3.1.7 Variable Templates

What was the problem

Up to C++11, the only things that could be templated were types and functions. So if you had a variable whose value was dependent on a template parameter, you had to wrap the variable in a type. Take for example this code that computes the number of bits set to 1 in a constant at compile time:

#include <cstdio> 
 
template<unsigned N> 
struct popcount; 
 
template<> 
struct popcount<0> 
{ 
  static constexpr unsigned value = 0; 
}; 
 
template<unsigned N> 
struct popcount 
{ 
  static constexpr unsigned value = N % 2 + popcount<(N >> 1)>::value; 
}; 
 
int main() 
{ 
  // In order to actually do the computation, we have to instantiate 
  // a type and get the value declared inside this type. 
  printf("%d\n", popcount<24>::value); 
}

How the Problem is Solved

Starting with C++14, all the types in the above code can be removed and replaced by a much more concise template variable, which also carries the intent more accurately.

#include <cstdio> 
 
// This is a variable, whose value depends upon a template argument. 
template<unsigned N> 
unsigned popcount = N % 2 + popcount<(N >> 1)>; 
 
// The variable can be specialized too! 
template<> 
unsigned popcount<0> = 0; 
 
int main() 
{ 
  // Here we can use a variable to represent a value. Much more clear. 
  printf("%d\n", popcount<24>); 
}

Note than in this specific case a constexpr function [2.1.4] is more suited to the task; plus it can also be called at run time.

You will probably not see many variable templates in the wild, as they are quite a niche feature. Mostly, they are used to store some domain-specific constants to the precision of a given type.