Can C 11 Determine if an Expression is a Compile-Time Constant?

Is constexpr Possible in C 11?

Question:

Can C 11 determine if an expression is a compile-time constant expression (constexpr)?

Answer:

Yes, it is possible to produce a compile-time boolean value based on this determination using the following technique:

template<typename T> 
constexpr typename remove_reference<T>::type makeprval(T &&& t) {
  return t;
}

#define isprvalconstexpr(e) noexcept(makeprval(e))

This macro checks whether the expression e is a prvalue constant expression by utilizing the noexcept operator. If noexcept(makeprval(e)) evaluates to true, the expression e is a prvalue constant expression.

Explanation:

Prvalue constant expressions have certain restrictions:

• Function calls must have non-throwing exception-specifications.
• Throw expressions are prohibited.
• Dynamic casts and typeid expressions must be in a non-throwable form.

The makeprval function is not declared as noexcept, so if e is a prvalue constant expression, the call makeprval(e) will be a constant expression. Thus, the noexcept operator will return true.

Limitations:

The isprvalconstexpr macro has some limitations:

• It may produce false negatives in cases where the expression is a constant expression but the sub-expressions are potentially evaluated but not actually evaluated.
• It only checks for prvalue constant expressions, not general constexpr expressions.

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