How to get the number of characters in a std::string?

If you're using a std::string, call length():

std::string str = "hello";
std::cout << str << ":" << str.length();
// Outputs "hello:5"

If you're using a c-string, call strlen().

const char *str = "hello";
std::cout << str << ":" << strlen(str);
// Outputs "hello:5"

Or, if you happen to like using Pascal-style strings (or f***** strings as Joel Spolsky likes to call them when they have a trailing NULL), just dereference the first character.

const char *str = "\005hello";
std::cout << str + 1 << ":" << *str;
// Outputs "hello:5"

When dealing with C++ strings (std::string), you're looking for length() or size(). Both should provide you with the same value. However when dealing with C-Style strings, you would use strlen().

#include <iostream>
#include <string.h>

int main(int argc, char **argv)
{
   std::string str = "Hello!";
   const char *otherstr = "Hello!"; // C-Style string
   std::cout << str.size() << std::endl;
   std::cout << str.length() << std::endl;
   std::cout << strlen(otherstr) << std::endl; // C way for string length
   std::cout << strlen(str.c_str()) << std::endl; // convert C++ string to C-string then call strlen
   return 0;
}

Output:

6
6
6
6

It depends on what string type you're talking about. There are many types of strings:

1. const char* - a C-style multibyte string
2. const wchar_t* - a C-style wide string
3. std::string - a "standard" multibyte string
4. std::wstring - a "standard" wide string

For 3 and 4, you can use .size() or .length() methods.

For 1, you can use strlen(), but you must ensure that the string variable is not NULL (=== 0)

For 2, you can use wcslen(), but you must ensure that the string variable is not NULL (=== 0)

There are other string types in non-standard C++ libraries, such as MFC's CString, ATL's CComBSTR, ACE's ACE_CString, and so on, with methods such as .GetLength(), and so on. I can't remember the specifics of them all right off the top of my head.

The STLSoft libraries have abstracted this all out with what they call string access shims, which can be used to get the string length (and other aspects) from any type. So for all of the above (including the non-standard library ones) using the same function stlsoft::c_str_len(). This article describes how it all works, as it's not all entirely obvious or easy.