'std::string': illegal type for non-type template parameter

[gedamial]

The code I'm trying to compile is this

template<typename T, T default_value = T{}>
struct Vec
{


};

This code is taken from Bjarne Stroustrup's "The C++ Programming Language" book at page 725

What this code does is obvious... but when I try to instantiate the struct with a std::string the error in the title appears

Vec<string> s;

Error: 'std::string': illegal type for non-type template parameter

Why?

---

I wanted to try this code because on Bjarne Stroustrup's book he makes this example, and he writes the following:

Vec<string>   // default_value is int*{} that is, nullptr 

I said: WHAT?

That's a string, why should T be evaluated as an int*?

So i compiled the code myself and got this error...

[mbozzi]

Error: 'std::string': illegal type for non-type template parameter

Template non-type parameters cannot be objects of class type. See, e.g.,: http://en.cppreference.com/w/cpp/language/template_parameters

I'm looking at my copy (the 4e) and I can't find any mention of this on page 725. I expect that you have the fourth edition too, thanks to mention of nullptr. With that in mind, I checked the errata (at http://www.stroustrup.com/4th_printing3.html ) and found nothing relevant.

Various text searches (I have a PDF copy) similarly yield nothing. In particular, grepping for Vec<string> yields exactly one result:
Vec<string> c22; // default_value is string{}; that is, ""

Where exactly does the issue appear? What book do you have?

[gedamial]

Yes I have the fourth edition too

I took some pictures of it, link to see them >> http://imgur.com/a/xQshl

Very strange situation...

Either way... the code should work... my code is the same as the one in the book...

Template non-type parameters cannot be objects of class type

In this case the non-type parameter is default_value that gets evaluated as string{}
Since string{} is an object of a class-type, it is not allowed.

Correct?

But... why then the book reports this example if it is not allowed?

[mbozzi]

But... why then the book reports this example if it is not allowed?

It's a typo. (Or both of us are misunderstanding it.)

[gedamial]

It's a typo.

Do you have these typos as well?

I see (and you can see too with the screenshots) that Bjarne uses the string class as non-type parameter twice

Vec<string,"">

Lots of typos?

[mbozzi]

You can use at least the following types as template non-type parameters [1, 2]:
- std::nullptr_t
- integral type;
- lvalue reference type (to object or to function);
- pointer type (to object or to function);
- pointer to member type (to member object or to member function);
- enumeration type.

String literals are not usable as template non-type arguments[3, 4]:; not even lvalue expressions with linkage satisfying LiteralType[4] are allowed[1]
Evidence:
http://coliru.stacked-crooked.com/a/9c72d6ed409a49e1

because string literals have no linkage - there is no name to link. In my copy of the book, this example is given (pp. 724):

X<int,"BMW323Ci"> x1; // error : string literal as template argument
char lx2[] = "BMW323Ci";
X<int,lx2> x2; // OK: lx2 has external linkage 

It seems like Stroustrup forgot that string literals are unusable in the context of a template non-type argument, or that behavior changed late during the standardization process.

1: http://en.cppreference.com/w/cpp/language/template_parameters#Template_non-type_arguments
2: http://eel.is/c++draft/temp.param#4
3: http://eel.is/c++draft/temp.arg.nontype#2
4: http://en.cppreference.com/w/cpp/concept/LiteralType

Do you have these typos as well?

Some: I have
Vec<string,"fortytwo"> c2; and
Vec<string,""> c2; and
Vec<string> c22;
And those are the only errors I spot. The strange thing is that Stroustrup mentions this exact thing is disallowed right before making the error. Perhaps someone else should check me on this.

[Enoizat]

You are not the only one who pointed out this issue:
https://stackoverflow.com/questions/20066104/is-a-string-literal-not-acceptable-as-a-template-argument-in-c/20066609
It seems there's a wrong example in that book.

[gedamial]

I don't have those examples in my book, check my screenshot of page 725 >> http://imgur.com/a/xQshl

It doesn't seem to be the same...