Defined in header <string>  

int stoi( const std::string& str, std::size_t* pos = 0, int base = 10 ); int stoi( const std::wstring& str, std::size_t* pos = 0, int base = 10 );  (1)  (since C++11) 
long stol( const std::string& str, std::size_t* pos = 0, int base = 10 ); long stol( const std::wstring& str, std::size_t* pos = 0, int base = 10 );  (2)  (since C++11) 
long long stoll( const std::string& str, std::size_t* pos = 0, int base = 10 ); long long stoll( const std::wstring& str, std::size_t* pos = 0, int base = 10 );  (3)  (since C++11) 
Interprets a signed integer value in the string str
.
Discards any whitespace characters (as identified by calling isspace()
) until the first nonwhitespace character is found, then takes as many characters as possible to form a valid basen (where n=base) integer number representation and converts them to an integer value. The valid integer value consists of the following parts:
0
) indicating octal base (applies only when the base is 8
or 0
) 0x
or 0X
) indicating hexadecimal base (applies only when the base is 16
or 0
) The set of valid values for base is {0,2,3,...,36}. The set of valid digits for base2 integers is {0,1
}, for base3 integers is {0,1,2
}, and so on. For bases larger than 10
, valid digits include alphabetic characters, starting from Aa
for base11 integer, to Zz
for base36 integer. The case of the characters is ignored.
Additional numeric formats may be accepted by the currently installed C locale
.
If the value of base is 0
, the numeric base is autodetected: if the prefix is 0
, the base is octal, if the prefix is 0x
or 0X
, the base is hexadecimal, otherwise the base is decimal.
If the minus sign was part of the input sequence, the numeric value calculated from the sequence of digits is negated as if by unary minus in the result type.
If pos
is not a null pointer, then a pointer ptr
 internal to the conversion functions  will receive the address of the first unconverted character in str.c_str()
, and the index of that character will be calculated and stored in *pos
, giving the number of characters that were processed by the conversion.
str    the string to convert 
pos    address of an integer to store the number of characters processed 
base    the number base 
The string converted to the specified signed integer type.
std::invalid_argument
if no conversion could be performed std::out_of_range
if the converted value would fall out of the range of the result type or if the underlying function (std::strtol or std::strtoll) sets errno
to ERANGE
. #include <iostream> #include <string> int main() { std::string str1 = "45"; std::string str2 = "3.14159"; std::string str3 = "31337 with words"; std::string str4 = "words and 2"; int myint1 = std::stoi(str1); int myint2 = std::stoi(str2); int myint3 = std::stoi(str3); // error: 'std::invalid_argument' // int myint4 = std::stoi(str4); std::cout << "std::stoi(\"" << str1 << "\") is " << myint1 << '\n'; std::cout << "std::stoi(\"" << str2 << "\") is " << myint2 << '\n'; std::cout << "std::stoi(\"" << str3 << "\") is " << myint3 << '\n'; //std::cout << "std::stoi(\"" << str4 << "\") is " << myint4 << '\n'; }
Output:
std::stoi("45") is 45 std::stoi("3.14159") is 3 std::stoi("31337 with words") is 31337
converts a byte string to an integer value (function) 

(C++11)(C++11)  converts a string to an unsigned integer (function) 
(C++11)(C++11)(C++11)  converts a string to a floating point value (function) 
(C++11)  converts an integral or floating point value to string (function) 
(C++17)  converts a character sequence to an integer or floatingpoint value (function) 
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