std::atoi, std::atol, std::atoll

Interprets an integer value in a byte string pointed to by str. The implied radix is always 10.

Discards any whitespace characters until the first non-whitespace character is found, then takes as many characters as possible to form a valid integer number representation and converts them to an integer value. The valid integer value consists of the following parts:

• (optional) plus or minus sign
• numeric digits

If the value of the result cannot be represented, i.e. the converted value falls out of range of the corresponding return type, the behavior is undefined.

[edit] Parameters

[edit] Return value

Integer value corresponding to the contents of str on success.

If no conversion can be performed, ​0​ is returned.

[edit] Possible implementation

template<typename T>
T atoi_impl(const char* str)
{
    while (std::isspace(static_cast<unsigned char>(*str)))
        ++str;
 
    bool negative = false;
 
    if (*str == '+')
        ++str;
    else if (*str == '-')
    {
        ++str;
        negative = true;
    }
 
    T result = 0;
    for (; std::isdigit(static_cast<unsigned char>(*str)); ++str)
    {
        int digit = *str - '0';
        result *= 10;
        result -= digit; // calculate in negatives to support INT_MIN, LONG_MIN,..
    }
 
    return negative ? result : -result;
}
 
int atoi(const char* str)
{
    return atoi_impl<int>(str);
}
 
long atol(const char* str)
{
    return atoi_impl<long>(str);
}
 
long long atoll(const char* str)
{
    return atoi_impl<long long>(str);
}

Actual C++ library implementations fall back to C library implementations of atoi, atoil, and atoll, which either implement it directly (as in MUSL libc) or delegate to strtol/strtoll (as in GNU libc).

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