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Implicit function template creation

I have the following code which I only figured out to practice function templates.

#include <iostream>

template <typename T>
T fun( const T &t ) { return t; }

struct A {
    int dataf;
    A( int a ) : dataf(a) { std::cout << "birth\n"; }
    friend A fun( const A & );
};

int main(){
    A a( 5 );
    fun( a );   
    return 0;
}

Although I am getting the following error:

code.cc:(.text+0x32): undefined reference to `fun(A const&)'
collect2: ld returned 1 exit status

I understand class templates well, but I'm still confused about functional templates.

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2 answers


Change your friend's ad to:

template <class T> friend T fun( const T & );


or:

friend A fun<A>( const A & );
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Normal functions are preferred over function templates during overloading. Declaring a free friend function inside A

is an exact match to call in main

. The declaration is all the compiler has to pick up, so it compiles fine, but the linker can't find the definition because, well, you never defined it.



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