取消引用字符串迭代器会产生int

Question

I get this error

comparison between pointer and integer ('int' and 'const char *')

For the following code

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

int main()
{
    std::string s("test string");
    for(auto i = s.begin(); i != s.end(); ++i)
    {
        cout << ((*i) != "s") << endl;
    }
}

Why does dereferencing the string iterator yield an int and not std::string ?

Answer 1

Actually, it does not yield an int , it yields a char (because a string iterator iterates over the characters in the string). Since the other operand of != is not a char (it's a const char[2] ), standard promotions and conversions are applied to the arguments:

• char is promoted to int via integral promotion
• const char[2] is converted to const char* via array-to-pointer conversion,

This is how you arrive at the int and const char* operands the compiler is complaining about.

You should compare the dereferenced iterator to a character, not to a string:

cout << ((*i) != 's') << endl;

"" encloses a string literal (type const char[N] ), '' encloses a character literal (type char ).