Can C 11's `noexcept` Trick Reliably Detect `constexpr` Expressions?

Detecting constexpr Expressions in C 11

In C 11, it is crucial to determine whether an expression qualifies as a constant expression (constexpr) at compile time. This knowledge aids in various optimization techniques and enables the generation of compile-time computed values.

Feasibility of is_constexpr

Concerns arise about the feasibility of producing a compile-time boolean value based on the constexpr nature of a C 11 expression. While previous Stack Overflow questions touch upon this topic, a definitive answer remains elusive.

A Solution: The noexcept Trick

A solution involves utilizing the noexcept expression, which evaluates to false if the provided expression contains specific elements that violate the requirements of constexpr. These elements include:

• Non-throwing exception specifications
• Throw expressions
• Throwable dynamic_cast or typeid

By deliberately making a function template non-noexcept and applying it to the expression, the noexcept(e) expression can be leveraged to detect constexpr expressions. This approach specifically identifies prvalue constant expressions.

Limitations

It is essential to note limitations associated with this method:

• The noexcept(e) expression is conservative, potentially leading to false negatives. It may indicate that an expression is not constexpr even though it technically qualifies.
• This technique primarily detects prvalue constant expressions, excluding other constant expression types.

Example Implementation

The following code snippet demonstrates the proposed solution:

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

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

Despite the limitations, this approach provides valuable insight into the constexpr nature of C 11 expressions, empowering programmers with greater control and optimization capabilities.

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