问题
I have the following test program:
#include <string>
#include <iostream>
int main()
{
std::string s;
std::string a = "sd";
std::cout << a==s ? "y" : "n";
return 0;
}
Trying to compile this with g++ test.cpp gives the following cryptic error:
error: no match for 'operator==' (operand types are 'std::basic_ostream<char>' and 'std::string {aka std::basic_string<char>}')
std::cout << a==s ? "y" : "n";
^
It seems that s is being correctly compiled as type std::string, while a is being compiled as std::basic_ostream<char>!? HELP!!
回答1:
The compiler parsed your statement as ((std::cout << a) == s) ? "y" : "n"; because of operators precedence : You need parentheses.
std::cout << (a==s ? "y" : "n");
回答2:
The error message from the compiler is very helpful here. It's saying the LHS of the operator is of type std::basic_ostream<char> while the RHS of the operator is of type std::string {aka std::basic_string<char>}.
i.e. the line
std::cout << a==s ? "y" : "n";
is being interpreted as
(std::cout << a) == s ? "y" : "n";
To change the compiler to take the right objects for the == operator, you'll have to use parantheses to override that behavior.
std::cout << (a==s ? "y" : "n");
来源：https://stackoverflow.com/questions/25558482/gcc-inconsistent-about-string-type