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

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< cpp‎ | string‎ | byte

Defined in header `<cstdlib>` |  |
int atoi( const char* str ); | (1) |
long atol( const char* str ); | (2) |
long long atoll( const char* str ); | (3) | (since C++11)

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

str | - | pointer to the null-terminated byte string to be interpreted

### 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

Run this code

    #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';
        }
    }

Possible output:

    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

stoi stol stoll (C++11)(C++11)(C++11) | converts a string to a signed integer (function)
stoul stoull (C++11)(C++11) | converts a string to an unsigned integer (function)
strtol strtoll (C++11) | converts a byte string to an integer value (function)
strtoul strtoull (C++11) | converts a byte string to an unsigned integer value (function)
strtoimax strtoumax (C++11)(C++11) | converts a byte string to std::intmax_t or std::uintmax_t (function)
from_chars (C++17) | converts a character sequence to an integer or floating-point value (function)

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