2.1.6 Compile-Time Assertions

What was the problem

Compile-time assertions, declared with a static_assert, are a way to verify a property at compile time.

For example, the code below declares a type with an array whose size, with respect to the type’s name, must be even:

template<typename T, unsigned N> 
struct even_sized_array 
{ 
  typedef T type[N]; 
};// The third template argument is expected to receive a flag 
// telling if N is even (i.e. valid). 
template<typename T, unsigned N, bool IsEven> 
struct even_sized_array_impl; 
 
// And we implement this type only if the flag is true. 
template<typename T, unsigned N> 
struct even_sized_array_impl<T, N, true> 
{ 
  typedef T type[N]; 
}; 
 
template<typename T, unsigned N> 
struct even_sized_array 
{ 
  // Now we trigger the check by actually instantiating the 
  // helper type, hoping that the flag will be true. 
  typedef 
  typename even_sized_array_impl<T, N, N % 2 == 0>::type 
  type; 
}; 
 
void test() 
{ 
  even_sized_array<int, 8>::type ok; 
  even_sized_array<int, 9>::type nok; 
}

How can we ensure that N is always even? Before C++11 a solution to that would have been to enter some template dance such that passing an odd value would instantiate an incomplete type.

Not only this is verbose, but these kind of template instantiation errors are well known for producing kilometer-long unbearable error messages.

How the Problem is Solved

Enters static_assert in C++11. Just like the good old assert(condition) would check that the given condition is true at run time, static_assert(condition, message) will check the condition during the compilation.

template<typename T, unsigned N> 
struct even_sized_array 
{ 
  // We can also put a message to repeat the condition, like 
  // professionals. Don’t forget the exclamation mark to 
  // show it is serious. 
  static_assert(N % 2 == 0, "The size must be even!"); 
  typedef T type[N]; 
}; 
 
// So much less code <3 
void test() 
{ 
  even_sized_array<int, 8>::type ok; 
  even_sized_array<int, 9>::type nok; 
}

This is very less verbose than previously and it actually carries the intent. The error message is also clearer, as it is just something like “static assertion failed: the message”.