问题来源

在牛客网的讨论群里,有人提出了这样的问题代码:

char *p  = NULL;
cout << p;
******
int *q = NULL;
cout << q;

上述代码在windows下面使用vs,一个会报错一个不会报错~ 但是在linux下面并不会报错~

猜想

• 字符指针是直接打印指针指向的值(字符串),访问了空指针的内容;
• int指针,输出是打印指针的值(null=0),所以不会报错.
• 而linux下面不会报错,那肯定是不同编译器的处理不一样了~

验证

• 程序在ubuntu14.04上跑的,使用g++
• 并没有去windows下面验证,但看过别人运行类似程序的结果
  

  程序如下:

#include<iostream>
#include<cstring>
using namespace std;
int main()
{
    char a[] = "hello,world!";
    cout << "a‘s address is " << &a << endl;
    char *p = a;
    cout <<"the p‘s value is: " << (void *)(p) << endl;
    cout << "the content is: " << p << endl;

    p = NULL;
    cout <<"the p‘s value is: " << (void *)(p) << endl;
    cout << "the content is: " << p << endl;
    if(cout.bad()) {        
        cout.clear();
        cout << endl;
        cout << "  If @p __sb is NULL, the stream will set failbit in its error state." << endl;
    }


    int *q = NULL;
    cout <<"q‘s value is " << q << endl;
    cout << "the content is " << *q << endl;
    return 0;
}

编译运行:

zy@zyPc:~/code/C/pointer$ ./a.out 
a‘s address is 0xbf897d7f
the p‘s value is: 0xbf897d7f
the content is: hello,world!
the p‘s value is: 0
the content is: 
  If @p __sb is NULL, the stream will set failbit in its error state.
q‘s value is 0
段错误 (核心已转储)

结论

• 如猜测的一样, cout 的<< 在重载时,遇到char * 和int *的输出解释是不一样的
• g++在遇到空字符指针时,会在错误状态中,置为fallbit,这个通过查看源码是可以找到的:
  

/**

• @brief Extracting from another streambuf.
• @param __sb A pointer to a streambuf
  *
• This function behaves like one of the basic arithmetic extractors,
• in that it also constructs a sentry object and has the same error
• handling behavior.
  *
  * If @p __sb is NULL, the stream will set failbit in its error state.
  *
• Characters are extracted from @p __sb and inserted into @c *this
• until one of the following occurs:
  *
• • the input stream reaches end-of-file,
• • insertion into the output sequence fails (in this case, the
• character that would have been inserted is not extracted), or
• • an exception occurs while getting a character from @p __sb, which
• sets failbit in the error state
  *
• If the function inserts no characters, failbit is set.
  */
  __ostream_type&
  operator<<(__streambuf_type* __sb);