# Convert hex string to integer using stoi() function in C++ STL

C++ STL stoi() function: Here, we are going to learn how to convert a given hexadecimal string to an integer using stoi() function.
Submitted by IncludeHelp, on March 11, 2019

Given a hex string, we have to convert it into an integer using stoi() function.

## C++ STL stoi() function

stoi() stands for string to integer, it is a standard library function in C++ STL, it is used to convert a given string in various formats (like binary, octal, hex or a simple number in string formatted) into an integer.

Syntax:

```    int stoi (const string&  str, [size_t* idx], [int base]);
```

Parameters:

• const string& str is an input string.
• size_t* idx is an optional parameter (pointer to the object whose value is set by the function), it's default value is 0 or we can assign it to nullptr.
• int base is also an optional parameter, its default is 10. It specifies the radix to determine the value type of input string (2 for binary, 8 for octal, 10 for decimal and 16 for hexadecimal).

Return value: It returns converted integer value.

Example:

```    Input:
string hex_string = "1F1FA2";

Function call:
stoi(hex_string, 0, 16);

Output:
2039714
```

## C++ STL code to convert a hex string into an integer

```#include <iostream>
#include <string>
using namespace std;

int main()
{
string hex_string = "1F1FA2";
int number =0;

number = stoi(hex_string, 0, 16);
cout<<"hex_string: "<<hex_string<<endl;
cout<<"number: "<<number<<endl;

hex_string = "12345";
number = stoi(hex_string, 0, 16);
cout<<"hex_string: "<<hex_string<<endl;
cout<<"number: "<<number<<endl;

return 0;
}
```

Output

```hex_string: 1F1FA2
number: 2039714
hex_string: 12345
number: 74565
```

Reference: std::stoi()

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