Interface with friend operator<<: why can't it link?

Question

I have created a small interface for objects that can be represented in text mode using the operator<< like so:

// ICliObject.h

class ICliObject
{
public:

    ~ICliObject() = default;
    friend std::ostream& operator<<(std::ostream& p_stream, const ICliObject& p_cliUiObject);


protected:
    virtual void print(std::ostream& p_stream) const = 0;

};


std::ostream& operator<<(std::ostream& p_stream, const ICliObject& p_cliUiObject)
{
    p_cliUiObject.print(p_stream);

    return p_stream;
}

When I inherit from this interface and try to build, the compilation works but I get the following linking error: In blablabla.cpp: multiple definition of operator<<(std::ostream& p_stream, const ICliObject& p_cliUiObject)

In all derived classes, I have took care of not re-defining/declaring the operator. The only way I can solve my problem is in inlining the operator in ICliObject.h. What is going on?

Note: I use GCC on Ubuntu.

Either mark the function definition in the header with `inline` keyword, or move it to some source file. — Igor Tandetnik, Jun 08 '17 at 22:37
Copy your linker error and search for it in SO. You might find one or more questions on the subject. — R Sahu, Jun 08 '17 at 22:39
I have, but it's mostly people that have forgotten not to re-define/declare. Also, I have found the inline solution, I just want to know why it is neccessary here. Thanks — BobMorane, Jun 08 '17 at 22:41
It seems to me the linker should know what to do here even without inlining. — BobMorane, Jun 08 '17 at 22:42
Telling the linker not to complain that the same symbol is defined in multiple translation units is the reason the `inline` keyword exists. — Miles Budnek, Jun 08 '17 at 22:46
@MilesBudnek: I'd argue that it's the reason the `inline` keyword _still_ exists; it doesn't seem likely to have been its original primary job, given the name. — Lightness Races in Orbit, Jun 08 '17 at 23:05
@IgorTandetnik: Please, do not answer questions in the comments section. Thank you. — Lightness Races in Orbit, Jun 08 '17 at 23:05
@BobMorane: "Don't put [non-inline] function definitions into header files" - have you ever heard about this basic rule? — AnT stands with Russia, Jun 08 '17 at 23:07

score 3 · Accepted Answer · answered Jun 08 '17 at 23:04

3

This has nothing to do with friend or with operator<< or with inheritance.

Like any function (or object) defined at namespace scope, if you do so multiple times in your program (including by having it in a header file that you #include in multiple translation units), you will get this error.

And, like in all those other cases, the solution is to either move the implementation to a "source file", or stick the inline keyword on it (which you already suggested, but it's literally the solution so I don't know why it's not acceptable).

answered Jun 08 '17 at 23:04

Lightness Races in Orbit

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Another possible solution would be to define the function inside the class definition (because that makes it implicitly `inline`) – M.M Jun 08 '17 at 23:12
So the definition I added outside the class (but in the same header file) for `operator<<` made it defined multiple times across the program, which caused the linking error. I get it, thanks. – BobMorane Jun 08 '17 at 23:45

Interface with friend operator<<: why can't it link?

1 Answers1