Constexpr string literals in anonymous namespace -> fix

bar.cpp

#include "bar.h"

#include <iostream>

namespace bar {

extern const char extern_delimiter[] = "</extern_delimiter>";

const char Foo::struct_delimiter[] = "</struct_delimiter>";

Bar::Bar() {
  std::cout << "Bar constructor!" << std::endl;
}

}

bar.h

#pragma once

#include <iostream>

/* This class reads until the "Delimiter" */
template<const char *Delimiter>
struct Reader {
  Reader() {
    std::cout << "Reader \"delimiter\" set to: " << Delimiter << std::endl;
  }
};

namespace bar {

// Option 1
extern const char extern_delimiter[];

// Option 2 - Gives "whose type uses the anonymous namespace" warning
static constexpr char constexpr_delimiter[] = "</constexpr_delimiter>";

// Option 3
struct Foo {
  static const char struct_delimiter[];
};

struct Bar {
  Bar();
  Reader<extern_delimiter> reader_extern_delimiter;
  Reader<constexpr_delimiter> reader_constexpr_delimiter;
  Reader<Foo::struct_delimiter> reader_struct_delimiter;
};

}// namespace bar

CMakeLists.txt

cmake_minimum_required(VERSION 3.17)
project(foo)

set(CMAKE_CXX_STANDARD 11)

add_executable(example main.cpp bar.cpp)

/* Build output: 

[ 66%] Building CXX object CMakeFiles/example.dir/main.cpp.o
[ 66%] Building CXX object CMakeFiles/example.dir/bar.cpp.o
In file included from main.cpp:2:0:
bar.h:27:8: warning: ‘bar::Bar’ has a field ‘bar::Bar::reader_constexpr_delimiter’ whose type uses the anonymous namespace [enabled by default]
 struct Bar {
        ^
In file included from bar.cpp:2:0:
bar.h:27:8: warning: ‘bar::Bar’ has a field ‘bar::Bar::reader_constexpr_delimiter’ whose type uses the anonymous namespace [enabled by default]
 struct Bar {
        ^
[100%] Linking CXX executable example
[100%] Built target example

Build finished

*/

main.cpp

#include "bar.h"

int main () {
  bar::Bar bar;
  return 0;
}

/* Console output:

Reader "delimiter" set to: </extern_delimiter>
Reader "delimiter" set to: </constexpr_delimiter>
Reader "delimiter" set to: </struct_delimiter>
Bar constructor!

*/