Is it possible to make a string as a template parameter?

Question

Is it possible to make a string as a template parameter and how? Like

A<"Okay"> is a type.

Any string (std::string or c-string) is fine.

Yes, but it is not terribly helpful because it doesn't behave as you would expect (it uses pointer equality, not string equality). — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Jul 11, 2013 at 15:40
Well... you could probably make a variadic template that takes in a bunch of characters as template inputs. At least it would give you real string equivalence. std::string will not work though because template arguments must be integral types. — Suedocode
– Suedocode, Commented Jul 11, 2013 at 15:59
Possible duplicate of Strings as template arguments?. See also Non-type template parameters — gx_
– gx_, Commented Jul 11, 2013 at 16:01
@Aggieboy Or pointers or references to objects with external linkage. (String literals don't work because they don't have external linkage.) — James Kanze
– James Kanze, Commented Jul 11, 2013 at 16:07
@JamesKanze Ahh details; a reference is just a pointer, a pointer is just an int, and an int is an integral. XD — Suedocode
– Suedocode, Commented Jul 11, 2013 at 16:47

James Kanze · Accepted Answer · 2013-07-11 16:05:48Z

2

Yes, but you need to put it in a variable with external linkage (or does C++11 remove the requirement for external linkage). Basically, given:

template <char const* str>
class A { /* ... */ };

this:

extern char const okay[] = "Okay";

A<okay> ...

works. Note thought that it is not the contents of the string which define uniqueness, but the object itself:

extern char const okay1[] = "Okay";
extern char const okay2[] = "Okay";

Given this, A<okay1> and A<okay2> have different types.

answered Jul 11, 2013 at 16:05

James Kanze

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Suedocode · Accepted Answer · 2013-07-11 16:41:29Z

Here's one way to make the string contents determine uniqueness

#include <windows.h> //for Sleep()
#include <iostream>
#include <string>

using namespace std;

template<char... str>
struct TemplateString{
    static const int n = sizeof...(str);
    string get() const {
        char cstr[n+1] = {str...}; //doesn't automatically null terminate, hence n+1 instead of just n
        cstr[n] = '\0'; //and our little null terminate
        return string(cstr); 
    }
};

int main(){
    TemplateString<'O','k','a','y'> okay;
    TemplateString<'N','o','t',' ','o','k','a','y'> notokay;
    cout << okay.get() << " vs " << notokay.get() << endl;
    cout << "Same class: " << (typeid(okay)==typeid(notokay)) << endl;
    Sleep(3000); //Windows & Visual Studio, sry
}

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