为什么g ++似乎混淆了数据类型？

Question

I am currently making a big integer library for my college assignment. I am experiencing a problem with the following code:

MyInteger::MyInteger(){ //default
    //some code
}
MyInteger::MyInteger(char *s){ //construct from string
    //some code
}
MyInteger::MyInteger(MyInteger &otherInt){ //copy constructor
    //some code
}
MyInteger MyInteger::parse(char *s){ //parse string and create new object
    return MyInteger(s);
}

I get the following error about the parse function:

MyInteger.cpp: In static member function ‘static MyInteger MyInteger::parse(char*)’:
MyInteger.cpp:34:20: error: no matching function for call to ‘MyInteger::MyInteger(MyInteger)’
  return MyInteger(s);
                    ^
MyInteger.cpp:34:20: note: candidates are:
MyInteger.cpp:23:1: note: MyInteger::MyInteger(MyInteger&)
 MyInteger::MyInteger(MyInteger &otherInt){ //copy constructor
 ^
MyInteger.cpp:23:1: note:   no known conversion for argument 1 from ‘MyInteger’ to ‘MyInteger&’
MyInteger.cpp:10:1: note: MyInteger::MyInteger(char*)
 MyInteger::MyInteger(char *s){ //construct from string
 ^
MyInteger.cpp:10:1: note:   no known conversion for argument 1 from ‘MyInteger’ to ‘char*’
MyInteger.cpp:4:1: note: MyInteger::MyInteger()
 MyInteger::MyInteger(){ //set string to 0
 ^
MyInteger.cpp:4:1: note:   candidate expects 0 arguments, 1 provided

Shouldn't it be using the 2nd constructor? Either it is confusing the string with a MyInteger, or the string is being converted to a MyInteger somehow and then the compiler is trying to convert it again using the 3 candidates that it has listed. A similar error is occuring with the overloaded + operator.

Please tell me what I am doing wrong.

Answer 1

It's not the MyInteger(s) that's the problem. It constructs that temporary object. It's the attempt to return this temporary object that is the problem. You are returning by value, which means that a copy needs to be made, yet your copy constructor takes a MyInteger& , which is unable to bind to temporary objects (rvalues). Your copy constructor should have parameter of type const MyInteger& instead, which will allow it to do so.