C++ Conversion to integers/floating point types


A std::string containing a number can be converted into an integer type, or a floating point type, using conversion functions.

Note that all of these functions stop parsing the input string as soon as they encounter a non-numeric character, so "123abc" will be converted into 123.

The std::ato* family of functions converts C-style strings (character arrays) to integer or floating-point types:

std::string ten = "10";

double num1 = std::atof(ten.c_str());
int num2 = std::atoi(ten.c_str());
long num3 = std::atol(ten.c_str());
long long num4 = std::atoll(ten.c_str());

However, use of these functions is discouraged because they return 0 if they fail to parse the string. This is bad because 0 could also be a valid result, if for example the input string was "0", so it is impossible to determine if the conversion actually failed.

The newer std::sto* family of functions convert std::strings to integer or floating-point types, and throw exceptions if they could not parse their input. You should use these functions if possible:

std::string ten = "10";

int num1 = std::stoi(ten);
long num2 = std::stol(ten);
long long num3 = std::stoll(ten);

float num4 = std::stof(ten);
double num5 = std::stod(ten);
long double num6 = std::stold(ten);

Furthermore, these functions also handle octal and hex strings unlike the std::ato* family. The second parameter is a pointer to the first unconverted character in the input string (not illustrated here), and the third parameter is the base to use. 0 is automatic detection of octal (starting with 0) and hex (starting with 0x or 0X), and any other value is the base to use

std::string ten = "10";
std::string ten_octal = "12";
std::string ten_hex = "0xA";

int num1 = std::stoi(ten, 0, 2); // Returns 2
int num2 = std::stoi(ten_octal, 0, 8); // Returns 10
long num3 = std::stol(ten_hex, 0, 16);  // Returns 10
long num4 = std::stol(ten_hex);  // Returns 0
long num5 = std::stol(ten_hex, 0, 0); // Returns 10 as it detects the leading 0x