模板友元函數查找

Question

下面的簡單代碼編譯罰款

class A { 
    int x[3]; 
public: 
    A() { x[0]=1; x[1]=2; x[2]=3; } 
    friend int const&at(A const&a, unsigned i) noexcept 
    { 
    return a.x[i]; 
    } 
    friend int foo(A const&a, unsigned i) noexcept 
    { 
    int tmp = at(a,i); 
    return tmp*tmp; 
    } 
}; 

但如果朋友們製作的模板

class A { 
    int x[3]; 
public: 
    A() { x[0]=1; x[1]=2; x[2]=3; } 

    template<unsigned I> 
    friend int const&at(A const&a) noexcept 
    { 
    static_assert(I<3,"array boundary exceeded"); 
    return a.x[I]; 
    } 

    template<unsigned I> 
    friend int foo(A const&a) noexcept 
    { 
    int tmp = at<I>(a); // <- error: use of undeclared identifier 'at' 
    return tmp*tmp; 
    } 
}; 

查找規則的改變和鐺抱怨與所述錯誤，但gcc和icpc不。誰是對的（C++ 11）？以及如何獲取叮噹的代碼？

Answer 1

解決方法是單獨的聲明和定義：

class A { 
    int x[3]; 
public: 
    A() { x[0]=1; x[1]=2; x[2]=3; } 

    template<unsigned I> 
    friend int const&at(A const&a) noexcept; 

    template<unsigned I> 
    friend int foo(A const&a) noexcept; 
}; 

template<unsigned I> 
int const&at(A const&a) noexcept 
{ 
    static_assert(I<3,"array boundary exceeded"); 
    return a.x[I]; 
} 

template<unsigned I> 
int foo(A const&a) noexcept 
{ 
    int tmp = at<I>(a); 
    return tmp*tmp; 
}