problem with compiling a simple program with c++

Question

I am really new to C++. I am trying to compile this simple program but I get an error

#include <iostream> 
int main(int argc, char** argv) 
{ 
   cout << "You have entered " << argc 
        << " arguments:" << "\n"; 
 
   for (int i = 0; i < argc; ++i) 
       cout << argv[i] << "\n"; 
 
   return 0; 
}

The error that I get is

(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

I try to compile it as g++ file1.cpp I also tried g++ -c file1.cpp and then g++ main.exe file1.o which does not work as well. What am I doing wrong?

This error should not occur provided that you *really* have the `main` function in `file1.cpp`. If, for example, you named it `Main` for some reason, you will see exactly this error. Please double check the contents of your `file1.cpp` file. — Andreas Fester, Dec 01 '20 at 14:47
The selected answer is considered bad practice, and shouldn't even be capable of fixing your issue. There is missing information here. I would have expected a different error for the code you're showing. — sweenish, Dec 01 '20 at 15:03
instead of using using namespace std; at the start I added std whenever it was needed. For example std::cout — Alex, Dec 01 '20 at 15:17
I believe the whole question should be deleted. I think the reason for the link error is somehow lost in changes to the code that happened after the error occurred. This code does have a compile error but that should not have given the link error. — drescherjm, Dec 01 '20 at 15:48

score 0 · Answer 1 · answered Dec 01 '20 at 14:53

Maybe try and create a new file in an empty directory name it if you want something like main.cpp and put the code in that you needed for namespace and everything else and run

#include <iostream>
using namespace std;
    int main(int argc, char** argv) 
        { 
           cout << "You have entered " << argc 
                << " arguments:" << "\n"; 
         
           for (int i = 0; i < argc; ++i) 
               cout << argv[i] << "\n"; 
         
           return 0; 
        }

It should work after and output a result Maybe check out this medium article for more about why you need using namespace std https://medium.com/breaktheloop/why-using-namespace-std-is-used-after-including-iostream-dc5ae45db652

Florent · Accepted Answer · 2020-12-01T16:06:21.577

-6

You can try specify namespace like this ->

#include <iostream>
using namespace std;

EDIT 'must' replaced by 'can try'. It works but why !

edited Dec 01 '20 at 16:06

answered Dec 01 '20 at 14:49

Florent

436
3
8

3

This is completely irrelevant for the issue mentioned in the question. – Andreas Fester Dec 01 '20 at 14:50
1

The moment I added using namespace it compiled just fine! Thank you – Alex Dec 01 '20 at 14:52
Ok @Alex so approve the answer – Florent Dec 01 '20 at 14:54
Now, I am courious how a missing `using namespace std;` can lead to a link time error as shown in the question ... are you sure that you have not modified something else in addition? – Andreas Fester Dec 01 '20 at 14:59
OP is not giving all the information is how. This answer is bogus, and a bad practice. – sweenish Dec 01 '20 at 15:01
Is it possible that the `.o` file was generated from a previous build? Or does gcc delete it if the current compile of a single file fails? I am talking about the case of `g++ -c file1.cpp` followed by `g++ main.exe file1.o` – drescherjm Dec 01 '20 at 15:11
I gess if the line using namespace std; is removed the error occur again. – Florent Dec 01 '20 at 15:12
@drescherjm OP claimed that even the all-in-one execution of `g++ file1.cpp` raised this error – Andreas Fester Dec 01 '20 at 15:13
I know, I don't understand that. – drescherjm Dec 01 '20 at 15:14
3

@Florent I would then expect a different (compile time) error like `error: 'cout' was not declared in this scope` – Andreas Fester Dec 01 '20 at 15:15
Agreed this should have had a compile time error not a linker error about ***undefined reference to `main'*** – drescherjm Dec 01 '20 at 15:16
Perhaps the signature of main ? --> int main (int argc, char *argv[]) – Florent Dec 01 '20 at 15:19
1

That is equivalent. – drescherjm Dec 01 '20 at 15:21
And even if it was not, it should not affect `main()`: https://stackoverflow.com/questions/53393585/why-name-mangling-has-no-effect-on-main-function-in-c – Andreas Fester Dec 01 '20 at 15:21
2

This is bad advice and "must" is wrong. It will also not fix the error OP is asking about. Read this: [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) – 463035818_is_not_an_ai Dec 01 '20 at 15:45
'must' replaced by 'can try' – Florent Dec 01 '20 at 16:24

problem with compiling a simple program with c++

2 Answers2