Linker unresolved external symbol for inline function - c++

Question

I'm developing a C++ solution with Visual Studio 2015.
I have a cpp source file and header file hpp with this declaration.
Header:
#ifndef MyLib__FREEFUNCTIONS__INCLUDE__
#define MyLib__FREEFUNCTIONS__INCLUDE__
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
using namespace std;
// Check if 'str' is null, empty or consists only of white-space characters.
inline bool IsNullOrWhiteSpace(string str);
// More functions
[ ... ]
#endif
And Source code:
#include "FreeFunctions.h"
inline bool IsNullOrWhiteSpace(string str)
{
return (str.empty() || (str.find_first_not_of(' ') == string::npos));
}
I use this function in a class:
#include "ConvertToOwnFormat.h"
#include "FreeFunctions.h"
ConvertToOwnFormat::ConvertToOwnFormat()
{
}
ConvertToOwnFormat::~ConvertToOwnFormat()
{
}
vector<Entry> ConvertToOwnFormat::ReadCatalogue(string path)
{
if (!IsNullOrWhiteSpace(path)
{
[ ... ]
}
}
And I get the following error in ConvertToOwnFormat::ReadCatalogue:
Error LNK2019 external symbol "bool __cdecl IsNullOrWhiteSpace(class
std::basic_string,class
std::allocator >)"
(?IsNullOrWhiteSpace##YA_NV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###Z)
unresolved referenced by the function "public: class std::vector > __cdecl
ConvertToOwnFormat::ReadCatalogue(class std::basic_string,class std::allocator >)"
(?ReadCatalogue#ConvertToOwnFormat##QEAA?AV?$vector#VEntry##V?$allocator#VEntry###std###std##V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##3##Z) MyProjectLib D:\Fuentes\Repos\MyProject\MyProjectLibConsoleTest\ConsoleMyProjectLib\Lib.lib(ConvertToOwnFormat.obj) 1

Answer 1

You have to put declaration of methods within the header. inline tells to compiler that it should replace call of function with the core of function. So, it need it at compilation time for every unit of compilation which use it
#ifndef MyLib__FREEFUNCTIONS__INCLUDE__
#define MyLib__FREEFUNCTIONS__INCLUDE__
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
// Check if 'str' is null, empty or consists only of white-space characters.
inline bool IsNullOrWhiteSpace(std::string str)
{
return (str.empty() || (str.find_first_not_of(' ') == std::string::npos));
}
// Others functions, prototype, ...
#endif
or remove inline in both source and header files
It's totally out of topic but never put a using namespace inside a header: a header should offer something but should not impose something like namespace. See also "using namespace" in c++ headers