Beispielprogramme zu C++

main.c

#include <initializer_list>
#include <iostream>
#include <ostream>
#include <string>
#include <vector>

using namespace ::std::literals;

struct sigma
{ sigma( ::std::string const s ): x{ 1 }{}
  sigma( ::std::initializer_list<::std::string> const s ): x{ 2 }{}
  operator bool(){ return false; }
  int x; };

sigma f( sigma x ){ return { "a"s, "b"s }; }

void try_out_sigma()
{ sigma s{ "abc"s };  ::std::cout << s.x << '\n';
  s = "abc"s;         ::std::cout << s.x << '\n';
  s ={ "abc"s };      ::std::cout << s.x << '\n';
  s ={ "a"s, "b"s };  ::std::cout << s.x << '\n';
  f( { "a"s, "b"s } );
  sigma t = sigma{ "a"s, "b"s }; ::std::cout << t.x << '\n';
  /* sigma u = static_cast< sigma >( { "a"s, "b"s }); *//* No! */
  sigma u = static_cast< sigma >( "a"s );
  ::std::vector< sigma >v;
  v.emplace_back( "a"s );
  /* v.emplace_back( { "a"s, "b"s } ); *//* No! Why? */
  if( s ){}
  if( sigma{ "a"s, "b"s } ){}
  if( sigma t = "abc"s ){}
  if( sigma t{ "a"s, "b"s } ){}
  ::std::cout << "done.\n"s;  }

struct eta
{ explicit eta( ::std::string const s ): x{ 1 }{}
  explicit eta( ::std::initializer_list<::std::string> const s ): x{ 2 }{}
  explicit operator bool(){ return false; }
  int x; };

void f( eta x ){ return; /* return { "a"s, "b"s }; */ /* No, explict! */ }

void try_out_eta()
{ eta s{ "abc"s };  ::std::cout << s.x << '\n'; /* still ok */
  /* s = "abc"s;         ::std::cout << s.x << '\n'; */ /* No, explict! */
  /* s ={ "abc"s };      ::std::cout << s.x << '\n'; */ /* No, explict! */
  /* s ={ "a"s, "b"s };  ::std::cout << s.x << '\n'; */ /* No, explict! */
  /* f( { "a"s, "b"s } ); */ /* No, explict! */
  eta t = eta{ "a"s, "b"s  }; ::std::cout << t.x << '\n'; /* still ok */
  eta u = static_cast< eta >( "a"s ); /* still ok */
  ::std::cout << u.x << '\n';
  ::std::vector< eta >v;
  v.emplace_back( "a"s ); /* still ok */
  if( s ){} /* still ok! Why? */ 
  if( eta{ "a"s, "b"s } ){}
  /* if( eta t = "abc"s ){} *//* No, explict! */
  if( eta t{ "a"s, "b"s } ){} /* still ok */
  ::std::cout << "done.\n"s;  }
 

int main()
{ try_out_sigma();
  try_out_eta(); }