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POmultiple definition in header file
 

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  1. POmultiple definition in header file
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    <p>Given this code sample:</p> <p><strong>complex.h :</strong></p> <pre><code>#ifndef COMPLEX_H #define COMPLEX_H #include &lt;iostream&gt; class Complex { public: Complex(float Real, float Imaginary); float real() const { return m_Real; }; private: friend std::ostream&amp; operator&lt;&lt;(std::ostream&amp; o, const Complex&amp; Cplx); float m_Real; float m_Imaginary; }; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; o, const Complex&amp; Cplx) { return o &lt;&lt; Cplx.m_Real &lt;&lt; " i" &lt;&lt; Cplx.m_Imaginary; } #endif // COMPLEX_H </code></pre> <p><strong>complex.cpp :</strong></p> <pre><code>#include "complex.h" Complex::Complex(float Real, float Imaginary) { m_Real = Real; m_Imaginary = Imaginary; } </code></pre> <p><strong>main.cpp :</strong></p> <pre><code>#include "complex.h" #include &lt;iostream&gt; int main() { Complex Foo(3.4, 4.5); std::cout &lt;&lt; Foo &lt;&lt; "\n"; return 0; } </code></pre> <p>When compiling this code, I get the following error:</p> <pre><code>multiple definition of operator&lt;&lt;(std::ostream&amp;, Complex const&amp;) </code></pre> <p>I've found that making this function <code>inline</code> solves the problem, but I don't understand why. Why does the compiler complain about multiple definition? My header file is guarded (with <code>#define COMPLEX_H</code>).</p> <p>And, if complaining about the <code>operator&lt;&lt;</code> function, why not complain about the <code>public real()</code> function, which is defined in the header as well?</p> <p>And is there another solution besides using the <code>inline</code> keyword?</p>
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    1. USKeith Pinson
    1. USJérôme
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    1. COYou can also make the function static. The inline specifier is commonly used to force a function to have internal linkage.
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      1. USAkanksh
    2. CO@Akanksh, actually that's exactly what "inline" is for.
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      1. USMichael Aaron Safyan
    3. CO@Akanksh: Using `static` for this purpose is deprecated in C++. `static` has been completely replaced by anonymous namespaces, although in this particular case, `inline` is the way to go.
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      1. USKonrad Rudolph
 

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