CODEWITHSUNDEEP
c++stringvalidationint
StarSweeper
StarSweeper

Reputation: 417

How do I validate int input using std::stoi() in C++?

Is there a way to check if string input is convertible to int using std::stoi() in C++? (For example, could I check to see if an invalid_argument exception will be thrown?)

An example that doesn't work, but hopefully explains what I'm trying to do:

    string response;
    cout << prompt;

    if (std::stoi(response) throws invalid_argument) { //Something like this
        return std::stoi(response);
    }
    else {
        badInput = true;
        cout << "Invalid input. Please try again!\n";
    }

Research:
I've found several ways to check if a string is an int, but I'm hoping there is a way to do it using std::stoi() which I haven't been able to find yet.

Upvotes: 3

Views: 12780

Answers (3)

davmac
davmac

Reputation: 20671

You should catch the exception when it is thrown, rather than trying to predetermine whether or not it will be thrown.

string response; 
cin >> response;

try {
    return std::stoi(response);
}
catch (...) {
    badInput = true;
    cout << "Invalid input. Please try again!\n";
}

Upvotes: 4

Matt Jordan
Matt Jordan

Reputation: 2181

std::stoi converts as much as possible, and only throws an exception if there is nothing to convert. However, std::stoi accepts a pointer parameter representing the start index, which is updated to the character that terminated conversion. See MSDN stoi docs here.

You can use stoi to test by passing 0 as the starting index, then verifying that the returned index is the same as the total length of the string.

Treat the following as pseudo-code, it should give you an idea of how to make it work, assuming response is a std::string:

std::size_t index = 0;
auto result = std::stoi(response, &index);
if(index == response.length()){
    // successful conversion
    return result;
}
else{
    // something in the string stopped the conversion, at index
}

Upvotes: -1

P1t_
P1t_

Reputation: 165

std::stoi() throws an exception if conversion could not be performed. Look at the "Exceptions" section in this c++ documentation http://www.cplusplus.com/reference/string/stoi/

Upvotes: 0

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