checking const char* as a non-type template argument

Question

using msvc 14, i would like to check a const char* argument as a non-type template argument.

template <typename T, const char Key[] = nullptr > class Base { public: Base() {} void f_not_null() { static_assert(Key != nullptr, "may be not null"); } void f_null() { static_assert(Key == nullptr, "may be null"); } }; extern const char A_STRING[] = "a string"; Base<int, A_STRING> test; test.f_not_null(); Base<int> test1; test1.f_null();

The code below does not compile due to error C2131: expression did not evaluate to a constant on static_assert(Key != nullptr, "may be not null") when declaring Base<int, A_STRING>.

If possible I would not like to use a wrapper as a workaround:

template <const char *str> class Str { }; Base<int, Str<A_STRING> > test; [...]

Is there a way to make this possible?

If your variable is extern, why is it declared here?

Matthieu Brucher
– Matthieu Brucher

2018-11-11 18:45:32 +00:00
Commented Nov 11, 2018 at 18:45 — Matthieu Brucher
– Matthieu Brucher, Commented Nov 11, 2018 at 18:45

catnip · Accepted Answer · 2018-11-11 18:56:59Z

1

As Matthieu says, this will work if (and only if) you change:

extern const char A_STRING[] = "a string";

to:

static const char A_STRING[] = "a string";

Live Demo

answered Nov 11, 2018 at 18:56

catnip

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3 Comments

E. Jorge Over a year ago

Unfortunately, i m not using gcc, i use msvc 14 and this does not work and fall into

error C2971: 'Base': template parameter 'Key': 'A_STRING': a variable with non-static storage duration cannot be used as a non-type argument

which is why i've used extern.

E. Jorge Over a year ago

my code sample is a brief sample of a code using extern on a const char* declared into another compilation unit and is set to use a static storage duration with external linkage.

catnip Over a year ago

Yes, you're right, MSVC won't compile the code I posted, see: rextester.com/DUZQB23817. I would call that an MSVC bug, since both gcc and clang are perfectly happy and the error message produced by MSVC makes no sense. I think you are stuck here.

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