How to Determine the True Length of a UTF-8 Encoded std::string in C ?

Determining the True Length of a UTF-8 Encoded std::string

In C , a std::string is an array of characters, each occupying one byte of memory. However, in the case of UTF-8 encoding, a single character may be represented using a sequence of multiple bytes. This leads to a discrepancy between the length of the string as reported by str.length() and its actual length in characters.

As per the UTF-8 character encoding standard, bytes are grouped into sequences, with the first byte indicating the length of the sequence:

• 0x00000000 - 0x0000007F: 1 byte
• 0x00000080 - 0x000007FF: 2 bytes
• 0x00000800 - 0x0000FFFF: 3 bytes
• 0x00010000 - 0x001FFFFF: 4 bytes

To determine the actual length of a UTF-8 encoded std::string, you can employ the following approach:

1. Iterate through the string character by character using the *s operator.
2. For each character, check if the first byte (using the & operator) matches the continuation byte pattern (10xxxxxx).

If the first byte does not match the continuation pattern, increment the length count. This indicates the start of a new character sequence.

Here's an example implementation:

<code class="c++">int len = 0;
while (*s) len += (*s++ & 0xc0) != 0x80;</code>

By following this approach, you can accurately determine the true length of a UTF-8 encoded std::string, which is essential for various operations, such as character counting, string manipulation, and data parsing.

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