iostream linker error

Question

I have some old C code that I would like to combine with some C++ code.

The C code used to have has the following includes:

#include <windows.h>
#include <stdio.h>
#include <string.h>
#include "mysql.h"

Now I'm trying to make it use C++ with iostream like this:

#include <windows.h>
#include <stdio.h>
#include <string>
#include <iostream>
#include "mysql.h"

But I keep getting the following linker errors when I compile:

[Linker error] undefined reference to `std::string::size() const'

[Linker error] undefined reference to `std::string::operator[](unsigned int) const'

[Linker error] undefined reference to `std::string::operator[](unsigned int) const'

[Linker error] undefined reference to `std::string::operator[](unsigned int) const'

[Linker error] undefined reference to `std::ios_base::Init::Init()'

[Linker error] undefined reference to `std::ios_base::Init::~Init()'

ld returned 1 exit status

How do I resolve this?

Edit: My compiler is Dev-C++ 4.9.9.2

Answer 1

You need to link against your C++ runtime. It depends on your platform and compiler, but adding -lc to your linker command might do it.

So might linking using your C++ compiler rather than ld.

In any case, you probably have to link using the C++ compiler rather than ld if you want your C++ code to work correctly -- it's often required for exceptions and static initializers to work correctly...

Answer 2

I got the same exact error when trying to compile with Cygwin (g++).

Adding the -L/usr/local/bin -L/usr/lib to the compilation command should work.

This may be specific to Cygwin but it might help solve your problem as well.

Answer 3

The C string.h header and the C++ string header are not interchangeable.

Overall, though, your problem is that the file is getting properly compiled, but the wrong runtime library is getting linked in.

Dev-C++ uses GCC. GCC can correctly determine the language in a file based on file extension, but won't link the right runtime library in unless you specifically ask it to (-lstdc++ at the command line). Calling GCC as "g++" (or, in your case, "mingwin32-g++") will also get the right language and will link the needed library.