Converting string to long

Question

I'm trying to convert a string to long. It sounds easy, but I still get the same error. I tried:

include <iostream>
include <string>    

using namespace std;

int main()
{
  string myString = "";
  cin >> myString;
  long myLong = atol(myString);
}

But always the error:

.../main.cpp:12: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'const char*' for argument '1' to 'long int atol(const char*)'

occured. The reference says following:

long int atol ( const char * str );

Any help?

Vlad from Moscow · Accepted Answer · 2014-03-29 15:48:44Z

15

Try

long myLong = std::stol( myString );

The function has three parameters

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

You can use the second parameter that to determine the position in the string where parsing of the number was stoped. For example

std::string s( "123a" );

size_t n;

std::stol( s, &n );

std::cout << n << std::endl;

The output is

The function can throw exceptions.

edited Mar 29, 2014 at 15:48

answered Mar 29, 2014 at 15:37

Vlad from Moscow

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Comments

Vishnu · Accepted Answer · 2022-03-30 09:19:44Z

8

Just write

long myLong = atol(myString.c_str());

edited Mar 30, 2022 at 9:19

Vishnu

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answered Mar 29, 2014 at 15:37

JohanTG

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1 Comment

gumuruh Over a year ago

is this c 11 standard?

Mr.C64 · Accepted Answer · 2014-03-29 15:40:00Z

4

atol() requires a const char*; there's no implicit conversion from std::string to const char*, so if you really want to use atol(), you must call the std::string::c_str() method to get the raw C-like string pointer to be passed to atol():

// myString is a std::string
long myLong = atol(myString.c_str());

A better C++ approach would be using stol() (available since C++11), without relying on a C function like atol():

long myLong = std::stol(myString);

answered Mar 29, 2014 at 15:40

Mr.C64

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Comments

Rontogiannis Aristofanis · Accepted Answer · 2014-03-29 15:48:01Z

1

atol gets as parameter a const char* (C-style string), but you are passing as parameter a std::string. The compiler is not able to find any viable conversion between const char* and std::string, so it gives you the error. You can use the string member function std::string::c_str(), which returns a c-style string, equivalent to the contents of you std::string. Usage:

string str = "314159265";
cout << "C-ctyle string: " << str.c_str() << endl;
cout << "Converted to long: " << atol(str.c_str()) << endl;

answered Mar 29, 2014 at 15:48

Rontogiannis Aristofanis

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