# std::mblen

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< cpp‎ | string‎ | multibyte

Defined in header `<cstdlib>` |  |
int mblen( const char* s, std::size_t n ); |  |

Determines the size, in bytes, of the multibyte character whose first byte is pointed to by `s`.

If `s` is a null pointer, resets the global conversion state and determined whether shift sequences are used.

This function is equivalent to the call std::mbtowc((wchar_t*)0, s, n), except that conversion state of std::mbtowc is unaffected.

### Notes

Each call to `mblen` updates the internal global conversion state (a static object of type std::mbstate_t, only known to this function). If the multibyte encoding uses shift states, care must be taken to avoid backtracking or multiple scans. In any case, multiple threads should not call `mblen` without synchronization: std::mbrlen may be used instead.

### Parameters

s | - | pointer to the multibyte character
n | - | limit on the number of bytes in s that can be examined

### Return value

If `s` is not a null pointer, returns the number of bytes that are contained in the multibyte character or -1 if the first bytes pointed to by `s` do not form a valid multibyte character or ​0​ if `s` is pointing at the null charcter '\0'.

If `s` is a null pointer, resets its internal conversion state to represent the initial shift state and returns ​0​ if the current multibyte encoding is not state-dependent (does not use shift sequences) or a non-zero value if the current multibyte encoding is state-dependent (uses shift sequences).

### Example

Run this code

    #include <clocale>
    #include <string>
    #include <iostream>
    #include <cstdlib>
    #include <stdexcept>

    // the number of characters in a multibyte string is the sum of mblen()'s
    // note: the simpler approach is std::mbstowcs(NULL, s.c_str(), s.size())
    std::size_t strlen_mb(const std::string& s)
    {
        std::size_t result = 0;
        const char* ptr = s.data();
        const char* end = ptr + s.size();
        std::mblen(NULL, 0); // reset the conversion state
        while (ptr < end) {
            int next = std::mblen(ptr, end-ptr);
            if (next == -1) {
                throw std::runtime_error("strlen_mb(): conversion error");
            }
            ptr += next;
            ++result;
        }
        return result;
    }

    int main()
    {
        // allow mblen() to work with UTF-8 multibyte encoding
        std::setlocale(LC_ALL, "en_US.utf8");
        // UTF-8 narrow multibyte encoding
        std::string str = u8"z\u00df\u6c34\U0001f34c"; // or u8"zß水🍌"
        std::cout << str << " is " << str.size() << " bytes, but only "
                  << strlen_mb(str) << " characters\n";
    }

Output:

    zß水🍌 is 10 bytes, but only 4 characters

### See also

mbtowc | converts the next multibyte character to wide character (function)
mbrlen | returns the number of bytes in the next multibyte character, given state (function)
C documentation for mblen

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