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#include <iostream>
using namespace std;

class Complex
{
  friend Complex operator + (Complex&, Complex&);
public:
  Complex();
  Complex(double);//½«Ë«¾«¶ÈÐÍת»»³ÉComplexÐÍ
  Complex(double, double);
  ...
private:
  double real;
  double imag;
};
Complex::Complex(double r)
{
  real = r;
  imag = 0;
}
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Complex c1(3,-4);
c2 = 4.0 + c2;
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$ g++ -c main.cpp
main.cpp: In function `int main()':
main.cpp:26: error: no match for 'operator+' in '4.0e+0 + c2'
complex.h:17: note: candidates are: Complex operator+(Complex&, Complex&)

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»¹ÓÐfriend Complex operator + (Complex&, Complex&);
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friend Complex operator + (Complex& c1, Complex& c2)
{
  return Complex(c1.real + c2.real, c1.imag + c2.imag);
}

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c2 = 4.0 + c2; ->   c2 = c2+4.0;
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