable. It's much of the source of the horrible error messages most of us associate with templates, where seemingly trivial typos can result in pages of unreadable dreck.

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If you are going to force an interface, then why use a template at all? You can simply do -

class SomeInterface //make this an interface by having pure virtual functions
{    
    public:
        RType SomeFunction(Param1 p1, Param2 p2) = 0; 
        /*You don't have to know how this method is implemented, 
          but now you can guarantee that whoever wants to create a type 
          that is SomeInterface will have to implement SomeFunction in 
          their derived class. 
        */
};

followed by

template <class C2>
class SomeClass
{
    //use SomeInterface here directly.       
};

Update -

A fundamental problem with this approach is that it only works for types that is rolled out by a user. If there is a standard library type that conforms to your interface specification, or a third party code or another library (like boost) that has classes that conform to SomeInterface, they won't work unless you wrap them in your own class, implement the interface and forward the calls appropriately. I'm somehow not liking my answer anymore.

---

Absent of concepts, a for now abandoned concept (pun not intended, but noted) for describing which requirements a template parameter must fulfill, the requirements are only enforced implicitly. That is, if whatever your users use as a template parameter doesn't fulfill them, the code won't compile. Unfortunately, the error message resulting from that are often quite gibberish. The only things you can do to improve matters is to

1. describe the requirements in your template's documentation
2. insert code that checks for those requirements early on in your template, before it delves so deep that the error messages your users get become unintelligibly. The latter can be quite complicated (static_assert to the rescue!) or even impossible, which is the reason concepts where considered to become a core-language feature, instead of a library.

Note that it is easy to overlook a requirement this way, which will only become apparent when someone uses a type as a template parameter that won't work. However, it is at least as easy to overlook that requirements are often quite lose and put more into the description than what the code actually calls for.
For example, + is defined not only for numbers, but also for std::string and for any number of user-defined types. Conesequently, a template add<T> might not only be used with numbers, but also with strings and an infinite number of user-defined types. Whether this is an unwanted side-effect of the code you want to suppress or a feature you want to support is up to you. All I'm saying is that it is not easy to catch this.

I don't think defining an interface in the form of an abstract base class with virtual functions is a good idea. This is run-time polymorphism, a main pillar classic OO. If you do this, then you don't need a template, just take the base class per reference.
But then you also lose one of the main advantages of templates, which is that they are, in some ways, more flexible (try to write an add() function classic OO which works with any type overloading + in) and faster, because the binding of the function calls take place not at run-time, but during compilation. (That brings more than it might look like at first due to the ability to inline, which usually isn't possible with run-time polymorphism.)