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

 

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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

stoistolstoll (C++11)(C++11)(C++11)	converts a string to a signed integer (function)
stoulstoull (C++11)(C++11)	converts a string to an unsigned integer (function)
strtolstrtoll (C++11)	converts a byte string to an integer value (function)
strtoulstrtoull (C++11)	converts a byte string to an unsigned integer value (function)
strtoimaxstrtoumax (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)
C documentation for atoi, atol, atoll