C++ - error while using atoi

Question

I am trying to use the atoi function in order to obtain conversion from string to int. The thing is that I have a string array which contains both integers and string values.

From what I've read, in order to get the error code from it, the function must return 0 :

string s = "ssss";
int i = atoi(s.c_str())
if (i == 0)
    cout<<"error"<<endl;
end;

How should I proceed if my string value is 0 ?

Another issue is with this string : string s = "001_01_01_041_00.png". The atoi function returns the value 1. Shouldn't it return 0. Why does it return 1?

Answer 1

That is why atoi is unsafe to use. It doesn't detect and inform the program if the input is invalid.

C++11 has introduced std:stoi which is safe, as it throws exception if input is invalid in some way. There are also two other variants : std::stol and std:stoll. See the online documentation for detail:

• std::stoi, std::stol, std::stoll

Your code would become this:

try {
     string s = "ssss";
     int  i = std::stoi(s); //don't call c_str() 
     //if (i == 0) no need to check!
     std::cout << i << endl;
}
catch(std::exception const & e)
{
     cout<<"error : " << e.what() <<endl;
}

Note that the runtime type of e could be either std::invalid_argument or std::out_of_range depending on the cause of the throw. You could just write two catch blocks if you want them to handle differently.

Answer 2

There are already good answers recommending the C++ APIs of std::stoi and boost::lexical_cast.

atoi() is a C API, and broken even in C because you can't tell failure apart from a successful parse of zero. If you're writing C, use strtol() and friends instead if you care about errors, because it reports them out-of-band in ERRNO.

Answer 3

Because the number in 001_ is 1, why should it return 0? If you want to only process one character, just use isdigit(s[0]) and s[0]-'0'. If you want better error checking to see how much of the string contains digit, use strtol.