Was macht std :: cout << std :: cin?

Question

Ich bin neu in C++ und habe mich mit einigen der Dinge beschäftigt, die ich gelernt habe. Also versuchte ich den folgenden Code:

#include <iostream> 
int main() { 
    std::cout << std::cin; 
} 

Also habe ich den Code erwartet einen Fehler zurückgibt, sondern bekam, was ich denke, ist eine Speicheradresse (0x6fccc408). Auch wenn ich es mehrfach ausführte, bekam ich die gleiche Speicheradresse, auch nach dem Neustart von cmd. Was genau bedeutet diese Speicheradresse?

Answer 1

Es scheint, als ob Sie C++ 98 oder C++ 03 verwenden. In diesen Versionen cin könnte implizit in eine void* konvertiert werden, was Sie sehen, ist diese Konvertierung.

Wenn Sie C++ 11 verwendet, die von der impliziten void* Umwandlung losgeworden und implementiert stattdessen eine explizite Umwandlung dieser in bool würde wie

einen Compiler-Fehler erzeugen

clang++ -std=c++11 -O2 -Wall -pedantic -pthread main.cpp && ./a.out 
main.cpp:4:15: error: invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'istream' (aka 'basic_istream<char>')) 
    std::cout << std::cin; 
    ~~~~~~~~~^~~~~~~~~ 
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:245:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const void *' for 1st argument; take the address of the argument with & 
     operator<<(const void* __p) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/system_error:209:5: note: candidate function [with _CharT = char, _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const std::error_code' for 2nd argument 
    operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:108:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to '__ostream_type &(*)(__ostream_type &)' (aka 'basic_ostream<char, std::char_traits<char> > &(*)(basic_ostream<char, std::char_traits<char> > &)') for 1st argument 
     operator<<(__ostream_type& (*__pf)(__ostream_type&)) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:117:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to '__ios_type &(*)(__ios_type &)' (aka 'basic_ios<char, std::char_traits<char> > &(*)(basic_ios<char, std::char_traits<char> > &)') for 1st argument 
     operator<<(__ios_type& (*__pf)(__ios_type&)) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:127:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'std::ios_base &(*)(std::ios_base &)' for 1st argument 
     operator<<(ios_base& (*__pf) (ios_base&)) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:166:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'long' for 1st argument 
     operator<<(long __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:170:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'unsigned long' for 1st argument 
     operator<<(unsigned long __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:174:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'bool' for 1st argument 
     operator<<(bool __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:178:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'short' for 1st argument 
     operator<<(short __n); 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:181:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'unsigned short' for 1st argument 
     operator<<(unsigned short __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:189:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'int' for 1st argument 
     operator<<(int __n); 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:192:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'unsigned int' for 1st argument 
     operator<<(unsigned int __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:201:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'long long' for 1st argument 
     operator<<(long long __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:205:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'unsigned long long' for 1st argument 
     operator<<(unsigned long long __n) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:220:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'double' for 1st argument 
     operator<<(double __f) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:224:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'float' for 1st argument 
     operator<<(float __f) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:232:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'long double' for 1st argument 
     operator<<(long double __f) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:270:7: note: candidate function not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to '__streambuf_type *' (aka 'basic_streambuf<char, std::char_traits<char> > *') for 1st argument 
     operator<<(__streambuf_type* __sb); 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:502:5: note: candidate function [with _CharT = char, _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'char' for 2nd argument 
    operator<<(basic_ostream<_CharT, _Traits>& __out, char __c) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:508:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'char' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, char __c) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:514:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'signed char' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, signed char __c) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:519:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'unsigned char' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:556:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const char *' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, const char* __s) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:569:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const signed char *' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:574:5: note: candidate function [with _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const unsigned char *' for 2nd argument 
    operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:628:5: note: candidate function [with _CharT = char, _Traits = std::char_traits<char>, _Tp = std::basic_istream<char>] not viable: no known conversion from 'ostream' (aka 'basic_ostream<char>') to 'basic_ostream<char, std::char_traits<char> > &&' for 1st argument 
    operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/bits/ostream.tcc:321:5: note: candidate function [with _CharT = char, _Traits = std::char_traits<char>] not viable: no known conversion from 'istream' (aka 'basic_istream<char>') to 'const char *' for 2nd argument 
    operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:497:5: note: candidate template ignored: deduced conflicting types for parameter '_CharT' ('char' vs. 'std::basic_istream<char>') 
    operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c) 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/bits/basic_string.h:5325:5: note: candidate template ignored: could not match 'basic_string' against 'basic_istream' 
    operator<<(basic_ostream<_CharT, _Traits>& __os, 
    ^
/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.1.0/../../../../include/c++/6.1.0/ostream:539:5: note: candidate template ignored: could not match 'const _CharT *' against 'istream' (aka 'basic_istream<char>') 
    operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s) 
    ^
1 error generated.