C++ – iostream linker error

ciostreamlinker

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

Best Answer

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.

Related Solutions

C++ – Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong

Because iostream::eof will only return true after reading the end of the stream. It does not indicate, that the next read will be the end of the stream.

Consider this (and assume then next read will be at the end of the stream):

while(!inStream.eof()){
  int data;
  // yay, not end of stream yet, now read ...
  inStream >> data;
  // oh crap, now we read the end and *only* now the eof bit will be set (as well as the fail bit)
  // do stuff with (now uninitialized) data
}

Against this:

int data;
while(inStream >> data){
  // when we land here, we can be sure that the read was successful.
  // if it wasn't, the returned stream from operator>> would be converted to false
  // and the loop wouldn't even be entered
  // do stuff with correctly initialized data (hopefully)
}

And on your second question: Because

if(scanf("...",...)!=EOF)

is the same as

if(!(inStream >> data).eof())

and not the same as

if(!inStream.eof())
    inFile >> data

C++ – experimental::filesystem linker error

The Filesystem TS is nothing to do with C++1z support, it is a completely separate specification not part of the C++1z working draft. GCC's implementation (in GCC 5.3 and later) is even available in C++11 mode.

You just need to link with -lstdc++fs to use it.

(The relevant library, libstdc++fs.a, is a static library, so as with any static library it should come after any objects that depend on it in the linker command.)

Update Nov 2017: as well as the Filesystem TS, GCC 8.x also has an implementation of the C++17 Filesystem library, defined in <filesystem> and in namespace std::filesystem (N.B. no "experimental" in those names) when using -std=gnu++17 or -std=c++17. GCC's C++17 support is not complete or stable yet, and until it's considered ready for prime time use you also need to link to -lstdc++fs for the C++17 Filesystem features.

Update Jan 2019: starting with GCC 9, the C++17 std::filesystem components can be used without -lstdc++fs (but you still need that library for std::experimental::filesystem).

Best Answer

Related Solutions

C++ – Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong

C++ – experimental::filesystem linker error

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