Difference between revisions of "cpp/string/byte/strtok"

Revision as of 08:53, 6 June 2023

Finds the next token in a null-terminated byte string pointed to by str. The separator characters are identified by null-terminated byte string pointed to by delim.

This function is designed to be called multiple times to obtain successive tokens from the same string.

• If str is not a null pointer, the call is treated as the first call to strtok for this particular string. The function searches for the first character which is not contained in delim.

• If no such character was found, there are no tokens in str at all, and the function returns a null pointer.
• If such character was found, it is the beginning of the token. The function then searches from that point on for the first character that is contained in delim.

• If no such character was found, str has only one token, and the future calls to strtok will return a null pointer.
• If such character was found, it is replaced by the null character '\0' and the pointer to the following character is stored in a static location for subsequent invocations.

• The function then returns the pointer to the beginning of the token.

• If str is a null pointer, the call is treated as a subsequent call to strtok: the function continues from where it left in previous invocation. The behavior is the same as if the previously stored pointer is passed as str.

Parameters

Return value

Pointer to the beginning of the next token or a nullptr if there are no more tokens.

Notes

This function is destructive: it writes the '\0' characters in the elements of the string str. In particular, a string literal cannot be used as the first argument of std::strtok.

Each call to this function modifies a static variable: is not thread safe.

Unlike most other tokenizers, the delimiters in std::strtok can be different for each subsequent token, and can even depend on the contents of the previous tokens.

Possible implementation

char* strtok(char* str, const char* delim)
{
    static char* buffer;
 
    if (str != nullptr)
        buffer = str;
 
    buffer += std::strspn(buffer, delim);
 
    if (*buffer == '\0')
        return nullptr;
 
    char* const tokenBegin = buffer;
 
    buffer += std::strcspn(buffer, delim);
 
    if (*buffer != '\0')
        *buffer++ = '\0';
 
    return tokenBegin;
}

Actual C++ library implementations of this function delegate to the C library, where it may be implemented directly (as in MUSL libc), or in terms of its reentrant version (as in GNU libc).

Example

#include <cstring>
#include <iomanip>
#include <iostream>
 
int main() 
{
    char input[] = "one + two * (three - four)!";
    const char* delimiters = "! +- (*)";
    char *token = std::strtok(input, delimiters);
    while (token)
    {
        std::cout << std::quoted(token) << ' ';
        token = std::strtok(nullptr, delimiters);
    }
 
    std::cout << "\nContents of the input string now:\n\"";
    for (std::size_t n = 0; n < sizeof input; ++n)
    {
        if (const char c = input[n]; c != '\0')
            std::cout << c;
        else
            std::cout << "\\0";
    }
    std::cout << "\"\n";
}

"one" "two" "three" "four" 
Contents of the input string now:
"one\0+ two\0* (three\0- four\0!\0"

See also