How do you convert a C++ string to an int?

Question

How do you convert a C++ string to an int?

Assume you are expecting the string to have actual numbers in it ("1", "345", "38944", for example).

Also, let's assume you don't have boost, and you really want to do it the C++ way, not the crufty old C way.

Answer 1

Use the C++ streams.

std::string       plop("123");
std::stringstream str(plop);
int x;

str >> x;

/* Lets not forget to error checking */
if (!str)
{
     // The conversion failed.
     // Need to do something here.
     // Maybe throw an exception
}

PS. This basic principle is how the boost library lexical_cast<> works.

My favorite method is the boost lexical_cast<>

int x = boost::lexical_cast<int>("123");

It provides a method to convert between a string and number formats and back again. Underneath it uses a string stream so anything that can be marshaled into a stream and then un-marshaled from a stream (Take a look at the >> and << operators).

Answer 2

Use atoi

Answer 3

// st is input string
int result;
stringstream(st) >> result;

Answer 4

in "stdapi.h"

StrToInt

This function tells you the result, and how much characters perticipated in the conversion.

Answer 5

I have used something like the following in C++ code before:

#include <sstream>
int main()
{
    char* str = "1234";
    std::stringstream s_str( str );
    int i;
    s_str >> i;
}

Answer 6

This is, more or less, a duplicate of How to parse a string to an int in C++?

Answer 7

Perhaps I am misunderstanding the question, by why exactly would you not want to use atoi? I see no point in reinventing the wheel.

Am I just missing the point here?

Answer 8

C++ FAQ Lite

[39.2] How do I convert a std::string to a number?

http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.2

Answer 9

Let me add my vote for boost::lexical_cast

int val = boost::lexical_cast<int>(strval) ;

It throws bad_lexical_cast on error.