'运算符<<（std :: ostream＆，int＆）'不明确

Question

Isn't this:

operator<<(std::cout, 0);

The same as this?

std::cout<<0;

I tried this piece of code:

#include<iostream>
int main()
{
    operator<<(std::cout,0);
    return 0;
}

But I got the following error message:

a.cpp: In function ‘int main()’:
a.cpp:11:28: error: call of overloaded ‘operator<<(std::ostream&, int)’ is ambiguous
a.cpp:11:28: note: candidates are:
/usr/include/c++/4.6/ostream:528:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:523:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:510:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/bits/ostream.tcc:323:5: note: std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:473:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:468:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:462:5: note: std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char) [with _Traits = std::char_traits<char>]
/usr/include/c++/4.6/ostream:456:5: note: std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char) [with _CharT = char, _Traits = std::char_traits<char>]

Can someone explain it please?

Answer 1

No, it is the same as:

std::cout.operator<<(0);

Using operator<<(std::cout, 0); makes Argument-Dependent Lookup (ADL) kick in, which finds multiple candidates that accept a std::basic_ostream<char> and an int (or a type that has a valid implicit conversion from int ) as input. Once ADL kicks in, all of these various overloads become valid candidates.