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akko
Asked: 2020-08-10 20:15:46 +0800 CST 2020-08-10 20:15:46 +0800 CST

How to convert a string to int in c++?

  • 772

Assuming I have a string variable with the text string of a hexadecimal number, how can I save it to a number?

string n = "0X54";
int a = int(n);
int b = static_cast<int>(n);

It throws me the error:invalid cast from type ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to type ‘int’

c++

2 Answers

  1. The error you receive is clear and concise: Cannot convert a std::stringto int. The conversion you used ( static_cast) is a type conversion; this conversion only allows conversions between certain fundamental types, conversions with classes that have conversion operators, or conversions between classes that are related by inheritance. For std::stringe int, none of the above cases holds.

    C++ has at least three tools to convert a character string into a numeric value, apart from the (already mentioned by Xam ) std::stoi( string to integer) we have the format reading std::stringstream:

    std::stringstream ss{"0X54"};
int valor;
ss >> std::hex >> valor;
std::cout << valor; // Muestra 84.

    In the code above, we told std::stringstreamthat we were going to read a hexadecimal value ( std::hex) into a variable of type int; would work with any integral data type.

    Also (since C++17) we have std::from_chars, which converts a closed sequence of values ​​to a number:

    char OX54[]{"0X54"};
int valor;
std::from_chars(std::begin(OX54), std::end(OX54), valor, 16);
std::cout << valor; // Muestra 84.
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  2. Best Answer

    As of the C++11 standard there is the function stoi, which can be accessed by including the header <string>.

    This function in its most general form has the signature: 

    int stoi (const string&  str, size_t* idx = 0, int base = 10);

    For your example, since your string contains a hexadecimal number, it is enough to change the last parameter (which defaults to base 10 numbers) to the value 16.

    Consequently, this should do the trick:

    #include <iostream>
#include <string>

using namespace std;

int main() 
{
    string n = "0x54";
    int a = stoi(n, nullptr, 16);
    cout << a << '\n'; //muestra en la consola el valor 84.
    return 0;
}

    For more reference you can check the following link .

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