什麼是std :: cout << std :: cin做什麼？

Question

我是C++的新手，並且對我學到的一些東西感到厭煩。 所以，我想下面的代碼：

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

所以我預期的代碼返回一個錯誤，而是得到了什麼，我認爲是一個內存地址（0x6fccc408）。而且，當多次運行它時，即使在重新啓動cmd之後，我也獲得了相同的內存地址。這個內存地址究竟意味着什麼？

Answer 1

看起來您正在使用C++ 98或C++ 03。在這些版本中，cin可能會隱式轉換爲void*，因此您所看到的是轉換。

如果您使用C++ 11，它擺脫了隱void*轉換，而是實現了一個顯式轉換爲bool，這將產生像

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.