if and switch with Initialization

if with initialization

• Any value initialized inside an if statement is valid until the end of the then and the else part. For example,

   #include <iostream>
   #include <vector>
   #include <fstream>
   #include <string>
     
   int main() {
      std::vector<std::string> coll{"Hello","World","Dear"};
      if (std::ofstream strm = std::ofstream("output.txt"); coll.empty())
      {
         strm << "no data\n";
      }else{
         for (const auto& e : coll) {
            strm << e << "\n";
         }
      }
   }
  

  The destructor for strm is called at the end of the whole if statement (at the end of the else part, if any, or at the end of the then part, otherwise).

• Multiple declarations with optional initializations are also allowed:

   if (auto x{qqq1()}, auto y{qqq2()}; x != y){
      std::cout << "x and y differs\n";
   }
  

• Consider inserting a new element into a map or unordered map.

   #include <iostream>
   #include <string>
   #include <map>
     
   int main() {
      std::map<int,std::string> m;
      if (auto [pos,ok] = m.insert({1,"Hi"}); ok){
         if (ok){
            auto& [key, val] = *pos;
            std::cout << key << ":" << val << "\n";
         }else{
            std::cout << "Failed\n";
         }
      }
   }
  

  switch with initialization

qqq/
├── aaa
│   └── bbb
├── ccc
│   └── bbb
└── xxx

#include <iostream>
#include <filesystem>
#include <string>

int main() {
   namespace fs = std::filesystem;

   std::string name{"qqq"};

   switch( fs::path p{name}; status(p).type() ) {

      case fs::file_type::not_found:
         std::cout << p << " not found\n";
         break;
      case fs::file_type::directory:
         std::cout << p << ": \n";
         for ( const auto& e : std::filesystem::directory_iterator{p} ) {
            std::cout  << "- " << e.path() << "\n";
         }
         break;
      default:
         std::cout << p << " is found but not a directory\n";
         break;
   }
}

Result:

"qqq":
- "qqq/aaa"
- "qqq/ccc"
- "qqq/xxx"