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c++one-definition-rule
Aditya Verma
Aditya Verma

Reputation: 177

C++: Defining class in multiple files

According to One Definition Rule (ODR):

In the entire program, an object or non-inline function cannot have more than one definition; if an object or function is used, it must have exactly one definition.

These are my files that I am trying to compile:

A.cpp

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

int main()
{
    greet();
    std::cin.get();
}

B.cpp

#include "Greet.h"

Greet.h

#include <iostream>

void greet()
{
    std::cout << "Hello World!" << std::endl;
}

I get a linker error as expected:

fatal error LNK1169: one or more multiply defined symbols found.

But when I put greet() function in a class. The code compiles fine and gives the output Hello World!.

A.cpp

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

int main()
{
    Greet G;
    G.greet();

    std::cin.get();
}

B.cpp

#include "Greet.h"

Greet.h

#include <iostream>

class Greet
{
public:
    void greet()
    {
        std::cout << "Hello World!" << std::endl;
    }
};

Why is the linker not complaining about multiple definitions of class Greet?

The behavior is the same for both MSVC and g++.

Upvotes: 4

Views: 354

Answers (1)

R Sahu
R Sahu

Reputation: 206747

But when I put greet() function in a class. The code compiles fine and gives the output Hello World!.

When a member function is defined inside a class definition, inline is implied. That is not so for non-member functions.

From the C++ Standard, class.mfct:

A member function may be defined ([dcl.fct.def]) in its class definition, in which case it is an inline member function ([dcl.fct.spec]), or it may be defined outside of its class definition if it has already been declared but not defined in its class definition.

Upvotes: 6

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