Testing template parameter for equality

Question

Let's say I have a template class defined as:

template < typename A, typename B >
class something { ... }

How can I test if types A and B are of the same type? I know this can be done at runtime with typeid, but I really need this to be a compile time test.

Also, How can I specialize the class if types A and B are equal?

In the real world, A would be an stl container of some sort, std::string for example, and B would be a char or wchar_t. Internally I already check the containers value_type (compile error if not what expected). If B is the same as the containers value_type, most of the code in the class will become redundant.

possible duplicate of How to check if two template parameters are exactly the same? — hmjd
– hmjd, Commented Jul 5, 2013 at 10:43
@hmjd: There is a similar question AND a non-similar one. This question asks too much, actually. — Sebastian Mach
– Sebastian Mach, Commented Jul 5, 2013 at 10:46
@hmjd Thanks for the link. I hate the search feature on this site. — Twifty
– Twifty, Commented Jul 5, 2013 at 10:47
@Waldermort: you can also type "site:stackoverflow.com <your query>" into google. You can even make that a simple shortcut in firefox (right click on any inputbox and use "add keyword for this search"). — Sebastian Mach
– Sebastian Mach, Commented Jul 5, 2013 at 11:01

Sebastian Mach · Accepted Answer · 2013-07-05 10:43:55Z

6

Also, How can I specialize the class if types A and B are equal?

By exactly that, specializing:

template <typename A>
class something<A,A> { ... }

Templates use pattern matching for their parameter lists, like also seen in many functional programming languages.

How can I test if types A and B are of the same type?

You can use std::is_same, or use specialization as above. It depends on your exact use case.

answered Jul 5, 2013 at 10:43

Sebastian Mach

39.3k8 gold badges97 silver badges131 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Alex1985 · Accepted Answer · 2013-07-05 10:45:44Z

4

You can check if the types are the same using:

std::is_same<A, B>::value

it will return true when they are.

answered Jul 5, 2013 at 10:45

Alex1985

6683 silver badges7 bronze badges

Comments

neuront · Accepted Answer · 2013-07-05 10:52:12Z

1

How about this

template <typename A, typename B>
struct EnsureSameType {};

template <typename A>
struct EnsureSameType<A, A> {
    typedef int the_template_types_are_different;
};

int main()
{
    /* this should compile */
    typedef EnsureSameType<std::string::value_type, char>::the_template_types_are_different _;
    /* this should fail, and you will see the compiler
       remind you that the_template_types_are_different */
    typedef EnsureSameType<std::string::value_type, wchar_t>::the_template_types_are_different _;
    return 0;
}

answered Jul 5, 2013 at 10:52

neuront

9,6526 gold badges46 silver badges74 bronze badges

6 Comments

Twifty Over a year ago

I really like your use of non std. One less #include I need to worry about.

Sebastian Redl Over a year ago

Seriously? Worrying about standard library includes? You need to rethink your priorities.

neuront Over a year ago

@Waldermort I'm not using STL solution, just because I could only get a bool value via std::is_same, but your question need a compile error.

Twifty Over a year ago

@neuront I understand. I prefer the compile error over the stl method.

Twifty Over a year ago

@SebastianRedl Seriously? Judging without the facts? Where strings come into play I try to stay away from STL since, in my experience, every compiler is different. For example ASCII vs UTF8 std::string.

|

Collectives™ on Stack Overflow

Testing template parameter for equality

3 Answers 3

Comments

Comments

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related