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

int       atoi( const char* str );

long      atol( const char* str );

long long atoll( const char* str );

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.

Parameters

Return value

Integer value corresponding to the contents of str on success.

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

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).

Example

#include <cstdlib>
#include <iostream>
 
int main()
{
    const auto data =
    {
        "42",
        "0x2A", // treated as "0" and junk "x2A", not as hexadecimal
        "3.14159",
        "31337 with words",
        "words and 2",
        "-012345",
        "10000000000" // note: out of int32_t range
    };
 
    for (const char* s : data)
    {
        const int i{std::atoi(s)};
        std::cout << "std::atoi('" << s << "') is " << i << '\n';
        if (const long long ll{std::atoll(s)}; i != ll)
            std::cout << "std::atoll('" << s << "') is " << ll << '\n';
    }
}

std::atoi('42') is 42
std::atoi('0x2A') is 0
std::atoi('3.14159') is 3
std::atoi('31337 with words') is 31337
std::atoi('words and 2') is 0
std::atoi('-012345') is -12345
std::atoi('10000000000') is 1410065408
std::atoll('10000000000') is 10000000000

See also