C++11 static_assert: Parameterized error messages

Question

In my previous question I wanted to use static_assert to restrict a template parameter to be a specific subtype. The question was answered, the code for archieving that is as follows:

template <typename T> 
struct X { 
    static_assert(std::is_base_of<Y,T>::value,"T must be derived from Y!");
};

Now, I want to make the error message more concise. I.e., I want to state which type is violating this constraint. E.g., if class A is not derived from Y and someone instanciates X<A>, then the error message should print "The type parameter must be derived from Y, but A isn't".

Is this somehow achievable with the standard library?

I see two challenges:

Assembling strings at compiletime without using boost::mpl
retrieving the name of the type with which T was instanciated. The name should be meaningful, ideally the same as used in the violating definition. I tried typeid(T).name() but it only returns the mangled name which is not acceptable. I remember that there was some kind of macro that returns the name of something, but I cannot recall that anymore.

Related (peraps even dupe): http://stackoverflow.com/q/6415186/34509 — Johannes Schaub - litb, Jun 15 '12 at 12:46

score 8 · Accepted Answer · answered Jun 15 '12 at 12:23

8

You cannot do this. static_assert wants a string literal. You have no way to assemble the semantic identity of T and Y into the string literal.

You can hope that the compiler gives an easy to read backtrace of the template instantiation stack and gives you the value of T and Y template parameters of the enclosing class template instantiation.

Other people thought about this too, see http://comments.gmane.org/gmane.comp.compilers.clang.devel/5073 for example.

answered Jun 15 '12 at 12:23

Johannes Schaub - litb

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As always for diagnosis, it's a QOI. – Matthieu M. Jun 15 '12 at 12:26
what a shame. Why a literal, a compile time constant string would be better :(. – gexicide Jun 15 '12 at 12:26
@gexicide It's not usually practical to form compile-time constant strings, anyway. (Besides literals, of course.) Printing the values of a sequence of arbitrary compile-time constant values and/or type-ids would be nice, though. – Potatoswatter Jun 17 '12 at 13:34
3

The link is dead now. – Ruslan Oct 17 '16 at 09:23

score 2 · Answer 2 · answered Jun 16 '12 at 09:50

You can still use BOOST_MPL_ASSERT_MSG. It accepts the generic types as parameters and tries to include the concrete type names in the error message.

More information and examples here: http://www.boost.org/doc/libs/1_49_0/libs/mpl/doc/refmanual/assert-msg.html

C++11 static_assert: Parameterized error messages

2 Answers2

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