如何实现constexpr std::string_view与数字的乘法操作？

需求 #

想要实现类似Python中"{}"*8的功能，让std::string_view与数字相乘，以此简化fmt::format格式字符串中{}的重复书写。

编写的代码 #

inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
  std::array<char, sv.length() * times> result;
  constexpr char* data = result.data();
  for(int i=0; i<times; ++i){
    for(int j=0; j<sv.length(); ++j, ++data){
      *data = sv.data()[j];
    }
  }
  return result;
}

编译器错误信息 #

/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:114: error: template argument 2 is invalid
   96 | inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
      |                                                                                                                  ^
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:114: error: template argument 2 is invalid
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:114: error: template argument 2 is invalid
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:114: error: template argument 2 is invalid
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:78: error: invalid template-id
   96 | inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
      |                                                                              ^~~
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:97: error: use of parameter outside function body before ‘.’ token
   96 | inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
      |                                                                                                 ^
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:114: error: use of parameter outside function body before ‘>’ token
   96 | inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
      |                                                                                                                  ^
/home/miki/dumpefs_cpp/include/fs/f3s_spec.h:96:23: error: deduced class type ‘array’ in function return type
   96 | inline constexpr auto operator*(const std::string_view& sv, size_t times) -> std::array<char, sv.length() * times> {
      |                       ^~~~~~~~
In file included from /usr/include/c++/11/tuple:39,
                 from /usr/include/c++/11/bits/unique_ptr.h:37,
                 from /usr/include/c++/11/memory:76,
                 from /home/miki/dumpefs_cpp/spdlog-1.11.0/include/spdlog/fmt/bundled/format.h:40,
                 from /home/miki/dumpefs_cpp/spdlog-1.11.0/include/spdlog/fmt/bundled/compile.h:11,
                 from /home/miki/dumpefs_cpp/include/fs/f3s_spec.h:23,
                 from /home/miki/dumpefs_cpp/src/dumpefs.cpp:15:
/usr/include/c++/11/array:95:12: note: ‘template<class _Tp, long unsigned int _Nm> struct std::array’ declared here
   95 |     struct array
      |            ^~~~~

疑问 #

• constexpr环境下似乎不能使用char指针，那该如何完成字符复制？
• 函数返回类型std::array<char, sv.length() * times>中的参数都是const属性，为什么模板参数2不被允许？

编译器信息 #

• C++编译器路径：/usr/bin/g++11
• C++编译器版本：11.3.0
• C++编译选项：-fdiagnostics-color=always -g -O0

内容的提问来源于stack exchange，提问作者MikimotoH