Re: [boost] RFC Expression Validity Asserts

On 1/22/2015 6:05 PM, Matt Calabrese wrote:

On Wed, Jan 21, 2015 at 4:52 PM, Matt Calabrese wrote:

...

I've found it useful, especially in tests, to easily be able to do expression validity checking via SFINAE in a static assert.

Bump, and also a couple of additional macros for checking constexpr-ness of an expression are linked farther down in this email, using similar tricks as the SFINAE checks int the previous email (pushing more complicated checks into the body of a lambda such that the macro expands to an expression).

Motivation for this is that often when writing templated code it is difficult to be certain that a specific instantiation is actually constexpr even if the constexpr keyword is used with the function template. For instance, for a matrix type whose value type is a template parameter T, it can be difficult to know if matrix<T>'s multiplication operation actually results in a constexpr object, even if it has constexpr inputs. Particularly when designing such a library, it is useful to be able to write simple tests that check the constexpr-ness of a specified expression in the form of a static_assert. A proof-of-concept implementation of this is linked below:

http://ideone.com/ko09KQ

As an example of such a usage:

////////////////////

int foo(int) { return 0; }

STATIC_ASSERT_CONSTEXPR ( (constexpr int) var = 5, foo(var) );

////////////////////

Produces the error:

prog.cpp:70:1: error: static assertion failed:

********************

Given: constexpr int var = 5;

The following expression was expected to be constexpr but was not: foo(var)

********************

This looks interesting but I am confused about where this implementation is. Is it available somewhere ? is it documented at all ? Are there examples/tests ? Is there a good general of when some of these macros should/would be used in template code ? It looks like you are saying that in place of BOOST_STATIC_ASSERT one can use your macros at compile time to produce compile time errors, but the majority of C++ expressions have to be evaluated at run-time. So is this strictly for compile-time expressions or what ? Your original OP looked like it worked with run-time expressions ( ++a, a+b etc. ).