#include <iostream>
#include <string>

int main()
{
        char s[] = "ABCD";
        std::string str(s);

        char *p = s;
        while(*p) {
                *p++ = tolower(*p);          // <-- incr after assignment
        }
        std::cout << s << std::endl;

        std::string::iterator it = str.begin(),end = str.end();
        while(it != end) {
                *it++ = tolower(*it);        // <-- incr before assignment ?
        }
        std::cout << str << std::endl;

        return 0;
}

它产生输出：

abcd
bcd

如果我们分配赋值操作和增量运算符：

while(it != end) {
  *it = tolower(*it);        // <-- incr before assignment ?
  it++;
}

输出将如预期.

原始代码有什么问题？

$g++ --version
g++ (GCC) 3.4.4 (cygming special,gdc 0.12,using dmd 0.125)
Copyright (C) 2004 Free Software Foundation,Inc.