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

< cpp‎ | string‎ | byte
Defined in header <cstdlib>
int       atoi( const char *str );
long      atol( const char *str );
long long atoll( const char *str );
(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.


[edit] Parameters

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

[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)))
    bool negative = false;
    if (*str == '+')
    else if (*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).

[edit] Example

#include <cstdlib>
#include <iostream>
int main()
    const auto data = { 
        "0x2A", // treated as "0" and junk "x2A", not as hexadecimal
        "31337 with words",
        "words and 2",
        "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

[edit] See also

converts a string to a signed integer
(function) [edit]
converts a string to an unsigned integer
(function) [edit]
converts a byte string to an integer value
(function) [edit]
converts a byte string to an unsigned integer value
(function) [edit]
converts a byte string to std::intmax_t or std::uintmax_t
(function) [edit]
converts a character sequence to an integer or floating-point value
(function) [edit]
C documentation for atoi, atol, atoll