重载运算符 C++：错误：没有可行的重载“=”

Question

My goal is to overload the '+' operator so that I can combine a Paragraph object and a Story object. This function should return a new Story object with the paragraph attached to the beginning.

Story Paragraph::operator+(const Story& story) {
    Paragraph paragraph;
    Story stry;

    Paragraph storyPara = story.paragraph;
    Sentence paraSentence = storyPara.sentence;

    paragraph.sentence = this->sentence + paraSentence;
    stry.paragraph = paragraph;

    return stry;
}

However, when I run all my code (A Story object should have a paragraph. A Paragraph object should have a sentence. A Sentence object should have a word, etc.) , I get this error:

error: no viable overloaded '='

This occurs when I try to do the following line:

paragraph.sentence = this->sentence + paraSentence;

I'm not quite sure how to add the sentences together to form a paragraph (to ultimately form & return a new Story). Does anyone know how to approach this problem?

Answer 1

you can assume that all my classes are defined properly

This is the wrong assumption which causes you this error. Sentence class has apparently no or wrong operator= and/or copy constructor defined

Answer 2

Paragraph operator+(const Sentence& sent);

This declares an operator such that adding two Sentence s results in a Paragraph .

 paragraph.sentence = this->sentence + paraSentence;

The right part of the assignment uses the operator from above, so you are trying to assing a Paragraph to a Sentence , as if you had written:

Paragraph additionResult = this->sentence + paraSentence;
paragraph.sentence = additionResult;

The problem is that you have not defined an assignment from Paragraph in Sentence . You could just add it to Sentence , of course:

Sentence& operator=(const Paragraph& para);

But how would you implement it? Can a paragraph logically be converted into a single sentence? This solution will not really work.

An alternative solution is to change the corresponding operator+ in Sentence to return a Sentence rather than a paragraph:

class Sentence {
    public:
        Sentence();     
        ~Sentence();        
        void show();
        Sentence operator+(const Sentence& sent); // <-- now returns a Sentence
        Paragraph operator+(const Paragraph& paragraph);
        Sentence operator+(const Word& word);

        Word word;              

};

When adding two Sentence s returns a Sentence , then the result of the addition can also be assigned to a Sentence , because copy assignment from the same type is automatically generated by the compiler (unless you explicitly delete it).

But this presents its own share of problems, because how can two sentences logically be combined into one?

The real problem can probably be found in this line:

Sentence sentence;      // Sentence in Paragraph

Your class definition effectively says that a paragraph always consists of exactly one sentence. This cannot be correct. The member variable should be of type std::vector<Sentence> to express the intention that one paragraph consists of 0 to n sentences. Once you have changed the member variable, rewrite all operator implementations to account for the new situation.

Of course, you have the same problem in Sentence (and I guess in your other classes, too).

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Generally, check your books/tutorials again and review the chapter about operator overloading. You are not following best practices. For example, you should define + in terms of += . And of course, an important question is whether operator overloading is actually helpful here or not.