multiple definition error including c++ header file with inline code from multiple sources

Question

I have a c++ header file containing a class. I want to use this class in several projects, bu I don't want to create a separate library for it, so I'm putting both methods declarations and definitions in the header file:

// example.h
#ifndef EXAMPLE_H_
#define EXAMPLE_H_
namespace test_ns{

class TestClass{
public:
    void testMethod();
};

void TestClass::testMethod(){
    // some code here...
}

} // end namespace test_ns
#endif

If inside the same project I include this header from more than one cpp file, I get an error saying "multiple definition of test_ns::TestClass::testMethod()", while if I put the method definition inside the class body this does not happen:

// example.h
#ifndef EXAMPLE_H_
#define EXAMPLE_H_
namespace test_ns{

class TestClass{
public:
    void testMethod(){
        // some code here...
    }
};

} // end namespace test_ns
#endif

Since the class is defined inside a namespace, shouldn't the two forms be equivalent? Why is the method considered to be defined twice in the first case?

Answer 1

These are not equivalent. The second example given has an implicit 'inline' modifier on the method and so the compiler will reconcile multiple definitions itself (most likely with internal linkage of the method if it isn't inlineable).

The first example isn't inline and so if this header is included in multiple translation units then you will have multiple definitions and linker errors.

Also, headers should really always be guarded to prevent multiple definition errors in the same translation unit. That should convert your header to:

#ifndef EXAMPLE_H
#define EXAMPLE_H

//define your class here

#endif