Is it possible in modern C++ to pass a string literal as a parameter to a C++ template?

Question

Is it possible in "modern C++" (C++17 or greater) to pass a string literal as a parameter to a C++ template?

I realize you could do this with constructor argument; I just thought it would be more convenient to have it as a template argument, rather than buried deep in the cpp file. I was curious if maybe this was a new feature of modern C++. See Pseudo code below of what I'm trying to do:

Pseudo-code Example:

// Header File ///////////////////////// template<constexpr string Name> class ModuleBase { public:     ModuleBase();     string name; };  class xyz : ModuleBase<"xyz"> { public:     xyz(); };  // Cpp File ////////////////////////// template<string_literal Name> ModuleBase<Name>::ModuleBase() {     name = Name; }  xyz::xyz() : ModuleBase() {  } 

Answer 1

Yes, in c++20.

The problem was that determining uniqueness of a template non-type argument was difficult.

c++20 adds in a <=> spaceship operator comparison. If it is non-user provided (and based only off non-user provided <=> in turn, repeat recursively) (and a few other requirements; see p0732), the type can be used as a non-type template argument.

Such types can be constructed from raw "strings" in constexpr constructors, including using c++17 deduction guides to make them auto-size themselves.

As the size of the data stored is probably going to be part of the type, you'll want to take the type as an auto typed non-type parameter or otherwise auto-deduced type.

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Note that placing the implementation of your template in a cpp file is usually a bad idea. But that is another question.

Answer 2

Until you get c++20 and if you have boost, you may find the following macro usefull:

#define C_STR(str_) boost::mpl::c_str< BOOST_METAPARSE_STRING(str_) >::value 

Then use as follows:

template<const char* str> structe testit{ }; testit<C_STR("hello")> ti;