C++ 如何使用strlen函数获取字符串的长度
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How to get length of a string using strlen function
提问by Akash Sharma
I have following code that gets and prints a string.
我有以下获取和打印字符串的代码。
#include<iostream>
#include<conio.h>
#include<string>
using namespace std;
int main()
{
string str;
cout << "Enter a string: ";
getline(cin, str);
cout << str;
getch();
return 0;
}
But how to count the number of characters in this string using strlen()function?
但是如何使用strlen()函数计算这个字符串中的字符数？
回答by templatetypedef
For C++ strings, there's no reason to use strlen. Just use string::length:
对于 C++ 字符串，没有理由使用strlen. 只需使用string::length：
std::cout << str.length() << std::endl;
You should stronglyprefer this to strlen(str.c_str())for the following reasons:
由于以下原因，您应该强烈倾向于这样做strlen(str.c_str())：
Clarity: The
length()(orsize()) member functions unambiguously give back the length of the string. While it's possible to figure out whatstrlen(str.c_str())does, it forces the reader to pause for a bit.Efficiency:
length()andsize()run in time O(1), whilestrlen(str.c_str())will take Θ(n) time to find the end of the string.Style: It's good to prefer the C++ versions of functions to the C versions unless there's a specific reason to do so otherwise. This is why, for example, it's usually considered better to use
std::sortoverqsortorstd::lower_boundoverbsearch, unless some other factors come into play that would affect performance.
清晰：（
length()或size()）成员函数明确地返回字符串的长度。虽然有可能弄清楚是什么strlen(str.c_str())，但它迫使读者暂停一下。效率：
length()并size()在 O(1) 时间内运行，而strlen(str.c_str())将花费 Θ(n) 时间来找到字符串的结尾。风格：除非有特殊原因，否则最好选择 C++ 版本的函数而不是 C 版本。这就是为什么，例如，通常认为使用
std::sortoverqsort或std::lower_boundover更好bsearch，除非一些其他因素会影响性能。
The only reason I could think of where strlenwould be useful is if you had a C++-style string that had embedded null characters and you wanted to determine how many characters appeared before the first of them. (That's one way in which strlendiffers from string::length; the former stops at a null terminator, and the latter counts all the characters in the string). But if that's the case, just use string::find:
我能想到哪里strlen有用的唯一原因是，如果您有一个嵌入空字符的 C++ 样式字符串，并且您想确定在第一个字符之前出现了多少个字符。（这是strlen与 不同的一种方式string::length；前者在空终止符处停止，后者计算字符串中的所有字符）。但如果是这种情况，只需使用string::find：
size_t index = str.find(0);
if (index == str::npos) index = str.length();
std::cout << index << std::endl;
Hope this helps!
希望这可以帮助！
回答by masoud
Function strlenshows the number of character before \0and using it for std::stringmay report wrong length.
函数strlen显示之前的字符数，\0使用它std::string可能会报告错误的长度。
strlen(str.c_str()); // It may return wrong length.
In C++, a string can contain \0within the characters but C-style-zero-terminated strings can not but at the end. If the std::stringhas a \0before the last character then strlenreports a length less than the actual length.
在 C++ 中，字符串可以包含\0在字符内，但 C 风格的零终止字符串不能只包含在末尾。如果在最后一个字符之前std::string有一个\0，则strlen报告长度小于实际长度。
Try to use .length()or .size(), I prefer second one since another standard containers have it.
尝试使用.length()or .size()，我更喜欢第二个，因为另一个标准容器有它。
str.size()
回答by Kiril Kirov
Use std::string::sizeor std::string::length(both are the same).
使用std::string::size或std::string::length（两者相同）。
As you insist to use strlen, you can:
当您坚持使用 时strlen，您可以：
int size = strlen( str.c_str() );
note the usage of std::string::c_str, which returns const char*.
注意 的用法std::string::c_str，它返回const char*。
BUTstrlencounts untill it hit \0char and std::stringcan store such chars. In other words, strlencould sometimes lie for the size.
但是strlen计数直到它击中\0字符并且std::string可以存储这样的字符。换句话说，strlen有时可能会因为大小而撒谎。
回答by KeithSmith
If you really, really want to use strlen(), then
如果你真的，真的想使用 strlen()，那么
cout << strlen(str.c_str()) << endl;
else the use of .length() is more in keeping with C++.
否则 .length() 的使用更符合 C++。
回答by user2972135
Manually:
手动：
int strlen(string s)
{
int len = 0;
while (s[len])
len++;
return len;
}
回答by Orange Blossom
#include<iostream>
#include<conio.h>
#include<string.h>
using namespace std;
int main()
{
char str[80];
int i;
cout<<"\n enter string:";
cin.getline(str,80);
int n=strlen(str);
cout<<"\n lenght is:"<<n;
getch();
return 0;
}
This is the program if you want to use strlen . Hope this helps!
如果您想使用 strlen ，这就是程序。希望这可以帮助！
回答by Jakarea Parvez
Simply use
只需使用
int len=str.length();
int len=str.length();
