Passing a string literal as a type argument to a class template

I want to declare a class template in which one of the template parameters takes a string literal, e.g. my_class<"string">.

Can anyone give me some compilable code which declares a simple class template as described?

Note: The previous wording of this question was rather ambiguous as to what the asker was actually trying to accomplish, and should probably have been closed as insufficiently clear. However, since then this question became multiple times referred-to as the canonical ‘string literal type parameter’ question. As such, it has been re-worded to agree with that premise.

You can have a const char* non-type template parameter, and pass it a const char[] variable with static linkage, which is not all that far from passing a string literal directly.

#include <iostream>    

template<const char *str> 
struct cts {
    void p() {std::cout << str;}
};

static const char teststr[] = "Hello world!";
int main() {
    cts<teststr> o;
    o.p();
}

http://coliru.stacked-crooked.com/a/64cd254136dd0272

Further from Neil's answer: one way to using strings with templates as you want is to define a traits class and define the string as a trait of the type.

#include <iostream>

template <class T>
struct MyTypeTraits
{
   static const char* name;
};

template <class T>
const char* MyTypeTraits<T>::name = "Hello";

template <>
struct MyTypeTraits<int>
{
   static const char* name;
};

const char* MyTypeTraits<int>::name = "Hello int";

template <class T>
class MyTemplateClass
{
    public:
     void print() {
         std::cout << "My name is: " << MyTypeTraits<T>::name << std::endl;
     }
};

int main()
{
     MyTemplateClass<int>().print();
     MyTemplateClass<char>().print();
}

prints

My name is: Hello int
My name is: Hello

Sorry, C++ does not currently support the use of string literals (or real literals) as template parameters.

But re-reading your question, is that what you are asking? You cannot say:

foo <"bar"> x;

but you can say

template <typename T>
struct foo {
   foo( T t ) {}
};

foo <const char *> f( "bar" );

C++20 fixed_string + "Class Types in Non-Type Template Parameters"

Apparently, a proposal for this was first accepted, but then removed: "String literals as non-type template parameters"

The removal was partly because it was deemed to be easy enough to do with another proposal that was accepted: "Class Types in Non-Type Template Parameters".

The accepted proposal contains an example with the following syntax:

template <std::basic_fixed_string Str>
struct A {};
using hello_A = A<"hello">;

I'll try to update this with an example that actually tells me anything once I see a compiler that supports it.

A Redditor has also shown that the following compiles on GCC master, provided you define your own version of basic_fixed_string which was not in the standard library yet: https://godbolt.org/z/L0J2K2

template<unsigned N>
struct FixedString {
    char buf[N + 1]{};
    constexpr FixedString(char const* s) {
        for (unsigned i = 0; i != N; ++i) buf[i] = s[i];
    }
    constexpr operator char const*() const { return buf; }
};
template<unsigned N> FixedString(char const (&)[N]) -> FixedString<N - 1>;

template<FixedString T>
class Foo {
    static constexpr char const* Name = T;
public:
    void hello() const;
};

int main() {
    Foo<"Hello!"> foo;
    foo.hello();
}

g++ -std=c++2a 9.2.1 from the Ubuntu PPA fails to compile that with:

/tmp/ccZPAqRi.o: In function `main':
main.cpp:(.text+0x1f): undefined reference to `_ZNK3FooIXtl11FixedStringILj6EEtlA7_cLc72ELc101ELc108ELc108ELc111ELc33EEEEE5helloEv'
collect2: error: ld returned 1 exit status

Bibliography: https://botondballo.wordpress.com/2018/03/28/trip-report-c-standards-meeting-in-jacksonville-march-2018/

Finally, EWG decided to pull the previously-approved proposal to allow string literals in non-type template parameters, because the more general facility to allow class types in non-type template parameters (which was just approved) is a good enough replacement. (This is a change from the last meeting, when it seemed like we would want both.) The main difference is that you now have to wrap your character array into a struct (think fixed_string or similar), and use that as your template parameter type. (The user-defined literal part of P0424 is still going forward, with a corresponding adjustment to the allowed template parameter types.)

This will be especially cool with the C++17 if constexpr: if / else at compile time in C++?

This kind of feature appears to be in line with the awesome "constexpr everything" proposals that went into C++20, such as: Is it possible to use std::string in a constexpr?

Passing a string literal as a type argument to a class template

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