cpp-h-expl.cpp

// definition - with `{}`
(1) Post processed cpp file.
// From header
class Foo // definition
{
    void f(); // declaration
};

// From implementation file
void Foo::f() // definition
{
}

// Ok

(2) Post processed cpp file.
// From header
class Foo // definition
{
    void f(); // declaration
};

// From implementation file

// From header
class Foo // Foo is 2nd time defined!
{
    void f(); // declaration
};

void Foo::f() // that's still ok.
{
}