How can I convert a std::string to int?

Question

I want to convert a string to an int and I don't mean ASCII codes.

For a quick run-down, we are passed in an equation as a string. We are to break it down, format it correctly and solve the linear equations. Now, in saying that, I'm not able to convert a string to an int.

I know that the string will be in either the format (-5) or (25) etc. so it's definitely an int. But how do we extract that from a string?

One way I was thinking is running a for/while loop through the string, check for a digit, extract all the digits after that and then look to see if there was a leading '-', if there is, multiply the int by -1.

It seems a bit over complicated for such a small problem though. Any ideas?

Answer 1

In C++11 there are some nice new convert functions from std::string to a number type.

So instead of

atoi( str.c_str() ) 

you can use

std::stoi( str ) 

where str is your number as std::string.

There are version for all flavours of numbers: long stol(string), float stof(string), double stod(string),... see http://en.cppreference.com/w/cpp/string/basic_string/stol

Answer 2

The possible options are described below:

1. sscanf()

    #include <cstdio>     #include <string>          int i;         float f;         double d;         std::string str;          // string -> integer         if(sscanf(str.c_str(), "%d", &i) != 1)             // error management          // string -> float         if(sscanf(str.c_str(), "%f", &f) != 1)             // error management              // string -> double          if(sscanf(str.c_str(), "%lf", &d) != 1)             // error management 

This is an error (also shown by cppcheck) because "scanf without field width limits can crash with huge input data on some versions of libc" (see here, and here).

2. std::sto()*

    #include <iostream>     #include <string>          int i;         float f;         double d;         std::string str;          try {             // string -> integer             int i = std::stoi(str);              // string -> float             float f = std::stof(str);              // string -> double              double d = std::stod(str);         } catch (...) {             // error management         }    

This solution is short and elegant, but it is available only on on C++11 compliant compilers.

3. sstreams

    #include <string>     #include <sstream>          int i;         float f;         double d;         std::string str;          // string -> integer         std::istringstream ( str ) >> i;          // string -> float         std::istringstream ( str ) >> f;          // string -> double          std::istringstream ( str ) >> d;          // error management ?? 

However, with this solution is hard to distinguish between bad input (see here).

4. Boost's lexical_cast

    #include <boost/lexical_cast.hpp>     #include <string>          std::string str;          try {             int i = boost::lexical_cast<int>( str.c_str());             float f = boost::lexical_cast<int>( str.c_str());             double d = boost::lexical_cast<int>( str.c_str());             } catch( boost::bad_lexical_cast const& ) {                 // Error management         } 

However, this is just a wrapper of sstream, and the documentation suggests to use sstream for better error management (see here).

5. strto()*

This solution is very long, due to error management, and it is described here. Since no function returns a plain int, a conversion is needed in case of integer (see here for how this conversion can be achieved).

6. Qt

    #include <QString>     #include <string>          bool ok;         std::string;          int i = QString::fromStdString(str).toInt(&ok);         if (!ok)             // Error management              float f = QString::fromStdString(str).toFloat(&ok);         if (!ok)             // Error management           double d = QString::fromStdString(str).toDouble(&ok);         if (!ok)     // Error management           

Conclusions

Summing up, the best solution is C++11 std::stoi() or, as a second option, the use of Qt libraries. All other solutions are discouraged or buggy.