Is My C 11 Expression a `constexpr`?

Detecting Constant Expressions in C 11

In C 11, it is possible to determine if an expression is a constant expression (constexpr) during compilation, which can be beneficial for optimizing code and ensuring certain properties.

Using noexcept(makeprval(e))

One way to determine if an expression is a constant expression is to use the following macro:

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

where makeprval is a template function that takes a reference to an expression as input and returns a prvalue of the same type.

Explanation

The noexcept(e) expression returns false if e contains certain operations that are not allowed in constant expressions, such as non-throwing function calls, throw expressions, and throwable dynamic casts or typeids. If the call to makeprval is not a constant expression, the noexcept expression will also return false.

Limitations

While isprvalconstexpr is generally effective in detecting prvalue constant expressions, it has a subtle limitation. It may give false negatives in cases where an expression is a constant expression but contains potentially evaluated subexpressions that are not allowed in constant expressions.

Example Usage

The following example demonstrates the usage of isprvalconstexpr:

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

In this example, atest is false because the expression (0 ? throw "fooled!" : 42) is not a constant expression, even though the initialization of a is successful. This is because the "evil" non-constant subexpression throw "fooled!" is potentially evaluated, even though it is never actually evaluated in this specific case.

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