c++ - Should static_assert be triggered with a typedef? -

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i noticed static assertions in class templates not triggered when instantiations typedef'ed.

#include <type_traits>  template <typename t> struct test_assert {     static_assert( std::is_same< t, int >::value, "should fail" ); };  typedef test_assert< float > t;

this code compiles without error. if try create instance, assertion fails:

t obj; // error: static assertion failed: "should fail"

finally, if replace condition false, assertion fails if don't instantiate class template:

template <typename t> struct test_assert {     static_assert( false, "always fails" ); };

i tried code on gcc-4.5.1 , gcc-4.7.0. behavior normal? @ time compiler supposed verify static assertions? guess two-phase lookup involved, shouldn't typedef trigger second phase?

i tried code on gcc-4.5.1 , gcc-4.7.0. behavior normal?

yes

at time compiler supposed verify static assertions?

this interesting question. during instantiation, first phase lookup non-dependent names , second lookup phase asserts depend on template arguments.

guess two-phase lookup involved, shouldn't typedef trigger second phase?

templates compiled on demand, typedef creates alias template , not trigger instantiation. consider following code:

template <typename t> class unique_ptr; typedef unique_ptr<int> int_unique_ptr;

the template declared, suffices typedef, generates alias type. on other side, if create object of type, template must instantiated (again on demand, member functions not instantiated).


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