如何去除模板類輸出過載的錯誤

Question

friend ostream& operator<< (ostream& os,const aList<T>&); 

template <class T> 
ostream & operator << (ostream &output,const aList<T>& a){ 
    output<<a.length; 
    return output; 
} 

................................ .....

error: g++ -c -g -MMD -MP -MF "build/Debug/Cygwin-Windows/aList.o.d" -o build/Debug/Cygwin-Windows/aList.o aList.cpp 
In file included from aList.cpp:2:0: 
aList.h:22:68: warning: friend declaration ‘std::ostream& operator<<(std::ostream&, const aList<T>&)’ declares a non-template function [-Wnon-template-friend] 
      friend ostream& operator<< (ostream& os,const aList<T>&); 
                    ^
aList.h:22:68: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) 
mkdir -p dist/Debug/Cygwin-Windows 
g++  -o dist/Debug/Cygwin-Windows/alist build/Debug/Cygwin-Windows/_ext/2ebfcef6/newstring.o build/Debug/Cygwin-Windows/aList.o build/Debug/Cygwin-Windows/alist1.o 
build/Debug/Cygwin-Windows/alist1.o: In function `main': 
/cygdrive/c/Users/Zunera Jamal/Documents/NetBeansProjects/aList/alist1.cpp:50: undefined reference to `operator<<(std::ostream&, aList<newstring> const&)' 
/cygdrive/c/Users/Zunera Jamal/Documents/NetBeansProjects/aList/alist1.cpp:50:(.text+0x2ca): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `operator<<(std::ostream&, aList<newstring> const&)' 

Answer 1

乍一看，你的朋友聲明指的是但不在模板聲明的事實看起來像你的第一個問題。

看看這個問題。這似乎有很大的相關性： friend declaration declares a non-template function