C++ if constexpr using a constexpr bool equivalent to plain if?

Question

Are the follow snippets functionally equivalent given the conditional variable that's being checked has been marked as constexpr?

Additionally, is this a correct usage of if constexpr or is it's intended application within a templatized context.

constexpr bool kOn = false;

// Snippet 1
if (kOn) { return true; }

// Snippet 2
if constexpr (kOn) { return true; }

Answer 1

No, the two ifs are not equivalent.

The 1st one is evaluated at runtime, and code in both of its branches must be well-formed at compile-time.

The 2nd one is evaluated at compile-time, and code in its unused branch can be ill-formed since its dead code that gets removed by the compiler.

if constexpr is not dependent on templates.

Answer 2

When you use if constexpr, the branch not taken (the else if the condition was true or the first statement if the condition was false) becomes a discarded statement and gains exactly 3 "special powers".

• If it is in a template and the condition was value-dependent (e.g., if constexpr (B) where you had a template-head template<bool B>), the discarded branch isn't instantiated at all when the specialization is instantiated.

• Entities inside a discarded branch are still odr-used but do not require a definition:

  int x();
  int main() {
      if constexpr (false) x();  // Does not require a definition of x
      if (false) x();  // Requires a definition of x
  }
  

• Return statements in a discarded branch do not affect auto type deduction:

  auto f() {
      if constexpr (false) return 0;
      return 0L;
  }  // OK: auto deduced as long
  
  auto g() {
      if (false) return 0;
      return 0L;
  }  // Error: two return statements with different types.
  

And, of course, the difference that if (x) doesn't force x to be a constant expression, whereas if constexpr (x) does.

So in a non-template context, there isn't much difference with your constexpr bool kOn = false;. The compiler will still check both branches to see if they compile with an if constexpr.