How to get length of a string using strlen function

For C++ strings, there's no reason to use strlen. Just use string::length:

std::cout << str.length() << std::endl;

You should strongly prefer this to strlen(str.c_str()) for the following reasons:

1. Clarity: The length() (or size()) member functions unambiguously give back the length of the string. While it's possible to figure out what strlen(str.c_str()) does, it forces the reader to pause for a bit.

2. Efficiency: length() and size() run in time O(1), while strlen(str.c_str()) will take Θ(n) time to find the end of the string.

3. 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::sort over qsort or std::lower_bound over bsearch, unless some other factors come into play that would affect performance.

The only reason I could think of where strlen would 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 strlen differs 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:

size_t index = str.find(0);
if (index == str::npos) index = str.length();
std::cout << index << std::endl;

Hope this helps!

Use std::string::size or std::string::length (both are the same).

As you insist to use strlen, you can:

int size = strlen( str.c_str() );

note the usage of std::string::c_str, which returns const char*.

BUT strlen counts untill it hit \0 char and std::string can store such chars. In other words, strlen could sometimes lie for the size.

Function strlen shows the number of character before \0 and using it for std::string may report wrong length.

strlen(str.c_str()); // It may return wrong length.

In C++, a string can contain \0 within the characters but C-style-zero-terminated strings can not but at the end. If the std::string has a \0 before the last character then strlen reports a length less than the actual length.

Try to use .length() or .size(), I prefer second one since another standard containers have it.

str.size()