将错误与模板类中的友元函数链接起来 [英] Linking error with friend function in template class

问题描述

我在使用自制的 Complex 类时遇到链接问题.

I have a linking problem when using a home-made Complex class.

类定义:

template<class T>
class Complex
{
public:
    Complex(const T real = 0, const T imag = 0);
    Complex(const Complex<T>& other);
    ~Complex(void) {};

    Complex<T> operator*(const Complex<T>& other) const;
    Complex<T> operator/(const Complex<T>& other) const;
    Complex<T> operator+(const Complex<T>& other) const;
    Complex<T> operator-(const Complex<T>& other) const;

    friend void operator*=(const Complex<T>& z,const Complex<T>& other);
    friend void operator/=(const Complex<T>& z,const Complex<T>& other);
    friend void operator+=(const Complex<T>& z,const Complex<T>& other);
    friend void operator-=(const Complex<T>& z,const Complex<T>& other);
    void operator=(const Complex<T>& other);

    friend T& real(Complex<T>& z);
    friend T& imag(Complex<T>& z);
    friend T abs(Complex<T>& z);
    friend T norm(Complex<T>& z);
private:
    T real_;
    T imag_;
};

abs的实现:

template<class T>
T abs(Complex<T>& z)
{
    return sqrt(z.real_*z.real_ + z.imag_*z.imag_);
}

我像这样使用 abs 函数:if(abs(z) <= 2).

I use the function abs like this : if(abs(z) <= 2).

这是我得到的一些错误:

Here are some errors I get:

Error   4   error LNK2001: unresolved external symbol "long double __cdecl abs(class Complex<long double> &)" (?abs@@YAOAAV?$Complex@O@@@Z) C:\Users\Lucas\Documents\Visual Studio 2012\Projects\Fractals\Fractals\Main.obj Fractals
Error   3   error LNK2001: unresolved external symbol "long double & __cdecl imag(class Complex<long double> &)" (?imag@@YAAAOAAV?$Complex@O@@@Z)   C:\Users\Lucas\Documents\Visual Studio 2012\Projects\Fractals\Fractals\Main.obj Fractals

我在使用 Complex 而不是 Complex 时遇到了同样的错误.我使用 Visual C++ 2012.如果你给我一些关于如何解决这个问题的提示，我会很高兴.谢谢.

I get the same errors when using Complex<float> instead of Complex<long double>. I work with Visual C++ 2012. I would be really glad if you give me some hint on how to fix this. Thank you.

推荐答案

函数声明为

template <typename T>
class Complex {
    // ...
    friend T abs(Complex<T>& z);
    // ...
};

不是一个函数模板！它看起来有点像，因为它嵌套在一个类模板中，但这还不够.以下是您可能要写的内容:

is not a function template! It looks a bit like one as it is nested within a class template but that isn't quite enough. Here is what you probably meant to write:

template <typename T> class Complex;
template <typename T> T abs(Complex<T>&);


template <typename T>
class Complex {
    // ...
    friend T abs<T>(Complex<T>& z);
    // ...
};

或者，您可以在将其声明为 friend 时实现 abs().

Alternatively, you could implement abs() when declaring it as friend.

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