operator overloading Error: cannot bind non-const lvalue reference of type ‘some-type’ to an rvalue of type ‘some-type’

Question

#include <iostream>
#include <string>

using namespace std;

namespace ComplexNumberNamespace
{
    typedef struct _Data
    {
        double real;
        double imag;
    }Data;

    class ComplexNumber
    {
    private:
        Data data{};
    public:
        ComplexNumber();
        ComplexNumber(float real, float imag);
        ComplexNumber(ComplexNumber&);
        string to_string();
        void operator = (ComplexNumber&);
        ComplexNumber operator + (ComplexNumber& rhs);
    };

    string ComplexNumber::to_string() {
        std::string str;
        str = std::to_string(data.real);
        str.append(" + i ");
        str.append(std::to_string(data.imag));

        return str;
    }

    ComplexNumber::ComplexNumber(ComplexNumber & rhs) {
        std::cout<<"copy constructor";
        data.real = rhs.data.real;
        data.imag = rhs.data.imag;
    }

    ComplexNumber::ComplexNumber() {
        data.real = 0.0;
        data.imag = 0.0;
    }

    void ComplexNumber::operator=(ComplexNumber &cplx) {
        std::cout<<"assignment operator";
        data.real = cplx.data.real;
        data.imag = cplx.data.imag;
    }

    ComplexNumber::ComplexNumber(float real, float imag) {
        data.real = real;
        data.imag = imag;
    }

    ComplexNumber ComplexNumber::operator+(ComplexNumber &rhs)
    {
        ComplexNumber temp(data.real + rhs.data.real, data.imag+rhs.data.imag);

        return temp;
    }
}

using namespace ComplexNumberNamespace;

int main()
{
    ComplexNumber c1(1, 2);

    ComplexNumber c2(4, 3);

    ComplexNumber sum = c1 + c2;

    std::cout<<sum.to_string();
}

编译器输出

====================[ Build | all | Debug ]=====================================
"C:\Program Files\JetBrains\CLion 2020.3.2\bin\cmake\win\bin\cmake.exe" --build C:\Users\pc\source\repos\Doxygen\input_cpp_files\cmake-build-debug --target all -- -j 3
Scanning dependencies of target input_cpp_files
[ 50%] Building CXX object CMakeFiles/input_cpp_files.dir/MyCppFile.cpp.obj
C:\Users\pc\source\repos\Doxygen\input_cpp_files\MyCppFile.cpp: In function 'int main()':
C:\Users\pc\source\repos\Doxygen\input_cpp_files\MyCppFile.cpp:79:28: error: cannot bind non-const lvalue reference of type 'ComplexNumberNamespace::ComplexNumber&' to an rvalue of type 'ComplexNumberNamespace::ComplexNumber'
     ComplexNumber sum = c1 + c2;
                         ~~~^~~~
C:\Users\pc\source\repos\Doxygen\input_cpp_files\MyCppFile.cpp:40:5: note:   initializing argument 1 of 'ComplexNumberNamespace::ComplexNumber::ComplexNumber(ComplexNumberNamespace::ComplexNumber&)'
     ComplexNumber::ComplexNumber(ComplexNumber & rhs) {
     ^~~~~~~~~~~~~
mingw32-make.exe[2]: *** [CMakeFiles\input_cpp_files.dir\build.make:83: CMakeFiles/input_cpp_files.dir/MyCppFile.cpp.obj] Error 1
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:95: CMakeFiles/input_cpp_files.dir/all] Error 2
mingw32-make.exe: *** [Makefile:103: all] Error 2

为什么我的源代码会产生这个错误？

我做错了什么？

我该如何解决这个问题？

Answer 1

您的构造函数和运算符重载以 ComplexNumber& 作为参数需要取而代之的是 const ComplexNumber&。

这一行 ComplexNumber sum = c1 + c2 实际上调用了您的 ComplexNumber 复制构造函数并给它 c1 + c2 表达式结果，这是一个临时的 ComplexNumber。由于无法在 C++ 中对临时 object 进行 non-const 引用，因此会出现此错误。

理想情况下，您的 ComplexNumber class 应该如下所示：

struct ComplexNumber {

  // ...

  // Copy 
  ComplexNumber(const ComplexNumber& other) { *this = other; } 
  ComplexNumber& operator=(const ComplexNumber& other) {
    if (this != &other) { /* copy your attributes from other to this */ }
    return *this;
  }

  // Move (optional - and not really useful in your case)
  ComplexNumber(ComplexNumber&& other);
  ComplexNumber& operator=(ComplexNumber&& other);

  // Unlike operator= we return a temporary value, 
  // so it's a ComplexNumber indeed (no &)
  // We'll modify neither 'this' nor 'rhs', so mark them const
  ComplexNumber operator+(const ComplexNumber& rhs) const;

  ~ComplexNumber();
};

更多详情：https://en.cppreference.com/w/cpp/language/rule_of_three

一些提示：

不要把 using namespace std 放在 header 中（我猜是 header）
typedef struct _Data { ... } Data; 是旧的 C 表示法。您可以简单地使用 struct Type { ... } myObj;

operator overloading Error: cannot bind non-const lvalue reference of type ‘some-type’ to an rvalue of type ‘some-type’

operator overloading Error: cannot bind non-const lvalue reference of type ‘some-type’ to an rvalue of type ‘some-type’

c++

operator-overloading

addition