std::mbrlen - cppreference.com
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    std::mbrlen

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    Cadenas multibyte terminadas en nulo
    Ancho / multibyte conversiones
    Original:
    Wide/multibyte conversions
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    Tipos
    Original:
    Types
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    Definido en el archivo de encabezado <cwchar> 
    std::size_t mbrlen( const char* s, std::size_t n, std::mbstate_t* ps);
    Determina el tamaño, en bytes, del resto de los caracteres multibyte cuyo primer byte es apuntado por s, dado el estado actual de conversión ps .
    Original:
    Determines the size, in bytes, of the remainder of the multibyte character whose first byte is pointed to by s, given the current conversion state ps.
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    Esta función es equivalente a la std::mbrtowc(nullptr, s, n, ps?ps:&internal) llamada para algunas internal objeto oculto de std::mbstate_t tipo, excepto que el ps expresión se evalúa sólo una vez .
    Original:
    This function is equivalent to the call std::mbrtowc(nullptr, s, n, ps?ps:&internal) for some hidden object internal of type std::mbstate_t, except that the expression ps is evaluated only once.
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    Parámetros

    s -
    puntero a un elemento de una cadena de caracteres multibyte
    Original:
    pointer to an element of a multibyte character string
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    n -
    limitar el número de bytes en s que puede ser examinado
    Original:
    limit on the number of bytes in s that can be examined
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    ps -
    puntero a la variable que contiene el estado de conversión
    Original:
    pointer to the variable holding the conversion state
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    Valor de retorno

    0 si el n siguiente o menos bytes completar el carácter nulo .
    Original:
    0 if the next n or fewer bytes complete the null character.
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    El número de bytes (entre 1 y n) que completan un carácter multibyte válida
    Original:
    The number of bytes (between 1 and n) that complete a valid multibyte character
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    (size_t)-1 si se produce error de codificación
    Original:
    (size_t)-1 if encoding error occurs
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    (size_t)-2 si los bytes n siguientes son parte de un carácter multibyte posiblemente válido, lo que es aún incompleta después de examinar todos los bytes n
    Original:
    (size_t)-2 if the next n bytes are part of a possibly valid multibyte character, which is still incomplete after examining all n bytes
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    Ejemplo

    Ejecuta este código
    #include <clocale>
#include <string>
#include <iostream>
#include <cwchar>

int main()
{
    // allow mbrlen() to work with UTF-8 multibyte encoding
    std::setlocale(LC_ALL, "en_US.utf8");
    // UTF-8 narrow multibyte encoding
    std::string str = u8"水"; // or u8"\u6c34" or "\xe6\xb0\xb4"
    std::mbstate_t mb = std::mbstate_t();
    int len1 = std::mbrlen(&str[0], 1, &mb);
    if(len1 == -2) {
        std::cout << "The first 1 byte of " << str
                  << " is an incomplete multibyte char (mbrlen returns -2)\n";
    }
    int len2 = std::mbrlen(&str[1], str.size()-1, &mb);
    std::cout << "The remaining " << str.size()-1 << " bytes of " << str
              << " hold " << len2 << " bytes of the multibyte character\n";
    std::cout << "Attempting to call mbrlen() in the middle of " << str
              << " while in initial shift state returns "
              << (int)mbrlen(&str[1], str.size(), &mb) << '\n';

}

    Salida: 

    The first 1 byte of 水 is an incomplete multibyte char (mbrlen returns -2)
The remaining 2 bytes of 水 hold 2 bytes of the multibyte character
Attempting to call mbrlen() in the middle of 水 while in initial shift state returns -1

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