Decimal to Octal recursively in C++. Output in std::string format

Question

I'm trying to decimal integers into Octal Strings recursively in C++. I have tried the following code with no success. The helper function to_string works good by itself. The problem seems to be in the recursive function call.

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

string to_string(long num)
{
    ostringstream os;
    os << num;
    return os.str();

}

string dec_2_oct(long dec)
{
    if (dec == 0)
    {
        string s = "";
        return s;
    }
    return dec_2_oct(dec / 8) + to_string(dec % 8);
}

int main() {
    long dec;
    cin >> dec;
    string s;
    s = dec_2_oct(dec);
    cout << s;
}

I'm getting pretty nasty looking errors.

Compiling failed with exitcode 1, compiler output:
prog.cpp: In function 'std::__cxx11::string dec_2_oct(long int)':
prog.cpp:21:50: error: call of overloaded 'to_string(long int)' is ambiguous
     return dec_2_oct(dec / 8) + to_string(dec % 8);
                                                  ^
prog.cpp:6:8: note: candidate: std::__cxx11::string to_string(long int)
 string to_string(long num)
        ^~~~~~~~~
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/7/bits/basic_string.h:6264:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(long double)
   to_string(long double __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6255:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(double)
   to_string(double __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6246:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(float)
   to_string(float __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6240:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(long long unsigned int)
   to_string(unsigned long long __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6234:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(long long int)
   to_string(long long __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6228:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(long unsigned int)
   to_string(unsigned long __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6223:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(long int)
   to_string(long __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6217:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(unsigned int)
   to_string(unsigned __val)
   ^~~~~~~~~
/usr/include/c++/7/bits/basic_string.h:6212:3: note: candidate: std::__cxx11::string std::__cxx11::to_string(int)
   to_string(int __val)
   ^~~~~~~~~

Any help would be appreciated!

Answer 1

Try to understand the error:

Compiling failed with exitcode 1, compiler output:
prog.cpp: In function 'std::__cxx11::string dec_2_oct(long int)':
prog.cpp:21:50: error: call of overloaded 'to_string(long int)' is ambiguous
     return dec_2_oct(dec / 8) + to_string(dec % 8);

The keywords in the error are overloaded, ambiguous and of course the function itself. When you understand the meanings of these words in C++ context, you will understand the reason for the error.

Then you can take one of the following approaches:

1. Change the name of your function from to_string to something like my_to_string. This is the naive approach.
2. Avoid importing all the std namespace at global scope or even function scope and follow the usage of std::function from now on.