错误：对非常量的引用的初始值必须为左值

Question

Getting another error with my string class. The intellisense will not let me use strcmp to compare an object with the self-invoking object (ie *this).

I tried making my own operator conversion function to help with this, but it is still giving me the error.

What do I have to change in my code to make this work??

//Overloaded comparison operators
bool &String::operator<(const String & obj)
{
    return strcmp(*this, obj) < 0 ? true : false;
}

//Operator conversion function
String::operator char const * () const
{
    return mStr;
}

Answer 1

You are returning a reference to a local variable. Return by value. You can also simplify the return expression, and make the method const , since comparing two objects should not change either of them:

bool String::operator<(const String & obj) const {
  return strcmp(*this, obj) < 0;
}

although I am not sure strcmp can deal with two String s, which is what you are passing it. Judgning from your previous question, you need

 
 
 
  
  return strcmp(mstr, obj.mStr) < 0;
 
  

Answer 2

Make it const and remove &

bool String::operator<(const String & obj) const
                                           ^^^^^

Returning references to temporary objects leads to undefined behavior.

Answer 3

错误消息显式指向strcmp()第一个参数，*这是一个右值，但是strcmp需要一个左值，请尝试在前面添加一个句子：

String thisObj = *this;