C++ – No == operator found while comparing structs in C++

ccomparison-operatorsstruct

Comparing two instances of the following struct, I receive an error:

struct MyStruct1 {
    MyStruct1(const MyStruct2 &_my_struct_2, const int _an_int = -1) :
        my_struct_2(_my_struct_2),
        an_int(_an_int)
    {}

    std::string toString() const;

    MyStruct2 my_struct_2;
    int an_int;
};

The error is:

error C2678: binary '==' : no operator
found which takes a left-hand operand
of type 'myproj::MyStruct1' (or there
is no acceptable conversion)

Why?

Best Answer

In C++, structs do not have a comparison operator generated by default. You need to write your own:

bool operator==(const MyStruct1& lhs, const MyStruct1& rhs)
{
    return /* your comparison code goes here */
}

Related Solutions

C++ – std::endl is of unknown type when overloading operator<<

std::endl is a function and std::cout utilizes it by implementing operator<< to take a function pointer with the same signature as std::endl.

In there, it calls the function, and forwards the return value.

Here is a code example:

#include <iostream>

struct MyStream
{
    template <typename T>
    MyStream& operator<<(const T& x)
    {
        std::cout << x;

        return *this;
    }


    // function that takes a custom stream, and returns it
    typedef MyStream& (*MyStreamManipulator)(MyStream&);

    // take in a function with the custom signature
    MyStream& operator<<(MyStreamManipulator manip)
    {
        // call the function, and return it's value
        return manip(*this);
    }

    // define the custom endl for this stream.
    // note how it matches the `MyStreamManipulator`
    // function signature
    static MyStream& endl(MyStream& stream)
    {
        // print a new line
        std::cout << std::endl;

        // do other stuff with the stream
        // std::cout, for example, will flush the stream
        stream << "Called MyStream::endl!" << std::endl;

        return stream;
    }

    // this is the type of std::cout
    typedef std::basic_ostream<char, std::char_traits<char> > CoutType;

    // this is the function signature of std::endl
    typedef CoutType& (*StandardEndLine)(CoutType&);

    // define an operator<< to take in std::endl
    MyStream& operator<<(StandardEndLine manip)
    {
        // call the function, but we cannot return it's value
        manip(std::cout);

        return *this;
    }
};

int main(void)
{
    MyStream stream;

    stream << 10 << " faces.";
    stream << MyStream::endl;
    stream << std::endl;

    return 0;
}

Hopefully this gives you a better idea of how these things work.

C++ – the “–>” operator in C/C++

--> is not an operator. It is in fact two separate operators, -- and >.

The conditional's code decrements x, while returning x's original (not decremented) value, and then compares the original value with 0 using the > operator.

To better understand, the statement could be written as follows:

while( (x--) > 0 )

Best Answer

Related Solutions

C++ – std::endl is of unknown type when overloading operator<<

C++ – the “–>” operator in C/C++

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