gcc main.cpp -o main.out not working [duplicate] - c++

Question

I write code like this in my Mac OS X v10.8 (Mountain Lion), and when I use "gcc use_new.cpp -o use_new" to compile it, it throws a wrong message like this:
Undefined symbols for architecture x86_64:
"std::basic_ostream<char, std::char_traits<char> >::operator<<(std::basic_ostream<char, std::char_traits<char> >& (*)(std::basic_ostream<char, std::char_traits<char> >&))", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >::operator<<(void const*)", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >::operator<<(double)", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >::operator<<(int)", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >::operator<<(unsigned long)", referenced from:
_main in ccr2vrRQ.o
"std::ios_base::Init::Init()", referenced from:
__static_initialization_and_destruction_0(int, int) in ccr2vrRQ.o
"std::ios_base::Init::~Init()", referenced from:
__static_initialization_and_destruction_0(int, int) in ccr2vrRQ.o
"std::cout", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)", referenced from:
_main in ccr2vrRQ.o
"std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)", referenced from:
_main in ccr2vrRQ.o
"operator delete(void*)", referenced from:
_main in ccr2vrRQ.o
"operator new(unsigned long)", referenced from:
_main in ccr2vrRQ.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
And when I use "g++ use_new.cpp -o use_new" it is OK. How can I fix this?
#include <iostream>
struct fish
{
float weight;
int id;
int kind;
};
int main()
{
using namespace std;
int* pt = new int;
*pt = 1001;
cout << "int: " << *pt << "in location: " << pt << endl;
double* pd = new double;
*pd = 100000001.0;
cout << "double: " << *pd << "in location: " << pd << endl;
cout << "int point pt is length " << sizeof(*pt) << endl;
cout << "double point pd is length " << sizeof(*pd) << endl;
delete pt;
delete pd;
cout << (int *)"How are you!" << endl;
return 0;
}

Answer 1

This is the case even with the old 4.2 GCC (I experienced this when I set up my unofficial iOS toolchain). gcc assumes C by default, and invokes the linker without linking to the C++ standard library; in contrast, g++ assumes C++ and links against the C++ standard library by default.
All in all - possible solutions:
gcc myprog.c -o myprog -lstdc++
or
g++ myprog.c -o myprog

Answer 2

The answer to this Stack Overflow question has the answer
gcc and g++ linker
Use
gcc -lstdc++ use_new.cpp -o use_new
The -lstdc++ flag tells the linker to include the C++ Standard Library.
I'm running Mac OS X v10.7.4 (Lion) and the library is located here:
/usr/lib/libstdc++.dylib

Answer 3

This isn't related to the code pasted by #user1582840, just my two cents, and from a different cause of the same problem in g++ when working on some of my own code:
I received the "ld: symbol(s) not found for architecture x86_64" error when using g++ 4.2.1 on OS X v10.8 (Mountain Lion)/Darwin 11 for a different reason.
I received this error because I had a Class defined. In the class definition, I declared member functions. However, when I defined the member functions I forgot to include the class modifier.
So for an example of what I'm talking about (code sample, not full program):
class NewClass
{
NewClass(); // Default constructor
};
Then later, when defining the NewClass() constructor (or any member function) I simply had:
// Don't do this. It will throw that error!!
NewClass()
{
// Do whatever
}
rather than:
// Proper way
NewClass::NewClass()
{
// Do whatever
}
This is a rather simple mistake, and I managed to catch it in a short amount of time luckily, but it could be easy for someone to miss (us newbies especially), and the solutions about gcc/g++ linkers, Xcode, etc. aren't any help for this :P