Why does namespace usage break MinGW compilation? - c++

Question

Through the act of separating a set of input and output related functions from other parts of a program, I have encountered a problem with compiling files when functions in a header are placed within a namespace. The following files compile:
main.cpp
#include "IO.h"
int main()
{
testFunction("yikes");
}
IO.h
#ifndef IO_H_INCLUDED
#define IO_H_INCLUDED
#include <string>
void testFunction(const std::string &text);
#endif
However, when testFunction is placed in a namespace:
#ifndef IO_H_INCLUDED
#define IO_H_INCLUDED
#include <string>
// IO.h
namespace IO
{
void testFunction(const std::string &text);
}
#endif
within IO.h, and then invoked as IO::testFunction, compilation fails, throwing
undefined reference to `IO::testFunction(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
collect2.exe: error: ld returned 1 exit status`
In each case, IO.cpp is
#include <string>
#include <iostream>
void testFunction(const std::string &text)
{
std::cout << text << std::endl;
}
and the compilation command is g++ -std=c++11 main.cpp IO.cpp, with the compiler being x86_64-w64-mingw32 from TDM-GCC, on Windows 10 Home.

Answer 1

If you change the declaration of your function to be in a namespace you need to implement the function within this namespace aswell.
The signature of your function will be IO::testFunction(...) but you only implemented testFunction(...) so there is no implementation for IO::testFunction(...)
The header file (IO.h):
namespace IO {
void testFunction(const std::string &text);
}
The cpp file (IO.cpp):
#include "IO.h"
// either do this
namespace IO {
void testFunction(const std::string &text) { ... }
// more functions in namespace
}
// or this
void IO::testFunction(const std::string &text) { ... }