std::atoi, std::atol, std::atoll - cppreference.com (original) (raw)

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 T atoi_impl(const char* str) { while (std::isspace(static_cast(*str))) ++str;   bool negative = false;   if (*str == '+') ++str; else if (*str == '-') { ++str; negative = true; }   T result = 0; for (; std::isdigit(static_cast(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(str); }   long atol(const char str) { return atoi_impl(str); }   long long atoll(const char str) { return atoi_impl(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 #include   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

[edit] See also