Why I can implement the class's member function in header file?

Question

I know that you should only declare a function in header and avoid define it because if more than one source file include this header ,the linker will tell you there are duplicate symbol.

I also know that it is recommended that declare a class in header and implement the member function in source file

But here is my question : I try to define the whole class in header (including all the implementation of member function) ,then I found that there was no error from linker when I include this header in two source files.

Here is my header.h file

class ctr
{
public:
    ctr();
    ctr(char *s);
    int showname(){return 0;}
private:
    char *name;

};

In other two file, I include header.h

//file1.cpp
#include header.h

//file2.cpp
#include header.h

Then compile them g++ file1.cpp file2.cpp

So can anyone tell me why normal function definition will give me an error but class definition is ok?

Answer 1

Member functions defined within the class body are implicitly inline [class.mfct]/p2:

A member function may be defined (8.4) in its class definition, in which case it is an inline member function [..]

The inline specifier allows a function to be defined across multiple translation units.