c++ 字符串长度计算

Question

如下代码：上下两段计算出来的sizeof(*p)均为1，但是后面注释（不是我写的）为什么一个是contains a null character，一个是didn't contain null character？
//string 转化为char*
string str="good boy";
const char *p=str.c_str(); //contains a null character
cout << str<< endl;
cout << *p << sizeof(*p)<< endl;

/*string str="good boy";
const char *p=str.data(); //didn't contain null character
cout << str<< endl;
cout << *p << sizeof(*p)<< endl;
*/
return 0;

Answer 1

c_str()返回的指针指向null-terminated(字符串结束符'\0')的字符数组，即最后位置是一个null字符。

data()在C++11之前返回数组是不以'\0'结束的，在C++11之后功能和c_str()相同了。

 

原文参考链接：

https://en.cppreference.com/w/cpp/string/basic_string/c_str

Returns a pointer to a null-terminated character array with data equivalent to those stored in the string.

The pointer is such that the range [c_str(); c_str() + size()] is valid and the values in it correspond to the values stored in the string with an additional null character after the last position.

 

https://en.cppreference.com/w/cpp/string/basic_string/data

The returned array is not required to be null-terminated.(until C++11)

The returned array is null-terminated, that is, data() and c_str() perform the same function.(since C++11)