When a method is defined inside class scope, project won't compile unless method is invoked somewhere else in the original cpp file

Question

Here's the body of main.cpp

#include "Header.h"

int main()
{
    auto f = Foo();
    f.bar();
    return 0;
}

Here's the body of Header.h

class Foo {
public:
    void bar();
};

Here's the body of Source.cpp

#include <iostream>

class Foo {
public:
    void bar() {
        std::cout << "Foo.bar() called\n";
    }
};

void t() {
    auto f = Foo();
    f.bar(); // If this line isn't here, the project won't compile
}

When I comment out f.bar(); in Source.cpp, I receive the following error upon compilation. It's telling me that f.bar() in main() is unresolved:

Error LNK2019 unresolved external symbol "public: void __thiscall Foo::bar(void)" (?bar@Foo@@QAEXXZ) referenced in function _main ...

I understand that it's common to define methods outside of class scope, and indeed the following version of Source.cpp compiles successfully--

#include <iostream>
#include "Header.h"

void Foo::bar() {
    std::cout << "Foo.bar() called\n";
}

Nonetheless, I don't understand what's wrong with the original version. It feels like something mysterious and magical is going on that I don't fully understand.

in main.cpp Foo::bar() is called but it doesn't have a body. — Hatted Rooster
your source.cpp looks pretty wrong. Thats not how you define class functions there. You may get some random overloading somewhere because you declare the class Foo twice. — Hayt

sp2danny sp2danny · Accepted Answer · 2016-09-19T08:03:14

This is an ODR violation either way, because the class Foo as defined in the header, and the class Foo as defined in the cpp file are not token by token identical.

out of line definition of bar should look like

#include <iostream>
#include "Header.h"

void Foo::bar() {
    std::cout << "Foo.bar() called\n";
}

Note that ODR violations are 'no diagnostics required'
(this is why one of the examples compiles, the compiler can't always detect ODR violations)

The best way to not accidentally create errors like this is to always include the header-file in the file/files where you implement the methods.

When we are talking about good practices, don't forget the include-guard in the header-file.

If a method is short enough to be inline defined, (style may vary, but for me that means a short oneliner), then that inline definition need to be present in the header-file, so that the compiler sees an identical type every time it encounters it.

When a method is defined inside class scope, project won't compile unless method is invoked somewhere else in the original cpp file

4 Answers