Question

I am having trouble inlining member functions. My code is as follows:

Main.cpp

#include "Foo.h"
int _tmain(int argc, _TCHAR* argv[])
{
    Foo foo;
    int a = foo.myInlinedFunc(2);
}

Foo.h

class Foo
{
public:
    Foo(void);
    ~Foo(void);
    inline int myInlinedFunc(int value);
};

Foo.cpp

#include "Foo.h"
Foo::Foo(void)
{
}
Foo::~Foo(void)
{
}
int Foo::myInlinedFunc(int value)
{
    return value * value;
}

I get the following error:

Tester.obj : error LNK2019: unresolved external symbol “public: int __thiscall Foo::myInlinedFunc(int)” (?myInlinedFunc@Foo@@QAEHH@Z) referenced in function _wmain
1>E:\Debug\Tester.exe : fatal error LNK1120: 1 unresolved externals

I have searched google for answers, but the only answers that show up, tells me that I should put the inline keyword in the header-file where it already is.

Answer 1

You need to put the function body, i.e. the definition, in the header file.

Your header needs to read like this:

Foo.h

class Foo
{
public:
    Foo(void);
    ~Foo(void);
    inline int myInlinedFunc(int value)
    {
        return value * value;
    }
};

And naturally you also have to remove the definition of myInlinedFunc from Foo.cpp.

Or, if you prefer, you can write your header code like this:

Foo.h

class Foo
{
public:
    Foo(void);
    ~Foo(void);
    int myInlinedFunc(int value);
};

inline int Foo::myInlinedFunc(int value)
{
    return value * value;
}

But the bottom line is that if you need your function inlined, and available to other translation units, its definition must be placed in the header file.

The C++ FAQ explains this and even predicts your unresolved external error.