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  1. // demo_string.h
  2.  
  3. #ifndef INCLUDED_STRING
  4. #define INCLUDED_STRING
  5.  
  6. #include <iosfwd>
  7. #include <string>
  8.  
  9. namespace demo
  10. {
  11.     // The class string is merely defined to have something which can be used
  12.     // to overload the output operator.
  13.     struct string
  14.         : std::string {
  15.         using std::string::string;
  16.     };
  17.     std::ostream& operator<< (std::ostream&, string const&);
  18.  
  19.     std::ostream& noquote(std::ostream&);
  20.     std::ostream& squote(std::ostream&);
  21.     std::ostream& dquote(std::ostream&);
  22.  
  23.     struct prefix
  24.         : std::string {
  25.         using std::string::string;
  26.     };
  27.     std::ostream& operator<< (std::ostream&, prefix const&);
  28. }
  29.  
  30. #endif
  31.  
  32. // demo_string.cpp
  33. // #include "demo/string.h"
  34. #include <ostream>
  35.  
  36. namespace
  37. {
  38.     int stringFormatIndex() {
  39.         static int rc = std::ios_base::xalloc();
  40.         return rc;
  41.     }
  42.  
  43.     void callback(std::ios_base::event ev, std::ios_base& s, int index) {
  44.         void* pword = s.pword(index);
  45.         switch (ev) {
  46.         case std::ios_base::erase_event:
  47.             delete static_cast<std::string*>(pword);
  48.             pword = 0;
  49.             break;
  50.         case std::ios_base::copyfmt_event:
  51.             pword = new std::string(*static_cast<std::string*>(pword));
  52.             break;
  53.         default:
  54.             break;
  55.         }
  56.     }
  57. }
  58.  
  59. std::ostream& demo::noquote(std::ostream& out) {
  60.     out.iword(stringFormatIndex()) = 0;
  61.     return out;
  62. }
  63. std::ostream& demo::squote(std::ostream& out) {
  64.     out.iword(stringFormatIndex()) = '\'';
  65.     return out;
  66. }
  67. std::ostream& demo::dquote(std::ostream& out) {
  68.     out.iword(stringFormatIndex()) = '"';
  69.     return out;
  70. }
  71.  
  72. std::ostream& demo::operator<< (std::ostream& out, demo::string const& str) {
  73.     int index{stringFormatIndex()};
  74.     if (out.pword(index)) {
  75.         out << *static_cast<std::string const*>(out.pword(index));
  76.     }
  77.     char quote(out.iword(index));
  78.     return quote? out << quote << str.c_str() << quote: out << str.c_str();
  79. }
  80.  
  81. std::ostream& demo::operator<< (std::ostream& out, demo::prefix const& p) {
  82.     void*& pword(out.pword(stringFormatIndex()));
  83.     if (pword) {
  84.         *static_cast<std::string*>(pword) = p;
  85.     }
  86.     else {
  87.         out.register_callback(&callback, stringFormatIndex());
  88.         pword = new std::string(p);
  89.     }
  90.     return out;
  91. }
  92.  
  93. // main.cpp
  94. // #include "demo/string.h"
  95. #include <iostream>
  96.  
  97. int main()
  98. {
  99.     demo::string value("whatever");
  100.     std::cout << demo::prefix("value=")
  101.               << value << ' '
  102.               << demo::squote  << value << ' '
  103.               << demo::dquote  << value << ' '
  104.               << demo::noquote << value << ' '
  105.               << '\n';
  106.         ;
  107. }
  108.  
Success #stdin #stdout 0s 3432KB
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value=whatever value='whatever' value="whatever" value=whatever
https://ideone.com/2T50aN
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

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