Can We Determine if a C 11 Expression is `constexpr` at Compile Time Without Declarations?

Utilizing constexpr in C 11: Determining Constant Expressions

Question:

Is it feasible to ascercerain whether a C 11 expression qualifies as a constant expression (constexpr) during compile-time without relying on declarations?

Answer:

Utilizing the noexcept operator, it is possible to construct a compile-time boolean indicating whether an expression constitutes a constexpr:

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

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

Explanation:

The noexcept(e) expression validates the following conditions:

• The absence of function calls with non-throwing exception specifications or variable calls that do not constitute constant expressions.
• No presence of "throw" expressions.
• No dynamic casts or typeids in the form of throwables.

Limitations:

Due to the conservative nature of "potentially evaluated" expressions, there may be instances where noexcept may provide false negatives. For example:

constexpr int a = (0 ? throw "fooled!" : 42);
constexpr bool atest = isprvalconstexpr((0 ? throw "fooled!" : 42)); // returns false

In this case, a is initialized successfully, but atest evaluates to false because the throw expression is potentially evaluated, even though it is never executed.

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