length()函数

length()函数用来获取字符串长度。

string s = "hello";
int len = s.length();
cout << "len = " << len << endl;

输出为：len = 5

size()函数

跟length()函数一样，可以获得字符串长度。但是size()函数还可以用来获取vector的长度。

string s = "hello";
vector<int> nums = {1,2,3,4};

int len1 = s.length();
int len2 = nums.size();

cout << "len1 = " << len1 << endl;
cout << "len2 = " << len2 << endl;

输出为：len1 = 5 len2 = 4

strlen()函数

只能用来计算字符型数组的长度。

char s[30] = "abcdefg";
int len = strlen(s);
cout << "len = " << len << endl;

输出为：len = 7

sizeof()函数

sizeof()是操作符，用来获取对象所占内存字节大小。

char s[30] = "abcdefg";
int n = sizeof(s);
cout << "n = " << n << endl;

输出为：n = 30