c++ template: intiantiating a variable out of a particular integer (as type)

Question

I've obviously misunderstood the code. Thanks for pointing out the error.

--original post--

I know for a while that integers can be used as types in c++ template programming. What surprises me is that when an integer such as 2 and 3 is used as a type, one can actually instantiate a variable out of the type, as shown in the example below extracted from gcc 4.8.1 random.h.

It seems to me that one can declare a variable

2 x; //not actual c++ code

and x will be an integer variable initialized to the value of 2.

Code for deterniming whether a number is a power of 2:

#include <iostream> template<typename _Tp> inline bool _Power_of_2(_Tp __x) { return ((__x - 1) & __x) == 0; }; int main() { std::cout << _Power_of_2(2) << std::endl; std::cout << _Power_of_2(3) << std::endl; }

Output:

1 0

Can someone please explain what's going on here in terms of types and domains?

Are there any similar features in other programming languages that can do the same, i.e. using a concrete value as a type?

Also, is this feature available for other types, such as struct, string or float?

Thanks,

Where do you see the use of an integer as a type in your code? — Constructor
– Constructor, Commented Mar 20, 2014 at 10:58

anderas · Accepted Answer · 2014-03-20 10:58:50Z

In your example, 2 is not used as a type, but as a function parameter. From that, the template parameter _tP is automatically deduced as int. So the lines inside main would be equivalent to:

std::cout << _Power_of_2<int>(2) << std::endl; std::cout << _Power_of_2<int>(3) << std::endl;

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