C++ string comparison

#include "stdafx.h"
#include "iostream"
#include "string"

using namespace std;

void main()
{
string a = "a";
string b(1, -70); /*constructor, create a string having 1 character that its value is equal to -70*/
cout<<((b>a)?b:a);
}

//output on screen: b was printed, not a (!)

Why b>a although the value of b is less than the value of a? How can I rectify this s开发者_如何转开发ituation?

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On VS2010, I found that the char was signed - hence not the desired explanation. Stepping through the debugger on the comparison, I eventually hit the code:

template<> struct char_traits<char>
 { // properties of a string or stream char element
 typedef char _Elem;
 typedef _Elem char_type;
 typedef int int_type;
 typedef streampos pos_type;
 typedef streamoff off_type;
 typedef _Mbstatet state_type;

 static int __CLRCALL_OR_CDECL compare(const _Elem *_First1, const _Elem *_First2,
  size_t _Count)
  { // compare [_First1, _First1 + _Count) with [_First2, ...)
  return (_CSTD memcmp(_First1, _First2, _Count));
  }
// etc
};

So the real comparision boils down to a memcmp. Checking that out, we find "evaluated as unsigned char values", hence the problem.

Cf. Arytom's answer - Interesting, I did not know this. Looking it up:

The 1998 standard 21.1.3.1:6 on char_traits states 'lt' is defined identically to the built in operator <.

The N3126 draft, 21.2.3.1:5 states it should be as for unsigned char.

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Strings are always compared as unsigned char regardless of the signess of actual character.

std::string took this behavior from C where strcmp uses characters in range 0-255 even if the char is in range -128 -- 127.

So basically -70 is 186 and 186 > 'a'

Edit: Reference to standard. I don't have C++98/2003 near me just C++0x, but:

"Working Draft, Standard for Programming Language C++", N2915, 21.2.3.1, remark 5 says:

The two-argument members eq and lt shall be defined identically to the built-in operators == and < for type unsigned char.

i.e. characters compared as unsigned char. (it references to character traits specialization for char)

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See the binary equivalent value of -70 as a 1 character.